View source for Astrobiology

{{Short description|Science concerned with life in the universe}}
{{redirect-distinguish|Exobiology|Bioastronautics|Xenobiology}}
{{redirect|Xenology|other uses|Xenology (disambiguation)}}
{{Use dmy dates|date=May 2020}}
{{Life in the Universe}}
[[File:DNA animation.gif|thumb|[[Nucleic acids]] may not be the only [[biomolecule]]s in the universe capable of coding for life processes.<ref name="AstroDNA">{{cite web|url=http://www.astrobio.net/debate/2168/launching-the-alien-debates |title=Launching the Alien Debates (part 1 of 7) | access-date=5 May 2014 | date=8 December 2006 |work=[[Astrobiology Magazine]] |publisher=NASA |archive-url=https://web.archive.org/web/20201023105824/https://www.astrobio.net/extreme-life/launching-the-alien-debates/ |archive-date=2020-10-23 |url-status=usurped}}</ref>]]

'''Astrobiology''' (also '''xenology''' or '''exobiology''') is a scientific field within the [[List of life sciences|life]] and [[environmental science]]s that studies the [[abiogenesis|origins]], [[Protocell|early evolution]], distribution, and future of [[life]] in the [[universe]] by investigating its deterministic conditions and contingent events.<ref name="about">{{cite web| url=http://astrobiology.nasa.gov/about-astrobiology/ |title=About Astrobiology |access-date=20 October 2008 |date=21 January 2008 |work=NASA Astrobiology Institute |publisher=NASA | url-status=dead | archive-url= https://web.archive.org/web/20081011192341/http://astrobiology.nasa.gov/about-astrobiology/| archive-date= 11 October 2008}}</ref> As a discipline, astrobiology is founded on the premise that life may exist beyond Earth.<ref>{{cite web |title=About Astrobiology |url=https://astrobiology.nasa.gov/about/ |website=NASA Astrobiology Institute |publisher=NASA |access-date=29 January 2023}}</ref>

Research in astrobiology comprises three main areas: the study of [[planetary habitability|habitable environments]] in the [[Solar System]] and beyond, the search for planetary [[biosignatures]] of past or present extraterrestrial life, and the study of the [[Abiogenesis|origin]] and [[Protocell|early evolution]] of life on Earth.

The field of astrobiology has its origins in the 20th century with the advent of [[space exploration]] and the discovery of [[exoplanets]]. Early astrobiology research focused on the search for extraterrestrial life and the study of the potential for life to exist on other planets.<ref name="about" /> In the 1960s and 1970s, NASA began its astrobiology pursuits within  the [[Viking program]], which was the first US mission to land on Mars and search for [[Viking biological experiments|signs of life]].<ref>{{cite book |title=The Living Universe: NASA and the Development of Astrobiology |url=https://archive.org/details/livinguniversena0000dick |url-access=registration |author1=Steven J. Dick  |author2=James E. Strick  |name-list-style=amp |publisher=Rutgers University Press |location=New Brunswick, NJ |date=2004}}</ref> This mission, along with other early space exploration missions, laid the foundation for the development of astrobiology as a discipline.

Regarding [[planetary habitability|habitable environments]], astrobiology investigates potential locations beyond Earth that could support life, such as [[Mars]], [[Europa (moon)|Europa]], and [[exoplanets]], through research into the [[extremophile]]s populating austere environments on Earth, like volcanic and deep sea environments. Research within this topic is conducted utilising the methodology of the geosciences, especially [[geobiology]], for astrobiological applications.

The search for [[biosignatures]] involves the identification of signs of past or present life in the form of [[List of interstellar and circumstellar molecules|organic compounds]], isotopic ratios, or microbial fossils. Research within this topic is conducted utilising the methodology of [[planetary science|planetary]] and [[environmental science]], especially [[atmospheric science]], for astrobiological applications, and is often conducted through [[remote sensing]] and in situ missions.

Astrobiology also concerns the study of the [[Abiogenesis|origin]] and [[Protocell|early evolution]] of life on Earth to try to understand the conditions that are necessary for life to form on other planets.<ref>{{cite web |url=https://link.springer.com/journal/11084 |title=Origins of Life and Evolution of Biospheres |work=Journal: Origins of Life and Evolution of Biospheres |access-date=6 April 2015 }}</ref> This research seeks to understand how life emerged from non-living matter and how it evolved to become the diverse array of organisms we see today. Research within this topic is conducted utilising the methodology of paleosciences, especially [[paleobiology]], for astrobiological applications.

Astrobiology is a rapidly developing field with a strong interdisciplinary aspect that holds many challenges and opportunities for scientists. Astrobiology programs and research centres are present in many universities and research institutions around the world, and space agencies like [[NASA]] and [[ESA]] have dedicated departments and programs for astrobiology research.

== Overview ==

The term astrobiology was first proposed by the [[Soviet Union|Russian]] astronomer [[Gavriil Adrianovich Tikhov|Gavriil Tikhov]] in 1953.<ref>{{Cite journal|last=Cockell |first=Charles S. |title='Astrobiology' and the ethics of new science |url=https://www.researchgate.net/publication/42790151 |date=2001 |journal=Interdisciplinary Science Reviews  |volume=26 |issue=2 |pages=90–96 |doi=10.1179/0308018012772533 }}</ref> It is etymologically derived from the [[Ancient Greek|Greek]] {{lang|grc|ἄστρον}}, "star"; {{lang|grc|βίος}}, "life"; and {{lang|grc|-λογία}}, ''[[wikt:-logia|-logia]]'', "study". A close synonym is exobiology from the [[Ancient Greek|Greek]] Έξω, "external"; {{lang|grc|βίος}}, "life"; and {{lang|grc|-λογία}}, ''[[wikt:-logia|-logia]]'', "study", coined by American molecular biologist [[Joshua Lederberg]]; exobiology is considered to have a narrow scope limited to search of life external to Earth.<ref>[http://profiles.nlm.nih.gov/ps/retrieve/Narrative/BB/p-nid/32 Launching a New Science: Exobiology and the Exploration of Space] ''The National Library of Medicine''.</ref> Another associated term is [[xenobiology]], from the Greek ξένος, "foreign"; {{lang|grc|βίος}}, "life"; and -λογία, "study", coined by American science fiction writer [[Robert Heinlein]] in his work ''[[The Star Beast (novel)|The Star Beast]]'';<ref>{{cite journal |vauthors=Heinlein R, Harold W|title=Xenobiology |journal=Science |date=21 July 1961 |pages=223–225 | doi=10.1126/science.134.3473.223 |pmid=17818726 |jstor=1708323 |volume=134 |issue=3473|bibcode = 1961Sci...134..223H }}</ref> xenobiology is now used in a more specialised sense, referring to 'biology based on foreign chemistry', whether of extraterrestrial or terrestrial (typically synthetic) origin.<ref>{{cite journal | author=Markus Schmidt | title=Xenobiology: A new form of life as the ultimate biosafety tool | journal=BioEssays | date=9 March 2010 | pages=322–331 | doi=10.1002/bies.200900147 | volume=32 | issue=4 | pmid=20217844 | pmc=2909387 }}</ref>

While the potential for extraterrestrial life, especially intelligent life, has been explored throughout human history within philosophy and narrative, the question is a verifiable [[hypothesis]] and thus a valid line of [[science|scientific]] inquiry;<ref name="NAT-20170216">{{cite journal |last=Livio |first=Mario |title=Winston Churchill's essay on alien life found |date=15 February 2017 |journal=[[Nature (journal)|Nature]] |volume=542 |issue=7641 |pages=289–291 |doi=10.1038/542289a |pmid=28202987 |bibcode = 2017Natur.542..289L |s2cid=205092694 |doi-access=free }}</ref><ref name="NYT-20170215">{{cite news |last=De Freytas-Tamura |first=Kimiko |title=Winston Churchill Wrote of Alien Life in a Lost Essay |url=https://www.nytimes.com/2017/02/15/world/europe/winston-churchill-aliens.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2017/02/15/world/europe/winston-churchill-aliens.html |archive-date=2022-01-01 |url-access=limited |date=15 February 2017 |work=[[The New York Times]] |access-date=18 February 2017 }}{{cbignore}}</ref> planetary scientist David Grinspoon calls it a field of natural philosophy, grounding speculation on the unknown in known scientific theory.<ref>Grinspoon 2004</ref>

The modern field of astrobiology can be traced back to the 1950s and 1960s with the advent of [[space exploration]], when scientists began to seriously consider the possibility of life on other planets. In 1957, the [[Soviet Union]] launched [[Sputnik 1]], the first artificial satellite, which marked the beginning of the [[Space Age]]. This event led to an increase in the study of the potential for life on other planets, as scientists began to consider the possibilities opened up by the new technology of space exploration. In 1959, NASA funded its first exobiology project, and in 1960, NASA founded the Exobiology Program, now one of four main elements of NASA's current Astrobiology Program.<ref>{{cite web |last1=Hubbard |first1=G. Scott |title=Astrobiology: Its Origins and Development |url=https://www.nasa.gov/50th/50th_magazine/astrobiology.html |website=NASA |access-date=29 January 2023 |archive-date=28 June 2022 |archive-url=https://web.archive.org/web/20220628064139/https://www.nasa.gov/50th/50th_magazine/astrobiology.html |url-status=dead }}</ref> In 1971, NASA funded [[Project Cyclops]],<ref>{{cite web |title=Project Cyclops: a Design Study of a System for Detecting Extraterrestrial Intelligent Life |date=January 1972 |url=https://ntrs.nasa.gov/citations/19730010095 |publisher=NASA |access-date=29 January 2023}}</ref> part of the [[SETI|search for extraterrestrial intelligence]], to search radio frequencies of the electromagnetic spectrum for interstellar communications transmitted by extraterrestrial life outside the Solar System. In the 1960s-1970s, NASA established the [[Viking program]], which was the first US mission to land on Mars and search for metabolic signs of present life; the results were inconclusive.

In the 1980s and 1990s, the field began to expand and diversify as new discoveries and technologies emerged. The discovery of microbial life in extreme environments on Earth, such as deep-sea hydrothermal vents, helped to clarify the feasibility of potential life existing in harsh conditions. The development of new techniques for the detection of biosignatures, such as the use of stable isotopes, also played a significant role in the evolution of the field.

The contemporary landscape of astrobiology emerged in the early 21st century, focused on utilising Earth and environmental science for applications within comparate space environments. Missions included the ESA's [[Beagle 2]], which failed minutes after landing on Mars, NASA's [[Phoenix lander|''Phoenix'' lander]], which probed the environment for past and present planetary habitability of microbial life on Mars and researched the history of water, and NASA's [[Curiosity rover|''Curiosity'' rover]], currently probing the environment for past and present planetary habitability of microbial life on Mars.

==Theoretical foundations==
=== Planetary habitability ===
{{Main|Planetary habitability}}

Astrobiological research makes a number of simplifying assumptions when studying the necessary components for planetary habitability.

'''Carbon and Organic Compounds''': Carbon is the [[Abundance of the chemical elements|fourth most abundant element]] in the universe and the energy required to make or break a bond is at just the appropriate level for building molecules which are not only stable, but also reactive. The fact that carbon atoms bond readily to other carbon atoms allows for the building of extremely long and complex molecules. As such, astrobiological research presumes that the vast majority of life forms in the Milky Way galaxy are based on [[Carbon-based life|carbon chemistries]], as are all life forms on Earth.<ref>{{cite web|url=http://library.thinkquest.org/C003763/index.php?page=interview07 |title=Polycyclic Aromatic Hydrocarbons: An Interview With Dr. Farid Salama |access-date=20 October 2008 |date=2000 |work=[[Astrobiology Magazine]] |url-status=dead |archive-url=https://web.archive.org/web/20080620075201/http://library.thinkquest.org/C003763/index.php?page=interview07 |archive-date=20 June 2008  }}</ref><ref name="Pace">{{cite journal| title=The universal nature of biochemist ry| journal=Proceedings of the National Academy of Sciences of the USA| date=30 January 2001|first=Norman R. |last=Pace| volume=98| issue=3| pages=805–808 | pmid=11158550| doi=10.1073/pnas.98.3.805| pmc=33372 |bibcode = 2001PNAS...98..805P | doi-access=free}}</ref> However, theoretical astrobiology entertains the potential for other [[organic compounds|organic molecular]] bases for life, thus astrobiological research often focuses on identifying environments that have the potential to support life based on the presence of organic compounds.

'''Liquid water''': Liquid [[water]] is a common molecule that provides an excellent environment for the [[Abiogenesis|formation]] of complicated carbon-based molecules, and is generally considered necessary for life as we know it to exist. Thus, astrobiological research presumes that extraterrestrial life similarly depends upon access to liquid water, and often focuses on identifying environments that have the potential to support liquid water.<ref>{{cite book| url=http://www.bookrags.com/research/astrobiology-spsc-04/ |title=Astrobiology |access-date=20 October 2008 |date=2006 |publisher=Macmillan Science Library: Space Sciences. }}</ref><ref name="SSR-20191212">{{cite journal 
|author=Camprubi, Eloi|display-authors=et al. |title= Emergence of Life |date=12 December 2019 |journal=[[Space Science Reviews(journal)|Space Science Reviews]] |volume=215|issue=56 |page=56 |doi=10.1007/s11214-019-0624-8|bibcode=2019SSRv..215...56C |doi-access=free }}</ref> Some researchers posit environments of water-[[ammonia]] mixtures as possible solvents for [[hypothetical types of biochemistry]].<ref>{{cite web| url=http://www.astrobio.net/news/article2057.html |title=The Ammonia-Oxidizing Gene |access-date=20 October 2008 |author=Penn State |date=19 August 2006 |publisher=[[Astrobiology Magazine]] |archive-url=https://web.archive.org/web/20110804221625/http://www.astrobio.net/pressrelease/2057/the-ammonia-oxidizing-gene |archive-date=2011-08-04 |url-status=usurped }}</ref>

'''Environmental stability''': Where organisms adaptively evolve to the conditions of the environments in which they reside, environmental stability is considered necessary for life to exist. This presupposes the necessity of a stable [[temperature]], pressure, and [[radiation]] levels; resultantly, astrobiological research focuses on planets orbiting [[Sun]]-like [[red dwarf]] [[star]]s.<ref>{{cite web|url=http://www.solstation.com/habitable.htm |title=Stars and Habitable Planets |access-date=20 October 2008 |date=2007 |publisher=Sol Company |archive-url=https://web.archive.org/web/20081001194624/http://www.solstation.com/habitable.htm |archive-date=1 October 2008 |url-status=dead }}</ref><ref name="Pace"/> This is because very large stars have relatively short lifetimes, meaning that life might not have time to emerge on planets orbiting them; very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be [[Tidal lock|tidally locked]] to the star;<ref>{{cite web |url=http://www.redorbit.com/news/display/?id=223364 |title=M Dwarfs: The Search for Life is On |access-date=20 October 2008 |date=29 August 2005 |publisher=Red Orbit & [[Astrobiology Magazine]] }}{{Dead link|date=September 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> whereas the long lifetimes of [[red dwarf]]s could allow the development of habitable environments on planets with thick atmospheres.<ref name = "AstroecologyJBIS2005">{{Cite journal|last = Mautner |first = Michael N. | title = Life in the cosmological future: Resources, biomass and populations  | journal = Journal of the British Interplanetary Society | date = 2005 | volume = 58 | pages = 167–180 | url=http://www.astro-ecology.com/PDFCosmologyJBIS2005Paper.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.astro-ecology.com/PDFCosmologyJBIS2005Paper.pdf |archive-date=2022-10-09 |url-status=live| bibcode = 2005JBIS...58..167M }}</ref> This is significant as red dwarfs are extremely common. (''See also'': [[Habitability of red dwarf systems]]).

'''Energy source''': It is assumed that any life elsewhere in the universe would also require an energy source. Previously, it was assumed that this would necessarily be from a [[sun]]-like star, however with developments within [[extremophile]] research contemporary astrobiological research often focuses on identifying environments that have the potential to support life based on the availability of an energy source, such as the presence of volcanic activity on a planet or moon that could provide a source of heat and energy.

It is important to note that these assumptions are based on our current understanding of life on Earth and the conditions under which it can exist. As our understanding of life and the potential for it to exist in different environments evolves, these assumptions may change.

==Methods==
===Studying terrestrial extremophiles===
Astrobiological research concerning the study of habitable environments in our solar system and beyond utilises methods within the geosciences. Research within this branch primarily concerns the geobiology of organisms that can survive in extreme environments on Earth, such as in volcanic or deep sea environments, to understand the limits of life, and the conditions under which life might be able to survive on other planets. This includes, but is not limited to:

'''Deep-sea extremophiles''': Researchers are studying organisms that live in the extreme environments of deep-sea hydrothermal vents and cold seeps.<ref>{{cite web |title=Life in the Extreme: Hydrothermal Vents |url=https://astrobiology.nasa.gov/news/life-in-the-extreme-hydrothermal-vents/#:~:text=Heat%20from%20this%20searing%20hot,what%20we%20call%20hydrothermal%20vents. |website=NASA Astrobiology Institute |publisher=NASA |access-date=29 January 2023}}</ref> These organisms survive in the absence of sunlight, and some are able to survive in high temperatures and pressures, and use chemical energy instead of sunlight to produce food.

'''Desert extremophiles''': Researchers are studying organisms that can survive in extreme dry, high temperature conditions, such as in deserts.<ref>{{cite journal |title=Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context |journal=Frontiers in Microbiology |date=15 April 2019 |volume=10 |doi=10.3389/fmicb.2019.00780 |doi-access=free |last1=Merino |first1=Nancy |last2=Aronson |first2=Heidi S. |last3=Bojanova |first3=Diana P. |last4=Feyhl-Buska |first4=Jayme |last5=Wong |first5=Michael L. |last6=Zhang |first6=Shu |last7=Giovannelli |first7=Donato |page=780 |pmid=31037068 |pmc=6476344 |bibcode=2019FrMic..10..780M }}</ref>

'''Microbes in extreme environments''': Researchers are investigating the diversity and activity of microorganisms in environments such as deep mines, subsurface soil, cold glaciers<ref>{{cite journal |title=Microbial Ecology and Functional Diversity of Natural Habitats |journal=The ISME Journal |date=7 February 2013 |volume=7 |pages=1211–1226 |doi=10.1038/ismej.2013.8|pmid=23389107 |last1=Mykytczuk |first1=N. C. |last2=Foote |first2=S. J. |last3=Omelon |first3=C. R. |last4=Southam |first4=G. |last5=Greer |first5=C. W. |last6=Whyte |first6=L. G. |issue=6 |pmc=3660685 }}</ref> and polar ice,<ref>{{cite journal |title=Sustainability of Extreme Microbial Ecosystems to the Comprehensive Impact of Physical Factors of the Martian Regolith |journal=Moscow University Soil Science Bulletin |date=28 September 2018 |volume=73 |pages=119–123 |doi=10.3103/S0147687418030043 |last1=Cheptsov |first1=V. S. |last2=Vorobyova |first2=E. A. |last3=Polyanskaya |first3=L. M. |last4=Gorlenko |first4=M. V. |last5=Pavlov |first5=A. K. |last6=Lomasov |first6=V. N. |issue=3 |bibcode=2018MUSSB..73..119C |s2cid=135443326 }}</ref> and high-altitude environments.

===Researching Earth's present environment===
Research also regards the long-term survival of life on Earth, and the possibilities and hazards of life on other planets, including:

'''Biodiversity and ecosystem resilience''': Scientists are studying how the diversity of life and the interactions between different species contribute to the resilience of ecosystems and their ability to recover from disturbances.<ref>{{cite web |title=Astrobiology Objectives: Microbial Ecology |url=https://astrobiology.nasa.gov/nai/media/roadmap/1998/objectives/o6_microbiology_ecology.html |website=Astrobiology at NASA |publisher=NASA |access-date=29 January 2023}}</ref>

'''[[Climate change]] and extinction''': Researchers are investigating the impacts of climate change on different species and ecosystems, and how they may lead to extinction or adaptation.<ref>{{cite web |title=Effects of Climate and Geology on Habitability |url=https://astrobiology.nasa.gov/nai/media/roadmap/1998/objectives/o12_climate_geology.html |website=Astrobiology at NASA |publisher=NASA |access-date=29 January 2023}}</ref> This includes the evolution of Earth's climate and geology, and their potential impact on the habitability of the planet in the future, especially for humans.

'''Human impact on the biosphere''': Scientists are studying the ways in which human activities, such as deforestation, pollution, and the introduction of invasive species, are affecting the biosphere and the long-term survival of life on Earth.<ref>{{cite web |title=Earth's Future Habitability |url=https://astrobiology.nasa.gov/nai/media/roadmap/1998/objectives/o15_future_habitability.html |website=Astrobiology at NASA |publisher=NASA |access-date=29 January 2023}}</ref>

'''Long-term preservation of life''': Researchers are exploring ways to preserve samples of life on Earth for long periods of time, such as cryopreservation and genomic preservation, in the event of a catastrophic event that could wipe out most of life on Earth.<ref>{{cite web |title=Bringing Life with Us beyond Earth |url=https://astrobiology.nasa.gov/nai/media/roadmap/1998/objectives/o16_bringing_life.html |website=Astrobiology at NASA |publisher=NASA |access-date=29 January 2023}}</ref>

===Finding biosignatures on other worlds===
Emerging astrobiological research concerning the search for planetary biosignatures of past or present extraterrestrial life utilise methodologies within planetary sciences. These include: 
{{Main|Geology of solar terrestrial planets}}

'''The study of microbial life in the subsurface of Mars''': {{Main|Life on Mars}}Scientists are using data from Mars rover missions to study the composition of the subsurface of [[Mars]], searching for biosignatures of past or present microbial life.<ref>{{cite journal |title=Earth-like Habitable Environments in the Subsurface of Mars |journal=Astrobiology |date=11 Jun 2021 |volume=21 |issue=6 |pages=741–756 |doi=10.1089/ast.2020.2386 |last1=Tarnas |first1=J.D. |last2=Mustard |first2=J.F. |last3=Sherwood Lollar |first3=B. |last4=Stamenković |first4=V. |last5=Cannon |first5=K.M. |last6=Lorand |first6=J.-P. |last7=Onstott |first7=T.C. |last8=Michalski |first8=J.R. |last9=Warr |first9=O. |last10=Palumbo |first10=A.M. |last11=Plesa |first11=A.-C. |pmid=33885329 |bibcode=2021AsBio..21..741T |s2cid=233352375 |url=https://hal.archives-ouvertes.fr/hal-03358460/file/Tarnas_ModernMarsHabitat_DraftForSubmission_Final_FormattedForAstrobiology_Revised_NoTrackedChanges.pdf }}</ref>

'''The study of liquid bodies on icy moons''': {{Main|Life on Europa|life on Titan|Enceladus#Exploration}} Discoveries of surface and subsurface bodies of liquid on moons such as [[Europa (moon)|Europa]],<ref name=Europa>{{cite web|url=http://people.msoe.edu/~tritt/sf/europa.life.html |title=Possibility of Life on Europa |access-date=20 October 2008 |last=Tritt |first=Charles S. |date=2002 |publisher=Milwaukee School of Engineering |url-status=dead |archive-url=https://web.archive.org/web/20070609150109/http://people.msoe.edu/~tritt/sf/europa.life.html |archive-date=9 June 2007 }}</ref><ref name=EuroUpdate>{{cite web|url=http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |title=Projects: Europa Mission Campaign |access-date=20 October 2008 |last=Friedman |first=Louis |date=14 December 2005 |publisher=The Planetary Society |archive-url=https://web.archive.org/web/20080920110348/http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |archive-date=20 September 2008 |url-status=dead  }}</ref><ref>{{cite web|url=http://www.space.com/news/europa_story_991109.html |title=Move Over Mars – Europa Needs Equal Billing |access-date=20 October 2008 |last=David |first=Leonard |date=10 November 1999 |publisher=Space.com }}</ref> [[Titan (moon)|Titan]]<ref name="Implications of Huygens">{{cite web | title=Life on Titan? | website=ESA | url=https://www.esa.int/Science_Exploration/Space_Science/Cassini-Huygens/Life_on_Titan | access-date=August 31, 2024}}</ref> and [[Enceladus]]<ref name="Kazan">{{cite web|last1=Kazan  |first1=Casey  |title=Saturn's Enceladus Moves to Top of "Most-Likely-to-Have-Life" List|url=http://www.dailygalaxy.com/my_weblog/2011/06/saturns-enceladus-moves-to-top-of-most-likely-to-have-life-list.html|date= 2 June 2011 |publisher=The Daily Galaxy|access-date=3 June 2011}}</ref><ref name="Lovett">{{cite journal |last1=Lovett |first1=Richard A. |title=Enceladus named sweetest spot for alien life |url=http://www.nature.com/news/2011/110531/full/news.2011.337.html |date=31 May 2011 |access-date=3 June 2011|doi=10.1038/news.2011.337 |journal=Nature}}</ref> showed possible habitability zones, making them viable targets for the search for extraterrestrial life. {{As of|2024|09}}, missions like ''[[Europa Clipper]]'' and ''[[Dragonfly (Titan space probe)|Dragonfly]]'' are planned to search for biosignatures within these environments. [[Image:PIA01130 Interior of Europa.jpg|thumb|right|The interior of [[Europa (moon)|Europa]]]]

'''The study of the atmospheres of planets''': {{Main|Life on Venus}} Scientists are studying the potential for life to exist in the atmospheres of planets, with a focus on the study of the physical and chemical conditions necessary for such life to exist, namely the detection of organic molecules and biosignature gases; for example, the study of the possibility of life in the atmospheres of exoplanets that orbit red dwarfs and the study of the potential for microbial life in the upper atmosphere of Venus.<ref>{{cite journal |title=Venus, an Astrobiology Target |journal=Astrobiology |date=7 October 2021 |volume=21 |issue=10 |pages=1163–1185 |doi=10.1089/ast.2020.2268 |last1=Limaye |first1=Sanjay S. |last2=Mogul |first2=Rakesh |last3=Baines |first3=Kevin H. |last4=Bullock |first4=Mark A. |last5=Cockell |first5=Charles |last6=Cutts |first6=James A. |last7=Gentry |first7=Diana M. |last8=Grinspoon |first8=David H. |last9=Head |first9=James W. |last10=Jessup |first10=Kandis-Lea |last11=Kompanichenko |first11=Vladimir |last12=Lee |first12=Yeon Joo |last13=Mathies |first13=Richard |last14=Milojevic |first14=Tetyana |last15=Pertzborn |first15=Rosalyn A. |last16=Rothschild |first16=Lynn |last17=Sasaki |first17=Satoshi |last18=Schulze-Makuch |first18=Dirk |last19=Smith |first19=David J. |last20=Way |first20=Michael J. |pmid=33970019 |s2cid=234344026 |doi-access=free |bibcode=2021AsBio..21.1163L }}</ref>

'''Telescopes and remote sensing of exoplanets''': The discovery of thousands of exoplanets has opened up new opportunities for the search for biosignatures. Scientists are using telescopes such as the [[James Webb Space Telescope]] and the [[Transiting Exoplanet Survey Satellite]] to search for biosignatures on exoplanets. They are also developing new techniques for the detection of biosignatures, such as the use of remote sensing to search for biosignatures in the atmosphere of exoplanets.<ref>{{cite journal |last1=Seager |first1=Sara |title=The future of spectroscopic life detection on exoplanets |journal=PNAS |date=4 August 2014 |volume=111 |issue=35 |pages=12634–12640 |doi=10.1073/pnas.1304213111 |pmid=25092345 |pmc=4156723 |doi-access=free |bibcode=2014PNAS..11112634S }}</ref>

===Talking to extraterrestrials===
{{Main|Communication with extraterrestrial intelligence}}
;'''[[SETI]] and [[Communication with extraterrestrial intelligence|CETI]]'''
Scientists search for signals from intelligent extraterrestrial civilizations using radio and optical telescopes within the discipline of [[Communication with extraterrestrial intelligence|extraterrestrial intelligence communications]] (CETI). CETI focuses on composing and deciphering messages that could theoretically be understood by another technological civilization. Communication attempts by humans have included broadcasting mathematical languages, pictorial systems such as the [[Arecibo message]], and computational approaches to detecting and deciphering 'natural' language communication. While some high-profile scientists, such as [[Carl Sagan]], have advocated the transmission of messages,<ref>Sagan, Carl. ''Communication with Extraterrestrial Intelligence''. MIT Press, 1973, 428 pp.</ref><ref>{{cite web| url=http://www.lightspeedmagazine.com/nonfiction/you-never-get-a-seventh-chance-to-make-a-first-impression-an-awkward-history-of-our-space-transmissions/| title=You Never Get a Seventh Chance to Make a First Impression: An Awkward History of Our Space Transmissions| work=Lightspeed Magazine| access-date=13 March 2015| date=March 2011}}</ref> theoretical physicist [[Stephen Hawking]] warned against it, suggesting that aliens may raid Earth for its resources.<ref>{{Cite news|url=http://www.huffingtonpost.com/2010/04/25/stephen-hawking-aliens_n_551035.html|title=Stephen Hawking: Humans Should Fear Aliens|date=25 June 2010|work=Huffington Post|access-date=27 May 2017}}</ref>

===Investigating the early Earth===
Emerging astrobiological research concerning the study of the origin and early evolution of life on Earth utilises methodologies within the palaeosciences. These include:

'''The study of the early atmosphere''': Researchers are investigating the role of the early atmosphere in providing the right conditions for the emergence of life, such as the presence of gases that could have helped to stabilise the climate and the formation of organic molecules.<ref>{{cite journal |title=Earth's Earliest Atmospheres |journal=Cold Spring Harbor Perspectives in Biology |date=October 2010 |volume=2 |issue=10 |doi=10.1101/cshperspect.a004895 |last1=Zahnle |first1=K. |last2=Schaefer |first2=L. |last3=Fegley |first3=B. |pages=a004895 |pmid=20573713 |pmc=2944365 }}</ref>

'''The study of the early magnetic field''': Researchers are investigating the role of the early magnetic field in protecting the Earth from harmful radiation and helping to stabilise the climate.<ref>{{cite journal |title=Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo |journal=Proceedings of the National Academy of Sciences |date=21 January 2020 |doi=10.1073/pnas.1916553117 |doi-access=free |last1=Tarduno |first1=John A. |last2=Cottrell |first2=Rory D. |last3=Bono |first3=Richard K. |last4=Oda |first4=Hirokuni |last5=Davis |first5=William J. |last6=Fayek |first6=Mostafa |last7=Erve |first7=Olaf van 't |last8=Nimmo |first8=Francis |last9=Huang |first9=Wentao |last10=Thern |first10=Eric R. |last11=Fearn |first11=Sebastian |last12=Mitra |first12=Gautam |last13=Smirnov |first13=Aleksey V. |last14=Blackman |first14=Eric G. |volume=117 |issue=5 |pages=2309–2318 |pmid=31964848 |pmc=7007582 |bibcode=2020PNAS..117.2309T }}</ref> This research has immense astrobiological implications where the subjects of current astrobiological research like Mars lack such a field.

'''The study of prebiotic chemistry''': Scientists are studying the chemical reactions that could have occurred on the early Earth that led to the formation of the building blocks of life- amino acids, nucleotides, and lipids- and how these molecules could have formed spontaneously under early Earth conditions.<ref>{{cite journal |title=Frontiers in Prebiotic Chemistry and Early Earth Environments |journal=Origins of Life and Evolution of Biospheres |date=7 July 2022 |volume=52 |pages=165–181 |doi=10.1007/s11084-022-09622-x |last1=Müller |first1=Ulrich F. |last2=Elsila |first2=Jamie |last3=Trail |first3=Dustin |last4=Dasgupta |first4=Saurja |last5=Giese |first5=Claudia-Corina |last6=Walton |first6=Craig R. |last7=Cohen |first7=Zachary R. |last8=Stolar |first8=Tomislav |last9=Krishnamurthy |first9=Ramanarayanan |last10=Lyons |first10=Timothy W. |last11=Rogers |first11=Karyn L. |last12=Williams |first12=Loren Dean |issue=1–3 |pmid=35796897 |pmc=9261198 |bibcode=2022OLEB...52..165M }}</ref> [[Image:We_are_star-stuff.png|thumb|right|Chart showing the theorized origin of the [[chemical elements]] that make up the human body]]

'''The study of impact events''': Scientists are investigating the potential role of impact events- especially meteorites- in the delivery of water and organic molecules to early Earth.<ref>{{cite journal |title=The Role of Meteorite Impacts in the Origin of Life |journal=Astrobiology |date=15 September 2020 |pages=1121–1149 |doi=10.1089/ast.2019.2203 |last1=Osinski |first1=G.R. |last2=Cockell |first2=C.S. |last3=Pontefract |first3=A. |last4=Sapers |first4=H.M. |volume=20 |issue=9 |pmid=32876492 |pmc=7499892 |bibcode=2020AsBio..20.1121O }}</ref>

'''The study of the primordial soup''': {{Main|Primordial soup}}Researchers are investigating the conditions and ingredients that were present on the early Earth that could have led to the formation of the first living organisms, such as the presence of water and organic molecules, and how these ingredients could have led to the formation of the first living organisms.<ref>{{cite journal |title=Nitrogen heterocycles form peptide nucleic acid precursors in complex prebiotic mixtures |journal=Scientific Reports |date=26 June 2019 |volume=9 |page=9281 |doi=10.1038/s41598-019-45310-z |last1=Rodriguez |first1=Laura E. |last2=House |first2=Christopher H. |last3=Smith |first3=Karen E. |last4=Roberts |first4=Melissa R. |last5=Callahan |first5=Michael P. |issue=1 |pmid=31243303 |pmc=6594999 |bibcode=2019NatSR...9.9281R }}</ref> This includes the role of water in the formation of the first cells and in catalysing chemical reactions.

'''The study of the role of minerals''': Scientists are investigating the role of minerals like clay in catalysing the formation of organic molecules, thus playing a role in the emergence of life on Earth.<ref>{{cite journal |last1=Kloprogge |first1=Jacob |last2=Hartman |first2=Hyman |title=Clays and the Origin of Life: The Experiments |journal=Life |date=9 February 2022 |volume=12 |issue=2 |page=259 |doi=10.3390/life12020259 |pmid=35207546 |pmc=8880559 |doi-access=free |bibcode=2022Life...12..259K }}</ref>

'''The study of the role of energy and electricity''': Scientists are investigating the potential sources of energy and electricity that could have been available on the early Earth, and their role in the formation of organic molecules, thus the emergence of life.<ref>{{cite journal |last1=Hess |first1=Benjamin |last2=Piazolo |first2=Sandra |last3=Harvey |first3=Jason |title=Lightning strikes as a major facilitator of prebiotic phosphorus reduction on early Earth |journal=Nature Communications |date=16 March 2021 |volume=12 |issue=1 |page=1535 |doi=10.1038/s41467-021-21849-2 |pmid=33727565 |pmc=7966383 |bibcode=2021NatCo..12.1535H }}</ref>

'''The study of the early oceans''': Scientists are investigating the composition and chemistry of the early oceans and how it may have played a role in the emergence of life, such as the presence of dissolved minerals that could have helped to catalyse the formation of organic molecules.<ref>{{cite journal |last1=Kempe |first1=Stephan |last2=Kazmierczak |first2=Jozef |title=Biogenesis and early life on Earth and Europa: favored by an alkaline ocean? |journal=Astrobiology |date=Spring 2002 |volume=2 |issue=1 |pages=123–130 |doi=10.1089/153110702753621394 |pmid=12449860 |bibcode=2002AsBio...2..123K }}</ref>

'''The study of hydrothermal vents''': Scientists are investigating the potential role of hydrothermal vents in the origin of life, as these environments may have provided the energy and chemical building blocks needed for its emergence.<ref>{{cite web |title=Sources of Organics on Earth |url=https://astrobiology.nasa.gov/nai/media/roadmap/1998/objectives/o1_sources_organics.html |website=Astrobiology at NASA |publisher=NASA |access-date=29 January 2023}}</ref>

'''The study of plate tectonics''': Scientists are investigating the role of plate tectonics in creating a diverse range of environments on the early Earth.<ref>{{cite journal |title=Initiation of modern-style plate tectonics recorded in Mesoarchean marine chemical sediments |journal=Geochimica et Cosmochimica Acta |date=15 July 2017 |volume=209 |pages=216–232 |doi=10.1016/j.gca.2017.04.024 |last1=Satkoski |first1=Aaron M. |last2=Fralick |first2=Philip |last3=Beard |first3=Brian L. |last4=Johnson |first4=Clark M. |doi-access=free |bibcode=2017GeCoA.209..216S }}</ref>

'''The study of the early biosphere''': Researchers are investigating the diversity and activity of microorganisms in the early Earth, and how these organisms may have played a role in the emergence of life.<ref>{{cite journal |last1=Lindsay |first1=John |last2=McKay |first2=David |last3=Allen |first3=Carlton |title=Earth's Earliest Biosphere—A Proposal to Develop a Collection of Curated Archean Geologic Reference Materials |journal=Astrobiology |date=Winter 2003 |volume=3 |issue=4 |pages=739–758 |doi=10.1089/153110703322736060 |pmid=14987479 |bibcode=2003AsBio...3..739L }}</ref>

'''The study of microbial fossils''': Scientists are investigating the presence of microbial fossils in ancient rocks, which can provide clues about the early evolution of life on Earth and the emergence of the first organisms.<ref>{{cite journal |last1=McMahon |first1=Sean |title=Earth's earliest and deepest purported fossils may be iron-mineralized chemical gardens |journal=Proceedings: Biological Sciences |date=4 December 2019 |volume=286 |issue=1916 |page=1916 |doi=10.1098/rspb.2019.2410 |pmid=31771469 |s2cid=208296652 |pmc=6939263 }}</ref>

==Research==
{{see also|Extraterrestrial life}}
The systematic search for possible life outside Earth is a valid multidisciplinary scientific endeavor.<ref>{{cite web| url=http://astrobiology.nasa.gov/nai/| archive-url=https://web.archive.org/web/20080323104548/http://astrobiology.nasa.gov/nai/| url-status=dead| archive-date=23 March 2008| title=NASA Astrobiology: Life in the Universe| access-date=13 March 2015}}</ref> However, hypotheses and predictions as to its existence and origin vary widely, and at the present, the development of hypotheses firmly grounded on science may be considered astrobiology's most concrete practical application. It has been proposed that [[viruses]] are likely to be encountered on other life-bearing planets,<ref name="Astro-20130814">{{cite journal |last=Griffin |first=Dale Warren |title=The Quest for Extraterrestrial Life: What About the Viruses? |date=14 August 2013 |journal=[[Astrobiology (journal)|Astrobiology]] |volume=13 |issue=8 |pages=774–783 |doi=10.1089/ast.2012.0959 |pmid=23944293 |bibcode = 2013AsBio..13..774G }}</ref><ref>{{cite journal | doi = 10.1089/ast.2017.1649 | pmid=29319335 | bibcode=2018AsBio..18..207B | volume=18 | issue=2 | title=Astrovirology: Viruses at Large in the Universe | year=2018 | journal=Astrobiology | pages=207–223 | last1 = Berliner | first1 = Aaron J. | last2 = Mochizuki | first2 = Tomohiro | last3 = Stedman | first3 = Kenneth M.| s2cid=4348200 | url=https://escholarship.org/uc/item/4d87k128 }}</ref> and may be present even if there are no biological cells.<ref>{{cite journal | doi = 10.1089/ast.2018.1851 | volume=18 | issue=12 | title=The Need for Including Virus Detection Methods in Future Mars Missions | year=2018 | journal=Astrobiology | pages=1611–1614 | last1 = Janjic | first1 = Aleksandar | bibcode=2018AsBio..18.1611J | s2cid=105299840 }}</ref>

=== Research outcomes ===
{{As of|2024}}, no evidence of extraterrestrial life has been identified.<ref>"[https://www.space.com/37312-nasa-alien-life-anonymous-video.html No, NASA Hasn't Found Alien Life]". Mike Wall, ''Space''.  26 June 2017.</ref> Examination of the [[Allan Hills 84001]] meteorite, which was recovered in [[Antarctica]] in 1984 and originated from [[Mars]], is thought by [[David S. McKay|David McKay]], as well as few other scientists, to contain [[Fossil#Microfossils|microfossils]] of extraterrestrial origin; this interpretation is controversial.<ref name=disbelief>{{cite web |title=Experts: Little Evidence of Life on Mars |url=http://dsc.discovery.com/news/2006/08/08/marslife_spa.html?category=space&guid=20060808100030 |last=Crenson |first=Matt |publisher=[[Associated Press]] |date=6 August 2006 |access-date=8 March 2011 |archive-url=https://web.archive.org/web/20110416094930/http://dsc.discovery.com/news/2006/08/08/marslife_spa.html?category=space&guid=20060808100030 |archive-date=16 April 2011 |url-status=dead  }}</ref><ref name="life">{{cite journal| title= Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001|author1=McKay DS |author2=Gibson E. K. |author3=Thomas-Keprta K. L. |author4=Vali H. |author5=Romanek C. S. |author6=Clemett S. J. |author7=Chillier X. D. F. |author8=Maechling C. R. |author9=Zare R. N. |s2cid=40690489 |journal=Science| volume= 273| pages=924–930| date=1996| doi= 10.1126/science.273.5277.924| pmid= 8688069| issue= 5277| bibcode = 1996Sci...273..924M }}</ref><ref name=McKay>{{cite journal |title=Life on Mars: new evidence from martian meteorites |author=McKay David S. |author2=Thomas-Keprta K. L. |author3=Clemett, S. J. |author4=Gibson, E. K. Jr |author5=Spencer L. |author6= Wentworth S. J. |journal=[[Proc. SPIE]] |volume=7441 |date=2009 |url=http://spiedigitallibrary.org/proceedings/resource/2/psisdg/7441/1/744102_1?isAuthorized=no |doi=10.1117/12.832317 |access-date=8 March 2011 |issue=1 |series=Proceedings of SPIE |pages=744102 |editor1-last=Hoover |editor1-first=Richard B. |editor2-last=Levin |editor2-first=Gilbert V. |editor3-last=Rozanov |editor3-first=Alexei Y. |editor4-last=Retherford |editor4-first=Kurt D. |bibcode=2009SPIE.7441E..02M|s2cid=123296237 }}</ref>
[[Image:951 Gaspra.jpg|thumb|left|Asteroid(s) may have transported life to [[Earth]].]]
[[Yamato 000593]], the [[List of Martian meteorites#List|second largest]] [[meteorite]] from [[Mars]], was found on Earth in 2000. At a microscopic level, [[Martian spherules|spheres]] are found in the meteorite that are rich in [[carbon]] compared to surrounding areas that lack such spheres. The carbon-rich spheres may have been formed by [[biotic material|biotic activity]] according to some NASA scientists.<ref name="NASA-20140227">{{cite web |last=Webster |first=Guy |title=NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars |url=http://www.jpl.nasa.gov/news/news.php?release=2014-065&1 |date=27 February 2014 |work=[[NASA]] |access-date=27 February 2014 |archive-date=1 March 2014 |archive-url=https://web.archive.org/web/20140301170153/http://www.jpl.nasa.gov/news/news.php?release=2014-065&1 |url-status=dead }}</ref><ref name="AJ-20140219">{{cite journal |last1=White |first1=Lauren M. |last2=Gibson |first2=Everett K. |last3=Thomnas-Keprta| first3=Kathie L. |last4=Clemett |first4=Simon J. |last5=McKay |first5=David |title=Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593|date=19 February 2014 |journal=[[astrobiology (journal)|Astrobiology]] |volume=14 |number=2 |pages=170–181 |doi=10.1089/ast.2011.0733|bibcode = 2014AsBio..14..170W |pmc=3929347 | pmid=24552234}}</ref><ref name="SP-20140228">{{cite web |last=Gannon |first=Megan |title=Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life| url=http://www.space.com/24834-strange-mars-meteorite-life-evidence-debate.html |date=28 February 2014 |work=[[Space.com]] |access-date=28 February 2014 }}</ref>

On 5 March 2011, [[Richard B. Hoover]], a scientist with the [[Marshall Space Flight Center]], speculated on the finding of alleged microfossils similar to [[cyanobacteria]] in [[CI1 fossils|CI1]] carbonaceous [[meteorites]] in the [[fringe science|fringe]] ''[[Journal of Cosmology]]'', a story widely reported on by [[mainstream media]].<ref name=Tenney>{{cite web |last=Tenney |first=Garrett |title=Exclusive: NASA Scientist Claims Evidence of Alien Life on Meteorite |url=https://www.foxnews.com/science/exclusive-nasa-scientist-claims-evidence-of-alien-life-on-meteorite |publisher=[[Fox News]] |date=5 March 2011 |access-date=6 March 2011 | archive-url= https://web.archive.org/web/20110306082448/http://www.foxnews.com/scitech/2011/03/05/exclusive-nasa-scientists-claims-evidence-alien-life-meteorite/| archive-date= 6 March 2011| url-status= live}}</ref><ref name=Hoover>{{cite journal |title=Fossils of Cyanobacteria in CI1 Carbonaceous Meteorites: Implications to Life on Comets, Europa, and Enceladus |last=Hoover |first=Richard B. |journal=Journal of Cosmology |volume=13 |pages=xxx |date=2011 |url=http://journalofcosmology.com/Life100.html |access-date=6 March 2011 |archive-url=https://web.archive.org/web/20110308034118/http://journalofcosmology.com/Life100.html |archive-date=8 March 2011 |url-status=dead  }}</ref> However, NASA formally distanced itself from Hoover's claim.<ref>{{cite news |last=Sheridan |first=Kerry |title = NASA shoots down alien fossil claims | date = 7 March 2011 | url = http://www.abc.net.au/news/stories/2011/03/08/3157645.htm | work = ABC News | access-date = 7 March 2011}}</ref> According to American astrophysicist [[Neil deGrasse Tyson]]: "At the moment, life on Earth is the only known life in the universe, but there are compelling arguments to suggest we are not alone."<ref>{{cite web | url = http://astrobiology.nasa.gov/articles/the-search-for-life-in-the-universe/ | title = The Search for Life in the Universe | access-date = 7 March 2011 | last = Tyson | first = Neil deGrasse | date = 23 July 2001 | work = Department of Astrophysics and Hayden Planetarium | publisher = NASA | url-status = dead | archive-url = https://web.archive.org/web/20110721050044/http://astrobiology.nasa.gov/articles/the-search-for-life-in-the-universe/ | archive-date = 21 July 2011 | df = dmy-all }}</ref>

===Elements of astrobiology===

==== Astronomy ====
{{Main|Astronomy}}
[[Image:OGLE-2005-BLG-390Lb planet.jpg|thumb|right|Artist's impression of the [[extrasolar planet]] [[OGLE-2005-BLG-390Lb]] orbiting its star 20,000 [[light-year]]s from [[Earth]]; this planet was discovered with [[gravitational microlensing]].]]
[[Image:Telescope Kepler-NASA.jpeg|thumb|right|The [[NASA]] [[Kepler mission]], launched in March 2009, searches for [[extrasolar planets]].]]
Most astronomy-related astrobiology research falls into the category of [[extrasolar planet]] (exoplanet) detection, the hypothesis being that if life arose on Earth, then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect Earth-sized exoplanets have been considered, most notably [[NASA]]'s [[Terrestrial Planet Finder]] (TPF) and [[European Space Agency|ESA's]] [[Darwin (ESA)|Darwin]] programs, both of which have been cancelled. NASA launched the [[Kepler (spacecraft)|Kepler mission]] in March 2009, and the [[French Space Agency]] launched the [[COROT]] space mission in 2006.<ref>{{cite web| url=http://kepler.nasa.gov/ |title=Kepler Mission |access-date=20 October 2008 |date=2008 |publisher=NASA | archive-url= https://web.archive.org/web/20081031070350/http://kepler.nasa.gov/| archive-date= 31 October 2008 | url-status= dead}}</ref><ref>{{cite web |url=http://smsc.cnes.fr/COROT/ |title=The COROT space telescope |access-date=20 October 2008 |date=17 October 2008 |publisher=CNES |archive-url=https://web.archive.org/web/20081108130307/http://smsc.cnes.fr/COROT/ |archive-date=8 November 2008 |url-status=dead  }}</ref> There are also several less ambitious ground-based efforts underway.

The goal of these missions is not only to detect Earth-sized planets but also to directly detect light from the planet so that it may be studied [[spectroscopy|spectroscopically]]. By examining planetary spectra, it would be possible to determine the basic composition of an extrasolar planet's atmosphere and/or surface.<ref name="NYT-20220915">{{cite news |last=Gertner |first=Jon |title=The Search for Intelligent Life Is About to Get a Lot More Interesting - There are an estimated 100 billion galaxies in the universe, home to an unimaginable abundance of planets. And now there are new ways to spot signs of life on them. |url=https://www.nytimes.com/2022/09/15/magazine/extraterrestrials-technosignatures.html |date=15 September 2022 |work=[[The New York Times]] |accessdate=15 September 2022 }}</ref> Given this knowledge, it may be possible to assess the likelihood of life being found on that planet. A NASA research group, the Virtual Planet Laboratory,<ref>{{cite web |url=http://vpl.astro.washington.edu/main/about_vpl.html |title=The Virtual Planet Laboratory |access-date=20 October 2008 |date=2008 |publisher=NASA }}</ref> is using computer modeling to generate a wide variety of virtual planets to see what they would look like if viewed by TPF or Darwin. It is hoped that once these missions come online, their spectra can be cross-checked with these virtual planetary spectra for features that might indicate the presence of life.

An estimate for the number of planets with intelligent ''communicative'' [[extraterrestrial life]] can be gleaned from the [[Drake equation]], essentially an equation expressing the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise:<ref>{{cite web|url=http://www.setileague.org/general/drake.htm |title=What is the Drake Equation? |access-date=20 October 2008 |last=Ford |first=Steve |date=August 1995 |publisher=SETI League | archive-url= https://web.archive.org/web/20081029212425/http://www.setileague.org/general/drake.htm| archive-date= 29 October 2008 | url-status= live}}</ref>
:<math>N = R^{*} ~ \times ~ f_{p} ~ \times ~ n_{e} ~ \times ~ f_{l} ~ \times ~ f_{i} ~ \times ~ f_{c} ~ \times ~ L</math>
where:
* '''''N''''' = The number of communicative civilizations
* '''''R*''''' = The rate of formation of suitable stars (stars such as the Sun)
* '''''f<sub>p</sub>''''' = The fraction of those stars with planets (current evidence indicates that planetary systems may be common for stars like the Sun)
* '''''n<sub>e</sub>''''' = The number of Earth-sized worlds per planetary system
* '''''f<sub>l</sub>''''' = The fraction of those Earth-sized planets where life actually develops
* '''''f<sub>i</sub>''''' = The fraction of life sites where intelligence develops
* '''''f<sub>c</sub>''''' = The fraction of communicative planets (those on which electromagnetic communications technology develops)
* '''''L''''' = The "lifetime" of communicating civilizations

However, whilst the rationale behind the equation is sound, it is unlikely that the equation will be constrained to reasonable limits of error any time soon. The problem with the formula is that it is not used to generate or support [[hypothesis|hypotheses]] because it contains factors that can never be verified. The first term, '''''R*''''', number of stars, is generally constrained within a few orders of magnitude. The second and third terms, ''f<sub>p</sub>'', stars with planets and ''f<sub>e</sub>'', planets with habitable conditions, are being evaluated for the star's neighborhood. Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference,<ref>{{cite web| url=http://www.planetary.org/explore/topics/seti/seti_history_07.html|author=Amir Alexander| title=The Search for Extraterrestrial Intelligence: A Short History – Part 7: The Birth of the Drake Equation}}</ref> but some applications of the formula had been taken literally and related to simplistic or [[pseudoscientific]] arguments.<ref name=BC>{{cite web | url = http://biocab.org/Astrobiology.html | title = Astrobiology | access-date = 17 January 2011 | date = 26 September 2006 | publisher = Biology Cabinet| archive-url= https://web.archive.org/web/20101212184044/http://biocab.org/Astrobiology.html| archive-date= 12 December 2010| url-status= live}}</ref> Another associated topic is the [[Fermi paradox]], which suggests that if intelligent life is common in the [[universe]], then there should be obvious signs of it.

Another active research area in astrobiology is [[planetary system]] formation. It has been suggested that the peculiarities of the [[Solar System]] (for example, the presence of [[Jupiter]] as a protective shield)<ref>{{cite web |url=http://www.europlanet-eu.org/demo/index.php?option=com_content&task=view&id=58&Itemid=999 |title=Jupiter: Friend or foe? |access-date=20 October 2008 |last=Horner |first=Jonathan |author2=Barrie Jones |date=24 August 2007 |publisher=Europlanet |url-status=dead |archive-url=https://web.archive.org/web/20120202152231/http://www.europlanet-eu.org/demo/index.php?option=com_content&task=view&id=58&Itemid=999 |archive-date=2 February 2012  }}</ref> may have greatly increased the probability of intelligent life arising on Earth.<ref>{{cite web |url=http://www.spaceref.com/news/viewsr.html?pid=3910 |archive-url=https://archive.today/20130104172047/http://www.spaceref.com/news/viewsr.html?pid=3910 |url-status=dead |archive-date=4 January 2013 |title=The Role of Astrobiology in Solar System Exploration |access-date=20 October 2008 |last1=Jakosky |first1=Bruce |author2=David Des Marais |date=14 September 2001 |work=NASA |publisher=SpaceRef.com |display-authors=etal }}</ref><ref>{{cite web|url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1222 |title=Coming Soon: "Good" Jupiters |access-date=20 October 2008 |last=Bortman |first=Henry |date=29 September 2004 |work=[[Astrobiology Magazine]] |archive-url=https://web.archive.org/web/20120208071539/http://astrobio.net/exclusive/1222/coming-soon-good-jupiters |archive-date=2012-02-08 |url-status=usurped}}</ref>

====Biology====
{{See also|Abiogenesis|Biology|Extremophile}}

[[Image:Blacksmoker in Atlantic Ocean.jpg|thumb|upright|[[Hydrothermal vent]]s support [[extremophile|extremophile bacteria]] on [[Earth]], provided an energy-rich environment for the [[origin of life]], and may also support life in other parts of the cosmos.]]
Biology cannot state that a process or phenomenon, by being mathematically possible, has to exist forcibly in an extraterrestrial body. Biologists specify what is speculative and what is not.<ref name=BC/> The discovery of [[extremophiles]], organisms able to survive in extreme environments, became a core research element for astrobiologists, as they are important to understand four areas in the limits of life in planetary context: the potential for [[panspermia]], forward contamination due to human exploration ventures, planetary colonization by humans, and the exploration of extinct and extant extraterrestrial life.<ref>"Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context." N. Merino, H.S. Aronson, D. Bojanova, J. Feyhl-Buska, et al. ''EarthArXiv.'' February 2019.</ref>

Until the 1970s, [[life]] was thought to be entirely dependent on energy from the [[Sun]]. Plants on Earth's surface capture energy from [[sunlight]] to [[photosynthesis|photosynthesize]] sugars from carbon dioxide and water, releasing oxygen in the process that is then consumed by oxygen-respiring organisms, passing their energy up the [[food chain]]. Even life in the ocean depths, where sunlight cannot reach, was thought to obtain its nourishment either from consuming [[marine snow|organic detritus rained down from the surface waters]] or from eating animals that did.<ref name="smoker">{{cite web |title= Black Smokers and Giant Worms |author=Chamberlin, Sean |date=1999 |work=Fullerton College |url=http://www.courseworld.com/ocean/smokers.html |access-date=11 February 2011 }}</ref> The world's ability to support life was thought to depend on its access to [[sunlight]]. However, in 1977, during an exploratory dive to the [[Galapagos Rift]] in the deep-sea exploration submersible ''[[DSV Alvin|Alvin]]'', scientists discovered colonies of [[giant tube worm]]s, [[clam]]s, [[crustacean]]s, [[mussel]]s, and other assorted creatures clustered around undersea volcanic features known as [[black smoker]]s.<ref name="smoker"/> These creatures thrive despite having no access to sunlight, and it was soon discovered that they form an entirely independent [[ecosystem]]. Although most of these multicellular lifeforms need dissolved oxygen (produced by oxygenic photosynthesis) for their aerobic cellular respiration and thus are not completely independent from sunlight by themselves, the basis for their food chain is a form of [[bacterium]] that derives its energy from [[Redox|oxidization]] of reactive chemicals, such as [[hydrogen]] or [[hydrogen sulfide]], that bubble up from the Earth's interior. Other lifeforms entirely decoupled from the energy from sunlight are green sulfur bacteria which are capturing geothermal light for anoxygenic photosynthesis or bacteria running chemolithoautotrophy based on the radioactive decay of uranium.<ref name="Trixler2013">{{cite journal |last=Trixler|first=F|title=Quantum tunnelling to the origin and evolution of life.|journal=Current Organic Chemistry|date=2013|volume=17|number=16|pages=1758–1770|doi=10.2174/13852728113179990083|pmc=3768233|pmid=24039543}}</ref> This [[chemosynthesis]] revolutionized the study of biology and astrobiology by revealing that life need not be sunlight-dependent; it only requires water and an energy gradient in order to exist.

Biologists have found extremophiles that thrive in ice, boiling water, acid, alkali, the water core of nuclear reactors, salt crystals, toxic waste and in a range of other extreme habitats that were previously thought to be inhospitable for life.<ref>{{cite web| url=http://www.livescience.com/animalworld/050207_extremophiles.html |title=Wild Things: The Most Extreme Creatures |access-date=20 October 2008 |last=Carey |first=Bjorn |date=7 February 2005 |work=Live Science }}</ref><ref name=Cavicchioli >{{cite journal| title=Extremophiles and the search for extraterrestrial life|journal=Astrobiology|date=Fall 2002 |first=R. |last=Cavicchioli |volume=2 |issue=3|pages=281–292 |pmid=12530238 |doi=10.1089/153110702762027862 |bibcode = 2002AsBio...2..281C |url=http://somosbacteriasyvirus.com/extremophiles.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://somosbacteriasyvirus.com/extremophiles.pdf |archive-date=2022-10-09 |url-status=live|citeseerx=10.1.1.472.3179}}</ref> This opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats. Characterization of these organisms, their environments and their evolutionary pathways, is considered a crucial component to understanding how life might evolve elsewhere in the universe. For example, some organisms able to withstand exposure to the vacuum and radiation of outer space include the lichen fungi ''[[Rhizocarpon geographicum]]'' and ''[[Rusavskia elegans]]'',<ref>{{cite web |url=https://www.newscientist.com/article/dn8297-hardy-lichen-shown-to-survive-in-space/| title= Hardy lichen shown to survive in space| access-date=17 January 2019|work = New Scientist|date = 10 November 2005|last =Young|first = Kelly}}</ref> the bacterium ''[[Bacillus safensis]]'',<ref name=ThePlanetaryReport >''The Planetary Report'', Volume XXIX, number 2, March/April 2009, "We make it happen! Who will survive? Ten hardy organisms selected for the LIFE project, by Amir Alexander</ref> ''[[Deinococcus radiodurans]]'',<ref name=ThePlanetaryReport /> ''[[Bacillus subtilis]]'',<ref name=ThePlanetaryReport /> yeast ''[[Saccharomyces cerevisiae]]'',<ref name=ThePlanetaryReport /> seeds from ''[[Arabidopsis thaliana]]'' ('mouse-ear cress'),<ref name=ThePlanetaryReport /> as well as the invertebrate animal [[Tardigrade]].<ref name=ThePlanetaryReport /> While [[tardigrade]]s are not considered true extremophiles, they are considered extremotolerant microorganisms that have contributed to the field of astrobiology. Their extreme radiation tolerance and presence of DNA protection proteins may provide answers as to whether life can survive away from the protection of the Earth's atmosphere.<ref>{{cite journal | last1 = Hashimoto | first1 = T. | last2 = Kunieda | first2 = T. | year = 2017 | title = DNA Protection protein, a novel mechanism of radiation tolerance: Lessons from Tardigrades | journal = Life | volume = 7 | issue = 2| page = 26| doi = 10.3390/life7020026 | pmid = 28617314 | pmc = 5492148 | doi-access = free | bibcode = 2017Life....7...26H }}</ref>

Jupiter's moon, [[Europa (moon)|Europa]],<ref name=Cavicchioli/><ref>{{cite web |title= Jupiter's Moon Europa Suspected of Fostering Life| url=http://www.unisci.com/stories/20021/0211026.htm| work=Daily University Science News| date=2002 |access-date=8 August 2009 }}</ref><ref>{{cite journal| title=Extremophiles and the search for extraterrestrial life| journal=Astrobiology| date=Fall 2002| first=R. |last=Cavicchioli| volume=2| issue=3| pages=281–292| pmid=12530238 |doi=10.1089/153110702762027862 |bibcode = 2002AsBio...2..281C|citeseerx=10.1.1.472.3179}}</ref><ref>{{cite news | first=Leonard | last=David |title=Europa Mission: Lost in NASA Budget | date=7 February 2006 |publisher=Space.com | url =http://www.space.com/news/060207_europa_budget.html |access-date = 8 August 2009 }}</ref><ref>{{cite news | title=Clues to possible life on Europa may lie buried in Antarctic ice | date=5 March 1998 | publisher=NASA | url=https://science.nasa.gov/newhome/headlines/ast05mar98_1.htm | work=Marshal Space Flight Center | access-date=8 August 2009 | archive-url=https://web.archive.org/web/20090731015842/https://science.nasa.gov/newhome/headlines/ast05mar98_1.htm | archive-date=31 July 2009 | url-status=dead | df=dmy-all }}</ref> and Saturn's moon, [[Enceladus]],<ref>{{cite journal |last1=Lovett |first1=Richard A. |title=Enceladus named sweetest spot for alien life| url=http://www.nature.com/news/2011/110531/full/news.2011.337.html| date=31 May 2011 | access-date=3 June 2011 |doi=10.1038/news.2011.337 |journal=Nature}}</ref><ref name="Kazan" /> are now considered the most likely locations for extant extraterrestrial life in the [[Solar System]] due to their [[Subsurface ocean#Extraterrestrial oceans|subsurface water oceans]] where radiogenic and tidal heating enables liquid water to exist.<ref name="Trixler2013" />

The origin of life, known as [[abiogenesis]], distinct from the [[Evolution#Evolution of life|evolution of life]], is another ongoing field of research. [[Oparin]] and [[J. B. S. Haldane|Haldane]] postulated that the conditions on the early Earth were conducive to the formation of [[organic compound]]s from [[inorganic chemistry|inorganic elements]] and thus to the formation of many of the chemicals common to all forms of life we see today. The study of this process, known as prebiotic chemistry, has made some progress, but it is still unclear whether or not life could have formed in such a manner on Earth. The alternative hypothesis of [[panspermia]] is that the first elements of life may have formed on another planet with even more favorable conditions (or even in interstellar space, asteroids, etc.) and then have been carried over to Earth.

The [[cosmic dust]] permeating the universe contains complex [[organic compound|organic]] compounds ("amorphous organic solids with a mixed [[aromatic]]-[[aliphatic]] structure") that could be created naturally, and rapidly, by [[stars]].<ref name="Space-20111026">{{cite web |last=Chow |first=Denise |title=Discovery: Cosmic Dust Contains Organic Matter from Stars |url=http://www.space.com/13401-cosmic-star-dust-complex-organic-compounds.html |date=26 October 2011 |publisher=[[Space.com]] |access-date=26 October 2011 }}</ref><ref name="ScienceDaily-20111026">{{cite web |author=[[ScienceDaily]] Staff |title=Astronomers Discover Complex Organic Matter Exists Throughout the Universe |url=https://www.sciencedaily.com/releases/2011/10/111026143721.htm |date=26 October 2011 |website=[[ScienceDaily]] |access-date=27 October 2011 }}</ref><ref name="Nature-20111026">{{cite journal |last1=Kwok |first1=Sun |last2=Zhang |first2=Yong |title=Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features |date=26 October 2011 |journal=[[Nature (journal)|Nature]] |doi=10.1038/nature10542 |bibcode = 2011Natur.479...80K |volume=479 |issue=7371 |pages=80–83 |pmid=22031328|s2cid=4419859 }}</ref> Further, a scientist suggested that these compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."<ref name="Space-20111026"/>

More than 20% of the [[carbon]] in the universe may be associated with [[polycyclic aromatic hydrocarbon|polycyclic aromatic hydrocarbons (PAHs)]], possible [[PAH world hypothesis|starting materials]] for the [[Abiogenesis#PAH world hypothesis|formation]] of [[Life#Extraterrestrial|life]]. PAHs seem to have been formed shortly after the [[Big Bang]], are widespread throughout the universe, and are associated with [[Star formation|new stars]] and [[exoplanets]].<ref name="NASA-20140221">{{cite web |last=Hoover |first=Rachel |title=Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That |url=http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |date=21 February 2014 |work=[[NASA]] |access-date=22 February 2014 |archive-date=6 September 2015 |archive-url=https://web.archive.org/web/20150906061428/http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |url-status=dead }}</ref> PAHs are subjected to [[interstellar medium]] conditions and are transformed through [[hydrogenation]], [[oxygenation (environmental)|oxygenation]] and [[hydroxylation]], to more complex [[organic compound|organics]]—"a step along the path toward [[amino acid]]s and [[nucleotide]]s, the raw materials of [[protein]]s and [[DNA]], respectively".<ref name="Space-20120920">{{cite web |author=Staff |title=NASA Cooks Up Icy Organics to Mimic Life's Origins |url=http://www.space.com/17681-life-building-blocks-nasa-organic-molecules.html |date=20 September 2012 |publisher=[[Space.com]] |access-date=22 September 2012 }}</ref><ref name="AJL-20120901">{{cite journal |last1=Gudipati |first1=Murthy S. |last2=Yang |first2=Rui |s2cid=5541727 |title=In-Situ Probing of Radiation-Induced Processing of Organics in Astrophysical Ice Analogs – Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies |date=1 September 2012 |journal=[[The Astrophysical Journal Letters]] |volume=756 |doi=10.1088/2041-8205/756/1/L24 |issue=1 |pages=L24 |bibcode = 2012ApJ...756L..24G }}</ref>

In October 2020, astronomers proposed the idea of detecting life on distant planets by studying the shadows of trees at certain times of the day to find patterns that could be detected through observation of exoplanets.<ref name="UT-20201006">{{cite news |last=Gough |first=Evan |title=Here's a Clever Idea, Looking for the Shadows of Trees On Exoplanets to Detect Multicellular Life |url=https://www.universetoday.com/148164/heres-a-clever-idea-looking-for-the-shadows-of-trees-on-exoplanets-to-detect-multicellular-life/ |date=6 October 2020 |work=[[Universe Today]] |access-date=7 October 2020 }}</ref><ref name="IJA_20201001">{{cite journal |author=Doughty, Christopher E. |display-authors=et al. |title=Distinguishing multicellular life on exoplanets by testing Earth as an exoplanet |date=1 October 2020 |journal=[[International Journal of Astrobiology]] |volume=19 |issue=6 |pages=492–499 |doi=10.1017/S1473550420000270 |arxiv=2002.10368 |bibcode=2020IJAsB..19..492D |doi-access=free }}</ref>

== Philosophy ==
David Grinspoon called astrobiology a field of natural philosophy.<ref name="Grinspoon">{{Cite book | first = David | last = Grinspoon  | title = Lonely Planets: The Natural Philosophy of Alien Life  | place = New York  | publisher = ECCO | date = 2004 | isbn = 978-0-06-018540-4 }}</ref> Astrobiology intersects with [[philosophy]] by raising questions about the nature and existence of life beyond Earth. Philosophical implications include the definition of [[life]] itself, issues in the [[philosophy of mind]] and [[cognitive science]] in case intelligent life is found, [[epistemology|epistemological]] questions about the nature of proof, [[ethics|ethical]] considerations of space exploration, along with the broader impact of discovering extraterrestrial life on human thought and society.

Dunér<ref>{{cite book
 | last = Dunér
 | first = David
 | chapter = Introduction
 | editor-last =  Dunér
 | editor-first = David
 | title = The History and Philosophy of Astrobiology: Perspectives on Extraterrestrial Life and the Human Mind
 | location = Newcastle upon Tyne
 | publisher = Cambridge Scholars Publishing
 | year = 2013
 | pages = 4
}}</ref> has emphasized philosophy of astrobiology as an ongoing existential exercise in individual and collective self-understanding, whose major task is constructing and debating concepts such as the concept of life. Key issues, for Dunér, are questions of resource money and monetary planning, [[epistemology|epistemological]] questions regarding astrobiological knowledge, [[linguistics]] issues about interstellar communication, [[cognitive science|cognitive]] issues such as the definition of [[intelligence]], along with the possibility of [[interplanetary contamination]].
Persson<ref>{{cite book
 | last1 = Persson
 | first1 = Erik
 | chapter = Philosophical Aspects of Astrobiology
 | editor-last = Dunér
 | editor-first = David
 | title = The History and Philosophy of Astrobiology: Perspectives on Extraterrestrial Life and the Human Mind
 | location = Newcastle upon Tyne
 | publisher = Cambridge Scholars Publishing
 | year = 2013
}}</ref> also emphasized key philosophical questions in astrobiology. They include ethical justification of resources, the question of life in general, the epistemological issues and knowledge about being alone in the universe, ethics towards extraterrestrial life, the question of [[politics]] and governing uninhabited worlds, along with questions of [[ecology]].

For von Hegner,<ref>{{cite journal |last1=Von Egner |first1=Ian |title=Astrobiology and astrophilosophy: Subsuming or bifurcating disciplines |journal=Philosophy and Cosmology |date=2019 |volume=23 |pages=62–79 | url=https://philarchive.org/archive/VONAAA-4 }}</ref> the question of astrobiology and the possibility of [[astrophilosophy]] differs. For him, the discipline needs to bifurcate into astrobiology and astrophilosophy since discussions made possible by astrobiology, but which have been astrophilosophical in nature, have existed as long as there have been discussions about extraterrestrial life. Astrobiology is a self-corrective interaction among observation, hypothesis, experiment, and theory, pertaining to the exploration of all natural phenomena. Astrophilosophy consists of methods of dialectic analysis and logical argumentation, pertaining to the clarification of the nature of reality. Šekrst<ref>{{cite journal |author=Kristina Šekrst |title=Astrobiology in Philosophy or Philosophy in Astrobiology |journal=Cosmos and History: The Journal of Natural and Social Philosophy|volume=20|issue=2|pages=405-415|url=https://cosmosandhistory.org/index.php/journal/article/view/1140/1782 }}</ref> argues that astrobiology requires the affirmation of astrophilosophy, but not as a separate cognate to astrobiology. The stance of conceptual speciesm, according to Šekrst, permeates astrobiology since the very name astrobiology tries to talk about not just [[biology]], but about life in a general way, which includes terrestrial life as a subset. This leads us to either redefine philosophy, or consider the need for astrophilosophy as a more general discipline, to which philosophy is just a subset that deals with questions such as the nature of the human mind and other [[anthropocentrism|anthropocentric]] questions.

Most of the philosophy of astrobiology deals with two main questions: the question of life and the ethics of space exploration. Kolb<ref>{{cite book
 | last1 = Kolb | first1 = Vera | chapter = Philosophy of Astrobiology: Some Recent Developments | editor-last = Hoover | editor-first = Richard B. | title = Proc. SPIE 9606, Instruments, Methods, and Missions for Astrobiology XVII | year = 2015 | pages = 960605-1-960605-6 }}</ref> specifically emphasizes the question of [[virus]]es, for which the question whether they are alive or not is based on the definitions of life that include [[self-replication]]. Schneider<ref>{{cite book
 | last1 = Schneider
 | first1 = Sean
 | chapter = Philosophical Problems of the Definition of Extraterrestrial Life and Intelligence
 | editor-last = Dunér
 | editor-first = David
 | title = The History and Philosophy of Astrobiology: Perspectives on Extraterrestrial Life and the Human Mind
 | location = Newcastle upon Tyne
 | publisher = Cambridge Scholars Publishing
 | year = 2013
 | page = 132
}}</ref> tried to defined exo-life, but concluded that we often start with our own prejudices and that defining extraterrestrial life seems futile using human concepts. For Dick, astrobiology relies on [[metaphysics|metaphysical]] assumption that there is extraterrestrial life, which reaffirms questions in the [[philosophy of cosmology]], such as [[Fine-tuning (physics)|fine-tuning]] or the [[anthropic principle]].

== Rare Earth hypothesis ==
{{Main|Rare Earth hypothesis}}

The Rare Earth hypothesis postulates that multicellular life forms found on Earth may actually be more of a rarity than scientists assume. According to this hypothesis, life on Earth (and more, multi-cellular life) is possible because of a conjunction of the right circumstances (galaxy and location within it, [[planetary system]], star, orbit, planetary size, atmosphere, etc.); and the chance for all those circumstances to repeat elsewhere may be rare. It provides a possible answer to the [[Fermi paradox]] which wonders: if extraterrestrial aliens are common, why aren't they obvious?  It is apparently in opposition to the [[principle of mediocrity]], assumed by famed astronomers [[Frank Drake]], [[Carl Sagan]], and others. The principle of mediocrity suggests that life on Earth is not exceptional, and it is more than likely to be found on innumerable other worlds.

== Missions ==
Research into the environmental limits of life and the workings of extreme [[ecosystem]]s is ongoing, enabling researchers to better predict what planetary environments might be most likely to harbor life. Missions such as the [[Phoenix (spacecraft)|''Phoenix'' lander]], [[Mars Science Laboratory]], [[ExoMars]], [[Mars 2020]] rover to Mars, and the [[Cassini–Huygens|''Cassini'' probe]] to [[Saturn]]'s moons aim to further explore the possibilities of life on other planets in the Solar System.

;Viking program
{{main|Viking lander biological experiments}}
The two [[Viking program|Viking landers]] each carried four types of biological experiments to the surface of Mars in the late 1970s. These were the only Mars landers to carry out experiments looking specifically for [[metabolism]] by current microbial [[Life on Mars (planet)|life on Mars]]. The landers used a robotic arm to collect soil samples into sealed test containers on the craft. The two landers were identical, so the same tests were carried out at two places on Mars' surface; ''[[Viking 1]]'' near the equator and ''[[Viking 2]]'' further north.<ref name="Chambers">{{Cite book
 | first = Paul
 | last = Chambers
 | title = Life on Mars; The Complete Story
 | place = London
 | publisher = Blandford
 | date = 1999
 | isbn = 978-0-7137-2747-0
 | url-access = registration
 | url = https://archive.org/details/lifeonmarscomple00cham
 }}</ref> The result was inconclusive,<ref>Levin, G and P. Straaf. 1976. "Viking Labeled Release Biology Experiment: Interim Results". ''Science'': 194. 1322–1329.</ref> and is still disputed by some scientists.<ref name="Bianciardi-2012">{{cite journal|last1=Bianciardi |first1=Giorgio |last2=Miller |first2=Joseph D. |last3=Straat |first3=Patricia Ann |last4=Levin |first4=Gilbert V. |title=Complexity Analysis of the Viking Labeled Release Experiments |journal=IJASS |date=March 2012 |volume=13 |issue=1 |pages=14–26 |bibcode=2012IJASS..13...14B |doi=10.5139/IJASS.2012.13.1.14 |doi-access=free }}</ref><ref name="Discovery-20120412">{{cite web |last=Klotz |first=Irene |title=Mars Viking Robots 'Found Life' |url=http://news.discovery.com/space/mars-life-viking-landers-discovery-120412.html |date=12 April 2012 |publisher=[[Discovery Channel|Discovery News]] |access-date=16 April 2012 |archive-date=14 April 2012 |archive-url=https://web.archive.org/web/20120414195922/http://news.discovery.com/space/mars-life-viking-landers-discovery-120412.html |url-status=dead }}</ref><ref name="Navarro">{{Cite journal|last1=Navarro-González| first1=R.| date=2006| title=The limitations on organic detection in Mars-like soils by thermal volatilization–gas chromatography – MS and their implications for the Viking results| journal=[[PNAS]]| volume=103| issue=44| pages=16089–16094| doi=10.1073/pnas.0604210103| pmid=17060639 |pmc=1621051| bibcode = 2006PNAS..10316089N| display-authors=1| last2=Navarro| first2=K. F.|last3=Rosa |first3=J. d. l.| last4=Iniguez| first4=E.| last5=Molina| first5=P.| last6=Miranda| first6=L. D.|last7=Morales| first7=P.| last8=Cienfuegos| first8=E.| last9=Coll| first9=P. | doi-access=free}}</ref><ref name="Paepe">{{Cite journal| title=The Red Soil on Mars as a proof for water and vegetation| journal=Geophysical Research Abstracts| date=2007| first=Ronald| last=Paepe| volume=9| issue=1794| url=http://www.cosis.net/abstracts/EGU2007/01794/EGU2007-J-01794.pdf?PHPSESSID=e| format=PDP| access-date=2 May 2012| archive-url=https://web.archive.org/web/20110613164620/http://www.cosis.net/abstracts/EGU2007/01794/EGU2007-J-01794.pdf?PHPSESSID=e| archive-date=13 June 2011| url-status=dead}}</ref>
 
[[Norman Horowitz]] was the chief of the [[Jet Propulsion Laboratory]] bioscience section for the [[Mariner program|Mariner]] and [[Viking program|Viking]] missions from 1965 to 1976.  Horowitz considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival of life on other planets.<ref name = Horowitz1986>Horowitz, N.H. (1986). Utopia and Back and the search for life in the solar system. New York: W.H. Freeman and Company. {{ISBN|0-7167-1766-2}}</ref>  However, he also considered that the conditions found on Mars were incompatible with carbon based life.

;''Beagle 2''
[[Image:Beagle 2 replica.jpg|thumb|Replica of the 33.2&nbsp;kg ''Beagle-2'' lander]]
[[File:Msl20110519 PIA14156-full.jpg|thumb|[[Mars Science Laboratory]] rover concept artwork]]
''[[Beagle 2]]'' was an unsuccessful [[United Kingdom|British]] Mars lander that formed part of the [[European Space Agency]]'s 2003 [[Mars Express]] mission. Its primary purpose was to search for signs of [[life on Mars]], past or present. Although it landed safely, it was unable to correctly deploy its solar panels and telecom antenna.<ref>{{cite web|url=http://beagle2.open.ac.uk/index.htm|title=Beagle 2 : the British led exploration of Mars|access-date=13 March 2015|archive-url=https://web.archive.org/web/20160304034552/http://beagle2.open.ac.uk/index.htm|archive-date=4 March 2016|url-status=dead}}</ref>

;EXPOSE
[[EXPOSE]] is a multi-user facility mounted in 2008 outside the [[International Space Station]] dedicated to astrobiology.<ref name="Rabbow">{{cite journal|title=Expose, an Astrobiological Exposure Facility on the International Space Station – from Proposal to Flight |journal=Orig Life Evol Biosph |date=9 July 2009 |author1=Elke Rabbow |author2=Gerda Horneck |author3=Petra Rettberg |author4=Jobst-Ulrich Schott |author5=Corinna Panitz |author6=Andrea L'Afflitto |author7=Ralf von Heise-Rotenburg |author8=Reiner Willnecker |author9=Pietro Baglioni |author10=Jason Hatton |author11=Jan Dettmann |author12=René Demets |author13=Günther Reitz |doi=10.1007/s11084-009-9173-6 |url=http://www.prism.gatech.edu/~alafflitto3/Documents/Rabbow_Horneck_LAfflitto_Origin_of_Life_and_Evolution_of_Biosphere.pdf |access-date=8 July 2013 |bibcode=2009OLEB...39..581R |pmid=19629743 |volume=39 |issue=6 |pages=581–598 |s2cid=19749414 |url-status=dead |archive-url=https://web.archive.org/web/20140110151139/http://www.prism.gatech.edu/~alafflitto3/Documents/Rabbow_Horneck_LAfflitto_Origin_of_Life_and_Evolution_of_Biosphere.pdf |archive-date=10 January 2014 }}</ref><ref name="Experimental methods">{{cite journal | title = Experimental methods for studying microbial survival in extraterrestrial environments | journal = Journal of Microbiological Methods | date = 23 October 2009 | author1 = Karen Olsson-Francis | author2 = Charles S. Cockell | volume = 80 | pages = 1–13 | url = http://www1.univap.br/~spilling/AB/Olsson-francis_cockel_2010_astrobiology_Exp.pdf | access-date = 31 July 2013 | doi = 10.1016/j.mimet.2009.10.004 | pmid = 19854226 | issue = 1 | archive-url = https://web.archive.org/web/20130918161906/http://www1.univap.br/~spilling/AB/Olsson-francis_cockel_2010_astrobiology_Exp.pdf | archive-date = 18 September 2013 | url-status = dead | df = dmy-all }}</ref> EXPOSE was developed by the [[European Space Agency]] (ESA) for [[Spaceflight|long-term spaceflights]] that allow exposure of [[organic chemical]]s and biological samples to [[outer space]] in [[low Earth orbit]].<ref name="home">{{cite web | url = http://smsc.cnes.fr/EXPOSE/ | title = Expose – home page | access-date = 8 July 2013 | publisher = Centre national d'études spatiales (CNES) | archive-url = https://web.archive.org/web/20130115000920/http://smsc.cnes.fr/EXPOSE/ | archive-date = 15 January 2013 | url-status = dead | df = dmy-all }}</ref>

;Mars Science Laboratory
The [[Mars Science Laboratory]] (MSL) mission landed the [[Curiosity (rover)|''Curiosity'']] [[Rover (space exploration)|rover]] that is currently in operation on [[Mars]].<ref name="MSLNameWebsite">{{cite web |title=Name NASA's Next Mars Rover |url=http://marsrovername.jpl.nasa.gov/ |publisher=NASA/JPL |date=27 May 2009 |access-date=27 May 2009 |archive-url=https://web.archive.org/web/20090522004939/http://marsrovername.jpl.nasa.gov/ |archive-date=22 May 2009 |url-status=dead }}</ref> It was launched 26 November 2011, and landed at [[Gale (crater)|Gale Crater]] on 6 August 2012. Mission objectives are to help assess Mars' [[Planetary habitability|habitability]] and in doing so, determine whether Mars is or has ever been able to support [[Life on Mars (planet)|life]],<ref name="MSL-main_page">{{cite web |title=Mars Science Laboratory: Mission  |url=http://marsprogram.jpl.nasa.gov/msl/mission/|archive-url=https://web.archive.org/web/20060305231951/http://marsprogram.jpl.nasa.gov/msl/mission/|url-status=dead|archive-date=5 March 2006| publisher=NASA/JPL | access-date=12 March 2010 }}</ref> collect data for a future [[Human mission to Mars|human mission]], study Martian geology, its climate, and further assess the role that [[Water on Mars|water]], an essential ingredient for life as we know it, played in forming minerals on Mars.

;''Tanpopo''
The [[Tanpopo (mission)|''Tanpopo'']] mission is an orbital astrobiology experiment investigating the potential interplanetary transfer of life, [[organic compound]]s, and possible terrestrial particles in the low Earth orbit. The purpose is to assess the [[panspermia]] hypothesis and the possibility of natural interplanetary transport of microbial life as well as prebiotic organic compounds. Early mission results show evidence that some clumps of microorganism can survive for at least one year in space.<ref name='AGU2017'>[http://blogs.agu.org/geospace/2017/05/19/early-tanpopo-mission-results-show-microbes-can-survive-space/ "Early Tanpopo mission results show microbes can survive in space"]. American Geophysical Union. ''Geospace''. Larry O'Hanlon. 19 May 2017.</ref> This may support the idea that clumps greater than 0.5 millimeters of microorganisms could be one way for life to spread from planet to planet.<ref name='AGU2017'/>

;''ExoMars'' rover
[[File:ExoMars model at ILA 2006.jpg|thumb|ExoMars rover model]]
''[[ExoMars]]'' is a robotic mission to Mars to search for possible [[biosignature]]s of [[Life on Mars|Martian life]], past or present. This astrobiological mission was under development by the [[European Space Agency]] (ESA) in partnership with the [[Russian Federal Space Agency]] (Roscosmos); it was planned for a 2022 launch;<ref name="still keen">{{cite news |last=Amos |first=Jonathan | title = Europe still keen on Mars missions | date = 15 March 2012 | url = https://www.bbc.co.uk/news/science-environment-17390576 | work = BBC News | access-date = 16 March 2012}}</ref><ref name="Svitak">{{cite news | last = Svitak | first = Amy | title = Europe Joins Russia on Robotic ExoMars | date = 16 March 2012 | url = http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=space&id=news/awx/2012/03/15/awx_03_15_2012_p0-437120.xml&headline=Europe%20Joins%20Russia%20on%20Robotic%20ExoMars | work = Aviation Week | access-date = 16 March 2012 }}{{Dead link|date=September 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name="Selding">{{cite news |last=Selding |first=Peter B. de | title = ESA Ruling Council OKs ExoMars Funding | date = 15 March 2012 | url = http://www.spacenews.com/civil/120315-esa-council-oks-exomars.html | archive-url = https://archive.today/20121206010858/http://www.spacenews.com/civil/120315-esa-council-oks-exomars.html | url-status = dead | archive-date = 6 December 2012 | work = Space News | access-date = 16 March 2012}}</ref> however, technical and funding issues and the [[Russian invasion of Ukraine]] have forced ESA to repeatedly delay the rover's delivery to 2028.<ref name=":0">{{Cite web |last=Foust |first=Jeff |date=2024-04-10 |title=ESA awards contract to Thales Alenia Space to restart ExoMars |url=https://spacenews.com/esa-awards-contract-to-thales-alenia-space-to-restart-exomars/ |access-date=2024-04-11 |website=SpaceNews |language=en-US}}</ref>

;Mars 2020
[[File:PIA23962-Mars2020-Rover&Helicopter-20200714.jpg|thumb|Artist's rendition of the [[Perseverance (rover)|''Perseverance'' rover]] on Mars, with the mini-helicopter [[Mars Helicopter Ingenuity|''Ingenuity'']] in front]]
[[Mars 2020]] successfully landed its rover ''[[Perseverance (rover)|Perseverance]]'' in [[Jezero Crater]] on 18 February 2021. It will investigate environments on Mars relevant to astrobiology, investigate its surface [[Geology of Mars|geological processes]] and history, including the assessment of its past [[Planetary habitability|habitability]] and potential for preservation of [[biosignature]]s and [[biomolecule]]s within accessible geological materials.<ref name="Cowing">{{cite web | url = http://spaceref.com/mars/science-definition-team-for-the-2020-mars-rover.html | archive-url = https://archive.today/20130203233556/http://spaceref.com/mars/science-definition-team-for-the-2020-mars-rover.html | url-status = dead | archive-date = 3 February 2013 | title = Science Definition Team for the 2020 Mars Rover | access-date = 21 December 2012 | last = Cowing | first = Keith | date = 21 December 2012 | work = NASA | publisher = Science Ref }}</ref> The Science Definition Team is proposing the rover collect and package at least 31 samples of rock cores and soil for a later mission to bring back for more definitive analysis in laboratories on Earth. The rover could make measurements and technology demonstrations to help designers of a [[Human mission to Mars|human expedition]] understand any hazards posed by Martian dust and demonstrate how to collect [[carbon dioxide]] (CO<sub>2</sub>), which could be a resource for making molecular oxygen (O<sub>2</sub>) and [[rocket fuel]].<ref name="goals">{{cite news | title = Science Team Outlines Goals for NASA's 2020 Mars Rover | date = 9 July 2013 | publisher = NASA | url = http://www.jpl.nasa.gov/news/news.php?release=2013-217 | work = Jet Propulsion Laboratory | access-date = 10 July 2013 }}</ref><ref name="FAQ">{{cite web | url = http://mars.nasa.gov/files/m2020/Mars2020FAQs.pdf | title = Mars 2020 Science Definition Team Report – Frequently Asked Questions | access-date = 10 July 2013 | date = 9 July 2013 | work = NASA}}</ref>

;''Europa Clipper''
''[[Europa Clipper]]'' is a mission launched by NASA on 14 October 2024 that will conduct detailed reconnaissance of [[Jupiter]]'s moon [[Europa (moon)|Europa]] beginning in 2030, and will investigate whether its internal ocean could harbor conditions suitable for life.<ref>{{Cite web |date=October 14, 2024 |title=NASA's Europa Clipper launches aboard SpaceX rocket, bound for Jupiter's icy ocean moon |url=https://www.latimes.com/science/story/2024-10-14/nasa-jupiter-probe-launches-aboard-spacex-rocket |access-date=October 14, 2024 |website=Los Angeles Times}}</ref><ref name="missions">{{cite news | title = Europa Clipper | date = November 2013 | publisher = NASA | url = http://solarsystem.nasa.gov/missions/profile.cfm?MCode=EuropaClipper&Display=ReadMore | work = Jet Propulsion Laboratory | access-date = 13 December 2013 | archive-url = https://web.archive.org/web/20131213193916/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=EuropaClipper&Display=ReadMore | archive-date = 13 December 2013 | url-status = dead | df = dmy-all }}</ref><ref name="FPE">{{cite news | first = Van | last = Kane | title = Europa Clipper Update | date = 26 May 2013 | url = http://futureplanets.blogspot.com/2013/05/europa-clipper-update.html | work = Future Planetary Exploration | access-date = 13 December 2013}}</ref> It will also aid in the selection of future [[Lander (spacecraft)|landing sites]].<ref name="Europa Lander 2013">{{cite journal | title = Science Potential from a Europa Lander | journal = Astrobiology | date = 2013 | first1 = Robert T. | last1 = Pappalardo |author2= S. Vance |author3=F. Bagenal |author4=B.G. Bills |author5=D.L. Blaney |author5-link= Diana Blaney |author6=D.D. Blankenship |author7=W.B. Brinckerhoff | volume = 13 | issue = 8| doi=10.1089/ast.2013.1003 | bibcode=2013AsBio..13..740P | pages=740–773 | pmid=23924246|display-authors=etal| hdl = 1721.1/81431 | s2cid = 10522270 | url=https://dspace.mit.edu/bitstream/1721.1/81431/2/Pappalardo_Science-potential.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://dspace.mit.edu/bitstream/1721.1/81431/2/Pappalardo_Science-potential.pdf |archive-date=2022-10-09 |url-status=live | hdl-access=free }}</ref><ref>{{citation | first = D. | last = Senske | contribution = Europa Mission Concept Study Update | title = Presentation to Planetary Science Subcommittee | date = 2 October 2012| url = http://www.lpi.usra.edu/pss/oct2012/presentations/5_Senske_Europa.pdf | access-date = 14 December 2013}}</ref>

;''Dragonfly''
''[[Dragonfly (Titan space probe)|Dragonfly]]'' is a NASA mission scheduled to land on [[Titan (moon)|Titan]] in 2036 to assess its microbial habitability and study its prebiotic chemistry. Dragonfly is a [[rotorcraft]] lander that will perform controlled flights between multiple locations on the surface, which allows sampling of diverse regions and geological contexts.<ref name='APL draft'>[http://dragonfly.jhuapl.edu/News-and-Resources/docs/34_03-Lorenz.pdf Dragonfly: A Rotorcraft Lander Concept for Scientific Exploration at Titan] Ralph D. Lorenz, Elizabeth P. Turtle, Jason W. Barnes, Melissa G. Trainer, Douglas S. Adams, Kenneth E. Hibbard, Colin Z. Sheldon, Kris Zacny, Patrick N. Peplowski, David J. Lawrence, Michael A. Ravine, Timothy G. McGee, Kristin S. Sotzen, Shannon M. MacKenzie, Jack W. Langelaan, Sven Schmitz, Larry S. Wolfarth, and Peter D. Bedini. 2018. Johns Hopkins APL Technical Digest, 34(3), 374-387</ref>

===Proposed concepts===

;''Icebreaker Life''
''[[Icebreaker Life]]'' is a lander mission that was proposed for NASA's [[Discovery Program]] for the 2021 launch opportunity,<ref name='Icebreaker2021'>{{cite journal | title = The ''Icebreaker Life'' Mission to Mars: A Search for Biomolecular Evidence for Life | journal = Astrobiology | date = 5 April 2013 | author1=Christopher P. McKay | author2=Carol R. Stoker | author3=Brian J. Glass | author4=Arwen I. Davé | author5=Alfonso F. Davila | author6=Jennifer L. Heldmann | author7=Margarita M. Marinova | author8=Alberto G. Fairen | author9=Richard C. Quinn | author10=Kris A. Zacny | author11=Gale Paulsen | author12=Peter H. Smith | author13=Victor Parro | author14=Dale T. Andersen | author15=Michael H. Hecht | author16=Denis Lacelle | author17=Wayne H. Pollard | display-authors = 6 | volume = 13 | issue = 4 | pages = 334–353 | doi = 10.1089/ast.2012.0878 | pmid = 23560417 |bibcode = 2013AsBio..13..334M}}</ref> but it was not selected for development. It would have had a stationary lander that would be a near copy of the successful 2008 [[Phoenix (spacecraft)|''Phoenix'']] and it would have carried an upgraded astrobiology scientific payload, including a 1-meter-long core drill to sample ice-cemented ground in the northern plains to conduct a search for [[organic molecule]]s and evidence of current or past [[life on Mars]].<ref name="AstrobioMag">{{cite news |last=Choi |first=Charles Q. | title = Icebreaker Life Mission | date = 16 May 2013 | url = http://www.astrobio.net/exclusive/5476/mars-icebreaker-life-mission | work = [[Astrobiology Magazine]] | access-date = 1 July 2013 |archive-url=https://web.archive.org/web/20151009000704/http://www.astrobio.net/news-exclusive/mars-icebreaker-life-mission/ |archive-date=2015-10-09 |url-status=usurped}}</ref><ref name="LPI 2012">{{cite book | author1 = C. P. McKay |author2= Carol R. Stoker |author3=Brian J. Glass |author4=Arwen I. Davé |author5=Alfonso F. Davila |author6=Jennifer L. Heldmann |author7=Margarita M. Marinova |author8=Alberto G. Fairen |author9=Richard C. Quinn |author10=Kris A. Zacny |author11=Gale Paulsen |author12=Peter H. Smith |author13=Victor Parro |author14=Dale T. Andersen |author15=Michael H. Hecht |author16=Denis Lacelle |author17=Wayne H. Pollard | display-authors = 6 | contribution = The Icebreaker Life Mission to Mars: A Search for Biochemical Evidence for Life | title = Concepts and Approaches for Mars Exploration | publisher = Lunar and Planetary Institute | date = 2012 | url = http://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4091.pdf | access-date = 1 July 2013}}</ref> One of the key goals of the ''Icebreaker Life'' mission is to test the [[hypothesis]] that the ice-rich ground in the polar regions has significant concentrations of organics due to protection by the ice from [[Oxidizing agent|oxidants]] and [[Sunlight|radiation]].

;''Journey to Enceladus and Titan''
''[[Journey to Enceladus and Titan]]'' (''JET'') is an astrobiology mission concept to assess the [[planetary habitability|habitability]] potential of [[Saturn]]'s moons [[Enceladus]] and [[Titan (moon)|Titan]] by means of an orbiter.<ref name="Sotin2011">{{cite conference |last1=Sotin |first1=C. |last2=Altwegg |first2=K.|author2-link=Kathrin Altwegg |last3=Brown |first3=R.H. |title=JET: Journey to Enceladus and Titan |url=http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1326.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1326.pdf |archive-date=2022-10-09 |url-status=live |conference=42nd Lunar and Planetary Science Conference |publisher=Lunar and Planetary Institute |date=2011 |display-authors=etal}}</ref><ref name="JET 204">{{cite news |last=Kane |first=Van |title=Discovery Missions for an Icy Moon with Active Plumes | url=http://www.planetary.org/blogs/guest-blogs/van-kane/20140402-discovery-missions-for-an-icy-moon-with-plumes.html| work=The Planetary Society |date=3 April 2014 |access-date=9 April 2015 }}</ref><ref name="LCPM-2013">{{cite conference |last1=Matousek |first1=Steve |last2=Sotin |first2=Christophe |last3=Goebel |first3=Dan |last4=Lang |first4=Jared |title=JET: Journey to Enceladus and Titan |url=http://lcpm10.caltech.edu/pdf/session-5/3_JET-LCPM-130618-Matousek-final.pdf |conference=Low Cost Planetary Missions Conference |publisher=California Institute of Technology |date=18–21 June 2013 |access-date=10 April 2015 |archive-url=https://web.archive.org/web/20160304072017/http://lcpm10.caltech.edu/pdf/session-5/3_JET-LCPM-130618-Matousek-final.pdf |archive-date=4 March 2016 |url-status=dead  }}</ref>

;''Enceladus Life Finder''
''[[Enceladus Life Finder]]'' (''ELF'') is a proposed astrobiology mission concept for a space probe intended to assess the [[Planetary habitability|habitability]] of the [[Extraterrestrial liquid water#Enceladus|internal aquatic ocean]] of [[Enceladus]], [[Saturn]]'s [[moons of Saturn|sixth-largest moon]].<ref name="2015 LPSC">{{cite conference |last1=Lunine |first1=Jonathan I. | last2=Waite | first2=Jack Hunter Jr. |last3=Postberg |first3=Frank |last4=Spilker |first4=Linda J. |title=Enceladus Life Finder: The search for life in a habitable moon |url=http://www.hou.usra.edu/meetings/lpsc2015/pdf/1525.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.hou.usra.edu/meetings/lpsc2015/pdf/1525.pdf |archive-date=2022-10-09 |url-status=live |conference=46th Lunar and Planetary Science Conference |publisher=Lunar and Planetary Institute |place=Houston (TX) |year=2015 }}</ref><ref name="contestants">{{cite news |last=Clark |first=Stephen |url=http://spaceflightnow.com/2015/04/06/diverse-destinations-considered-for-new-interplanetary-probe/ |title=Diverse destinations considered for new interplanetary probe |work=Space Flight Now |date=6 April 2015 |access-date=7 April 2015 }}</ref>

;''Life Investigation For Enceladus''
''[[Life Investigation For Enceladus]]'' (''LIFE'') is a proposed astrobiology sample-return mission concept. The spacecraft would enter into [[Saturn]] orbit and enable multiple flybys through Enceladus' icy plumes to collect icy plume particles and volatiles and return them to Earth on a capsule. The spacecraft may sample Enceladus' plumes, the [[Rings of Saturn#E Ring|E ring of Saturn]], and the upper atmosphere of [[Titan (moon)|Titan]].<ref name="Astro2012">{{cite journal |title= Life Investigation For Enceladus A Sample Return Mission Concept in Search for Evidence of Life. |journal=Astrobiology |date=August 2012 |last1=Tsou |first1=Peter |last2=Brownlee |first2=D.E. |first3=Christopher |last3=McKay |last4=Anbar |first4=A.D. |last5= Yano |first5=H. |volume=12 |issue=8 |pages=730–742 |doi=10.1089/ast.2011.0813 |bibcode = 2012AsBio..12..730T |pmid=22970863}}</ref><ref name="Porco 2014">{{cite journal |title=Life – Enceladus Plume Sample Return via Discovery |journal=45th Lunar and Planetary Science Conference |year=2014 |last1=Tsou |first1=Peter |last2=Anbar |first2=Ariel |last3=Atwegg |first3=Kathrin |last4=Porco |first4=Carolyn |last5=Baross |first5=John |last6=McKay |first6=Christopher |issue=1777 |pages=2192 |bibcode=2014LPI....45.2192T |url=http://www.hou.usra.edu/meetings/lpsc2014/pdf/2192.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.hou.usra.edu/meetings/lpsc2014/pdf/2192.pdf |archive-date=2022-10-09 |url-status=live |access-date=10 April 2015 }}</ref><ref name="Doc 2013">{{cite journal|url=http://discoveringenceladus.com/downloads/LIFE%20-%20Life%20Investigation%20For%20Enceladus%20-%20A%20Sample%20Return%20Mission%20Concept%20in%20Search%20for%20Evidence%20of%20Life.doc |format=.doc |title=Life Investigation For Enceladus – A Sample Return Mission Concept in Search for Evidence of Life. |last=Tsou |first=Peter |journal=Jet Propulsion Laboratory |date=2013 |volume=12 |issue=8 |pages=730–742 |doi=10.1089/ast.2011.0813 |access-date=10 April 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150901121008/http://discoveringenceladus.com/downloads/LIFE%20-%20Life%20Investigation%20For%20Enceladus%20-%20A%20Sample%20Return%20Mission%20Concept%20in%20Search%20for%20Evidence%20of%20Life.doc |archive-date=1 September 2015  |pmid=22970863 |bibcode=2012AsBio..12..730T }}</ref>

;''Oceanus''
[[Oceanus (Titan orbiter)|''Oceanus'']] is an orbiter proposed in 2017 for the [[New Frontiers program|New Frontiers]] mission No. 4. It would travel to the moon of [[Saturn]], [[Titan (moon)|Titan]], to assess its [[Planetary habitability|habitability]].<ref name=sotin>{{cite conference |last1= Sotin|first1= C.|last2= Hayes|first2= A.|last3= Malaska|first3= M.|last4= Nimmo|first4= F.|last5= Trainer|first5= M. |first6 =M.|last6= Mastrogiuseppe|first7= J.|last7= Soderblom|first8= P.|last8= Tortora|first9= J.|last9= Hofgartner|first10=O.|last10= Aharonson|first11=J. W.|last11= Barnes|first12= R.|last12= Hodyss|first13= L.|last13 = Iess|first14 = R.|last14= Kirk|first15= P.|last15 = Lavvas|first16 = R.|last16 = Lorenz |first17=Jonathan I. |last17=Lunine |first18=E.|last18 = Mazarico|first19=A.|last19= McEwen|first20= C. |last20 = Neish|first21=C.|last21= Nixon|first22=E.|last22= Turtle|first23 =  V.|last23= Vuitton|first24= R.|last24= Yelle|display-authors = 6 |url = http://www.hou.usra.edu/meetings/lpsc2017/pdf/2306.pdf|title= Oceanus: A New Frontiers orbiter to study Titan's potential habitability |conference= 48th Lunar and Planetary Science Conference|date= 20–24 March 2017 |location = The Woodlands, Texas}}</ref> ''Oceanus''{{'}} objectives are to reveal Titan's [[organic chemistry]], geology, gravity, topography, collect 3D reconnaissance data, catalog the [[Organic compound|organics]] and determine where they may interact with liquid water.<ref>{{cite conference|last1 =Tortora |first1= P.|last2= Zannoni|first2= M.|last3= Nimmo|first3= F.|last4= Mazarico|first4= E.|last5= Iess|first5= L.|last6= Sotin|first6= C. |last7= Hayes|first7= A.|last8= Malaska|first8= M.|title=Titan gravity investigation with the Oceanus mission|journal= EGU General Assembly Conference Abstracts|volume= 19|conference= 19th EGU General Assembly, EGU2017|date= 23–28 April 2017 |page = 17876|bibcode= 2017EGUGA..1917876T}}</ref>

;''Explorer of Enceladus and Titan''
''[[Explorer of Enceladus and Titan]]'' ('''E<sup>2</sup>T''') is an orbiter mission concept that would investigate the evolution and [[Planetary habitability|habitability]] of the Saturnian satellites [[Enceladus (moon)|Enceladus]] and [[Titan (moon)|Titan]]. The mission concept was proposed in 2017 by the [[European Space Agency]].<ref name='E2T 2017'>{{cite journal |title=Explorer of Enceladus and Titan (E2T): Investigating the habitability and evolution of ocean worlds in the Saturn system |journal=American Astronomical Society |year=2017|url=https://e2tmission.wordpress.com/ |author1=Mitri, Giuseppe |author2=Postberg, Frank |author3=Soderblom, Jason M. |author4=Tobie, Gabriel |author5=Tortora, Paolo |author6=Wurz, Peter |author7=Barnes, Jason W. |author8=Coustenis, Athena |author9=Ferri, Francesca |author10=Hayes, Alexander |author11=Hayne, Paul O. |author12=Hillier, Jon |author13=Kempf, Sascha |author14=Lebreton, Jean-Pierre |author15=Lorenz, Ralph |author16=Orosei, Roberto |author17=Petropoulos, Anastassios |author18=Yen, Chen-wan |author19=Reh, Kim R. |author20=Schmidt, Jürgen |author21=Sims, Jon |author22=Sotin, Christophe |author23=Srama, Ralf |volume=48 |pages=225.01 |bibcode=2016DPS....4822501M |display-authors=6 |access-date=16 September 2017 }}</ref>

==See also==
{{Div col|colwidth=22em}}
* {{annotated link|Abiogenesis}}
* {{annotated link|Active SETI}}
* {{annotated link|Astrobotany}}
* {{annotated link|Astrochemistry}}
* {{Annotated link|Astrovirology}}
* {{annotated link|Cosmic dust}}
* {{annotated link|Detecting Earth from distant star-based systems}}
* {{annotated link|Exoplanetology}}
* {{annotated link|Extraterrestrial life}}
* {{annotated link|Extraterrestrial sample curation}}
* {{annotated link|Forward-contamination}}
* {{annotated link|Hypothetical types of biochemistry}}
* {{annotated link|List of microorganisms tested in outer space}}
* {{annotated link|MERMOZ}}
* {{annotated link|Nexus for Exoplanet System Science}}
* {{annotated link|Planetary habitability}}
* {{annotated link|Planetary protection}}
* {{annotated link|Planet Simulator}}
* {{annotated link|Quiet and loud aliens}}
* {{annotated link|Synthetic biology}}
* ''[[The Living Cosmos]]''
* {{annotated link|Xenobiology}}
{{div col end}}

==Citations==
<!--You may want to use the automated reference generator for acceptable reference format: http://toolserver.org/~magnus/makeref.php  -->
{{Reflist}}

==General references==
* The [http://www.journals.cambridge.org/jid_IJA ''International Journal of Astrobiology''] {{Webarchive|url=https://web.archive.org/web/20080725030239/http://www.journals.cambridge.org/jid_IJA |date=25 July 2008 }}, published by [[Cambridge University Press]], is the forum for practitioners in this interdisciplinary field.
* [https://web.archive.org/web/20040831120442/http://www.liebertpub.com/publication.aspx?pub_id=99 ''Astrobiology''], published by [[Mary Ann Liebert, Inc.]], is a peer-reviewed journal that explores the origins of life, evolution, distribution, and destiny in the universe.
* {{cite book |title=Astrobiology: A Very Short Introduction |last=Catling |first=David C. |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-958645-5 }}
* {{cite book |title=Astrobiology: Understanding Life in the Universe |last=Cockell |first=Charles S. |date=2015 |publisher=Wiley-Blackwell |location=NJ |isbn=978-1-118-91332-1 }}
* {{cite book |title=Astrobiology: An Evolutionary Approach |editor-last=Kolb |editor-first=Vera M. |date=2015 |publisher=CRC Press |location=Boca Raton |isbn=978-1-4665-8461-7 }}
* {{cite book |title=Handbook of Astrobiology |editor-last=Kolb |editor-first=Vera M. |date=2019 |publisher=CRC Press |location=Boca Raton |isbn=978-1-138-06512-3 }}
*[[Avi Loeb|Loeb, Avi]] (2021). ''[[Extraterrestrial: The First Sign of Intelligent Life Beyond Earth]]''. [[Houghton Mifflin Harcourt]]. {{ISBN|978-0358278146}}
* {{cite book |title=The Living Universe: NASA and the Development of Astrobiology |last=Dick |first=Steven J. |author2=James Strick |date=2005 |publisher=Rutgers University Press |location=Piscataway, NJ |isbn=978-0-8135-3733-7 }}
* {{cite book |title=Lonely planets. The natural philosophy of alien life |last=Grinspoon |first=David |date=2004 |orig-year=2003 |publisher=ECCO |location=New York |isbn=978-0-06-018540-4 }}
* {{Cite book | last = Mautner | first = Michael N. | title = Seeding the Universe with Life: Securing Our Cosmological Future | location = Washington D. C. | date = 2000 |  isbn = 978-0-476-00330-9 | url = http://www.astro-ecology.com/PDFSeedingtheUniverse2005Book.pdf |publisher=Legacy Books }}
* {{cite book |title=Science, Society, and the Search for Life in the Universe |url=https://archive.org/details/sciencesocietyse0000jako |url-access=registration |last=Jakosky |first=Bruce M. |date=2006 |publisher=University of Arizona Press |location=Tucson |isbn=978-0-8165-2613-0 }}
* {{cite book |title=Astrobiology. A Multidisciplinary Approach |last=Lunine |first=Jonathan I. |date=2005 |publisher=Pearson Addison-Wesley |location=San Francisco |isbn=978-0-8053-8042-2 }}
* {{cite book |title=An introduction to astrobiology |last=Gilmour |first=Iain |author2=Mark A. Sephton |date=2004 |publisher=Cambridge Univ. Press |location=Cambridge |isbn=978-0-521-83736-1 }}
* {{cite book |title=Rare Earth: Why Complex Life is Uncommon in the Universe |last1=Ward |first1=Peter |author2=Brownlee, Donald |date=2000 |publisher=Copernicus |location=New York |isbn=978-0-387-98701-9 }}
* {{Cite journal
| last1 = Chyba | first1 = C. F.
| last2 = Hand | first2 = K. P.
| s2cid = 2084246
| doi = 10.1146/annurev.astro.43.051804.102202
| title = ASTROBIOLOGY: The Study of the Living Universe
| journal = Annual Review of Astronomy and Astrophysics
| volume = 43
| issue = 1
| pages = 31–74
| year = 2005
|bibcode = 2005ARA&A..43...31C }}

==Further reading==
* {{Cite journal|title=The Astrobiology Primer v2.0|journal=[[Astrobiology (journal)|Astrobiology]]|editor-last=Domagal-Dorman|editor-first=Shawn|language=en|doi=10.1089/ast.2015.1460 |date=2016|volume=16|issue=8|pages=561–653|pmid=27532777|bibcode=2016AsBio..16..561D|last1=Domagal-Goldman|first1=Shawn D.|last2=Wright|first2=Katherine E.|last3=Adamala|first3=Katarzyna|last4=Arina de la Rubia|first4=Leigh|last5=Bond|first5=Jade|last6=Dartnell|first6=Lewis R.|last7=Goldman|first7=Aaron D.|last8=Lynch|first8=Kennda|last9=Naud|first9=Marie-Eve|last10=Paulino-Lima|first10=Ivan G.|last11=Singer|first11=Kelsi|last12=Walter-Antonio|first12=Marina|last13=Abrevaya|first13=Ximena C.|last14=Anderson|first14=Rika|last15=Arney|first15=Giada|last16=Atri|first16=Dimitra|last17=Azúa-Bustos|first17=Armando|last18=Bowman|first18=Jeff S.|last19=Brazelton|first19=William J.|last20=Brennecka|first20=Gregory A.|last21=Carns|first21=Regina|last22=Chopra|first22=Aditya|last23=Colangelo-Lillis|first23=Jesse|last24=Crockett|first24=Christopher J.|last25=Demarines|first25=Julia|last26=Frank|first26=Elizabeth A.|last27=Frantz|first27=Carie|last28=de la Fuente|first28=Eduardo|last29=Galante|first29=Douglas|last30=Glass|first30=Jennifer|pmc=5008114|s2cid=4425585|display-authors=1}}
* D. Goldsmith, T. Owen, ''The Search For Life in the Universe'', Addison-Wesley Publishing Company, 2001 (3rd edition). {{ISBN|978-1891389160}}
* [[Andy Weir]]'s 2021 novel, ''[[Project Hail Mary]]'', centers on astrobiology.

==External links==
{{sister project links|d=no|voy=no|species=no|b=no|s=no|n=no|q=no|wikt=astrobiology|commons=Category:Astrobiology|v=The Department of Astrobiology}}
* [https://web.archive.org/web/20110221183914/http://astrobiology.nasa.gov/ Astrobiology.nasa.gov]
* [http://astrobiology.ac.uk/ UK Centre for Astrobiology] {{Webarchive|url=https://web.archive.org/web/20121020132805/http://www.astrobiology.ac.uk/ |date=20 October 2012 }}
* [http://cab.inta-csic.es/ Spanish Centro de Astrobiología]
* [https://www.loc.gov/loc/kluge/fellowships/NASA-astrobiology.html Astrobiology Research at The Library of Congress]
* [https://web.archive.org/web/20120308172650/http://www.astrobiologysurvey.org/ Astrobiology Survey] – An introductory course on astrobiology
* [https://www.nasa.gov/feature/nasa-s-search-for-life-astrobiology-in-the-solar-system-and-beyond Summary - Search For Life Beyond Earth] {{Webarchive|url=https://web.archive.org/web/20230424182916/https://www.nasa.gov/feature/nasa-s-search-for-life-astrobiology-in-the-solar-system-and-beyond/ |date=24 April 2023 }} ([[NASA]]; 25 June 2021)

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