List of interstellar and circumstellar molecules

Infrared spectrum of HH 46/47 (image in inset), with vibrational bands of several molecules labelled in colour

This is a list of molecules that have been detected in the interstellar medium and circumstellar envelopes, grouped by the number of component atoms. The chemical formula is listed for each detected compound, along with any ionized form that has also been observed.

Background

Idealised example of the rotational spectrum (bottom) produced by transitions between different rotational energy levels (top) of a simple linear molecule. $B$ is the rotational constant of the molecule, $J$ is the rotational quantum number, $J^{\prime }$ is the upper level and $J^{″}$ is the lower level.

The molecules listed below were detected through astronomical spectroscopy. Their spectral features arise because molecules either absorb or emit a photon of light when they transition between two molecular energy levels. The energy (and thus the wavelength) of the photon matches the energy difference between the levels involved. Molecular electronic transitions occur when one of the molecule's electrons moves between molecular orbitals, producing a spectral line in the ultraviolet, optical or near-infrared parts of the electromagnetic spectrum. Alternatively, a vibrational transition transfers quanta of energy to (or from) vibrations of molecular bonds, producing signatures in the mid- or far-infrared. Gas-phase molecules also have quantised rotational levels, leading to transitions at microwave or radio wavelengths.

Sometimes a transition can involve more than one of these types of energy level e.g. ro-vibrational spectroscopy changes both the rotational and vibrational energy level. Occasionally all three occur together, as in the Phillips band of C₂ (diatomic carbon), in which an electronic transition produces a line in the near-infrared, which is then split into several vibronic bands by a simultaneous change in vibrational level, which in turn are split again into rotational branches.

The spectrum of a particular molecule is governed by the selection rules of quantum chemistry and by its molecular symmetry. Some molecules have simple spectra which are easy to identify, whilst others (even some small molecules) have extremely complex spectra with flux spread among many different lines, making them far harder to detect. Interactions between the atomic nuclei and the electrons sometimes cause further hyperfine structure of the spectral lines. If the molecule exists in multiple isotopologues (versions containing different atomic isotopes), the spectrum is further complicated by isotope shifts.

Detection of a new interstellar or circumstellar molecule requires identifying a suitable astronomical object where it is likely to be present, then observing it with a telescope equipped with a spectrograph working at the required wavelength, spectral resolution and sensitivity. The first molecule detected in the interstellar medium was the methylidyne radical (CH^•) in 1937, through its strong electronic transition at 4300 angstroms (in the optical). Advances in astronomical instrumentation have led to increasing numbers of new detections. From the 1950s onwards, radio astronomy began to dominate new detections, with sub-mm astronomy also becoming important from the 1990s.

The inventory of detected molecules is highly biased towards certain types which are easier to detect. For example, radio astronomy is most sensitive to small linear molecules with a high molecular dipole. The most common molecule in the Universe, H₂ (molecular hydrogen), is completely invisible to radio telescopes because it has no dipole; its electronic transitions are too energetic for optical telescopes, so detection of H₂ required ultraviolet observations with a sounding rocket. Vibrational lines are often not specific to an individual molecule, allowing only the general class to be identified. For example, the vibrational lines of polycyclic aromatic hydrocarbons (PAHs) were identified in 1984, showing the class of molecules is very common in space, but it took until 2021 to identify any specific PAHs through their rotational lines.

The carbon star CW Leonis. The visible shells of circumstellar material were ejected by the central star over thousands of years.

One of the richest sources for detecting interstellar molecules is Sagittarius B2 (Sgr B2), a giant molecular cloud near the centre of the Milky Way. About half of the molecules listed below were first found in Sgr B2, and many of the others have been subsequently detected there. A rich source of circumstellar molecules is CW Leonis (also known as IRC +10216), a nearby carbon star, where about 50 molecules have been identified. There is no clear boundary between interstellar and circumstellar media, so both are included in the tables below.

The discipline of astrochemistry includes understanding how these molecules form and explaining their abundances. The extremely low density of the interstellar medium is not conducive to the formation of molecules, making conventional gas-phase reactions between neutral species (atoms or molecules) inefficient. Many regions also have very low temperatures (typically 10 kelvin inside a molecular cloud), further reducing the reaction rates, or high ultraviolet radiation fields, which destroy molecules through photochemistry. Explaining the observed abundances of interstellar molecules requires calculating the balance between formation and destruction rates using gas-phase ion chemistry (often driven by cosmic rays), surface chemistry on cosmic dust, radiative transfer including interstellar extinction, and sophisticated reaction networks. The use of molecular lines to determine the physical properties of astronomical objects is known as molecular astrophysics.

Molecules

The following tables list molecules that have been detected in the interstellar medium or circumstellar matter, grouped by the number of component atoms. Neutral molecules and their molecular ions are listed in separate columns; if there is no entry in the molecule column, only the ionized form has been detected. Designations (names of molecules) are those used in the scientific literature describing the detection; if none was given that field is left empty. Mass is listed in atomic mass units. Deuterated molecules, which contain at least one deuterium (²H) atom, have slightly different masses and are listed in a separate table. The total number of unique species, including distinct ionization states, is indicated in each section header.

Most of the molecules detected so far are organic. The only detected inorganic molecule with five or more atoms is SiH₄. Molecules larger than that all have at least one carbon atom, with no N−N or O−O bonds.

Carbon monoxide is frequently used to trace the distribution of mass in molecular clouds.

Diatomic (43)

Molecule	Designation	Mass	Ions
AlCl	Aluminium monochloride	62.5	—
AlF	Aluminium monofluoride	46	—
AlO	Aluminium monoxide	43	—
—	Argonium	37	ArH+
C2	Diatomic carbon	24	—
—	Fluoromethylidynium	31	CF+
CH	Methylidyne radical	13	CH+
CN	Cyano radical	26	CN+, CN−
CO	Carbon monoxide	28	CO+
CP	Carbon monophosphide	43	—
CS	Carbon monosulfide	44	—
FeO	Iron(II) oxide	82	—
—	Helium hydride ion	5	HeH+
H2	Molecular hydrogen	2	—
HCl	Hydrogen chloride	36.5	HCl+
HF	Hydrogen fluoride	20	—
HO	Hydroxyl radical	17	OH+
KCl	Potassium chloride	75.5	—
NH	Imidogen radical	15	—
N2	Molecular nitrogen	28	—
NO	Nitric oxide	30	NO+
NS	Nitrogen sulfide	46	—
NaCl	Sodium chloride	58.5	—
—	Magnesium monohydride cation	25.3	MgH+
O2	Molecular oxygen	32	—
PN	Phosphorus mononitride	45	—
PO	Phosphorus monoxide	47	—
SH	Sulfur monohydride	33	SH+
SO	Sulfur monoxide	48	SO+
SiC	Carborundum	40	—
SiN	–	42	—
SiO	Silicon monoxide	44	—
SiS	Silicon monosulfide	60	—
TiO	Titanium(II) oxide	63.9	—

[[File:Trihydrogen-cation-3D-vdW.png|right|thumb|The [[Protonated molecular hydrogen|Template:Chem]] cation is one of the most abundant ions in the universe. It was first detected in 1993.]]

Triatomic (44)

Molecule	Designation	Mass	Ions
AlNC	Aluminium isocyanide	53	—
AlOH	Aluminium hydroxide	44	—
C3	Tricarbon	36	—
C2H	Ethynyl radical	25	—
CCN	Cyanomethylidyne	38	—
C2O	Dicarbon monoxide	40	—
C2S	Thioxoethenylidene	56	—
C2P	—	55	—
CO2	Carbon dioxide	44	—
CaNC	Calcium isocyanide	92	—
FeCN	Iron cyanide	82	—
—	Protonated molecular hydrogen	3	Template:Chem
H2C	Methylene radical	14	—
—	Chloronium	37.5	H2Cl+
H2O	Water	18	H2O+
HO2	Hydroperoxyl	33	—
H2S	Hydrogen sulfide	34	—
HCN	Hydrogen cyanide	27	—
HNC	Hydrogen isocyanide	27	—
HCO	Formyl radical	29	HCO+
HCP	Phosphaethyne	44	—
HCS	Thioformyl	45	HCS+
—	Diazenylium	29	Template:Chem
HNO	Nitroxyl	31	—
—	Isoformyl	29	HOC+
HSC	Isothioformyl	45	—
KCN	Potassium cyanide	65	—
MgCN	Magnesium cyanide	50	—
MgNC	Magnesium isocyanide	50	—
NH2	Amino radical	16	—
N2O	Nitrous oxide	44	—
NaCN	Sodium cyanide	49	—
NaOH	Sodium hydroxide	40	—
OCS	Carbonyl sulfide	60	—
O3	Ozone	48	—
SO2	Sulfur dioxide	64	—
c-SiC2	c-Silicon dicarbide	52	—
SiCSi	Disilicon carbide	68	—
SiCN	Silicon carbonitride	54	—
SiNC		54	—
TiO2	Titanium dioxide	79.9	—

Formaldehyde is an organic molecule that is widely distributed in the interstellar medium.

Four atoms (30)

Molecule	Designation	Mass	Ions
CH3	Methyl radical	15	Template:Chem2
l-C3H	Propynylidyne	37	l-C3H+
c-C3H	Cyclopropynylidyne	37	—
C3N	Cyanoethynyl	50	C3N−
C3O	Tricarbon monoxide	52	—
C3S	Tricarbon sulfide	68	—
—	Hydronium	19	H3O+
C2H2	Acetylene	26	—
H2CN	Methylene amidogen	28	H2CN+
H2NC	Aminocarbyne	28	—
H2CO	Formaldehyde	30	—
H2CS	Thioformaldehyde	46	—
HCCN	—	39	—
HCCO	Ketenyl	41	—
—	Protonated hydrogen cyanide	28	HCNH+
—	Protonated carbon dioxide	45	HOCO+
HCNO	Fulminic acid	43	—
HOCN	Cyanic acid	43	—
CNCN	Isocyanogen	52	—
HOOH	Hydrogen peroxide	34	—
HNCO	Isocyanic acid	43	—
HNCN	Cyanomidyl radical	41	—
HNCS	Isothiocyanic acid	59	—
NH3	Ammonia	17	—
HSCN	Thiocyanic acid	59	—
SiC3	Silicon tricarbide	64	—
HMgNC	Hydromagnesium isocyanide	51.3	—
HNO2	Nitrous acid	47	—

Methane, the primary component of natural gas, has also been detected on comets and in the atmosphere of several planets in the Solar System.

Five atoms (20)

Molecule	Designation	Mass	Ions
—	Ammonium ion	18	Template:Chem
CH4	Methane	16	—
CH3O	Methoxy radical	31	—
c-C3H2	Cyclopropenylidene	38	—
l-H2C3	Propadienylidene	38	—
H2CCN	Cyanomethyl	40	—
H2C2O	Ketene	42	—
H2CNH	Methylenimine	29	—
HNCNH	Carbodiimide	42	—
—	Protonated formaldehyde	31	H2COH+
C4H	Butadiynyl	49	C4H−
HC3N	Cyanoacetylene	51	—
HCC-NC	Isocyanoacetylene	51	—
HCOOH	Formic acid	46	—
NH2CN	Cyanamide	42	—
NH2OH	Hydroxylamine	37	—
—	Protonated cyanogen	53	NCCNH+
HC(O)CN	Cyanoformaldehyde	55	—
C5	Linear C5	60	—
SiC4	Silicon-carbide cluster	92	—
SiH4	Silane	32	—

In the ISM, formamide (above) can combine with methylene to form acetamide.

Six atoms (16)

Molecule	Designation	Mass	Ions
c-H2C3O	Cyclopropenone	54	—
E-HNCHCN	E-Cyanomethanimine	54	—
C2H4	Ethylene	28	—
CH3CN	Acetonitrile	40	—
CH3NC	Methyl isocyanide	40	—
CH3OH	Methanol	32	—
CH3SH	Methanethiol	48	—
l-H2C4	Diacetylene	50	—
—	Protonated cyanoacetylene	52	HC3NH+
HCONH2	Formamide	44	—
HOCOOH	Carbonic acid		—
C5H	Pentynylidyne	61	—
C5N	Cyanobutadiynyl radical	74	—
HC2CHO	Propynal	54	—
HC4N	—	63	—
CH2CNH	Ketenimine	40	—
C5S	—	92	—

Acetaldehyde (above) and its isomers vinyl alcohol and ethylene oxide have all been detected in interstellar space.

Seven atoms (13)

Molecule	Designation	Mass	Ions
c-C2H4O	Ethylene oxide	44	—
CH3C2H	Methylacetylene	40	—
H3CNH2	Methylamine	31	—
CH2CHCN	Acrylonitrile	53	—
HCCCHNH	Propargylimine	53	—
H2CHCOH	Vinyl alcohol	44	—
C6H	Hexatriynyl radical	73	C6H−
HC4CN	Cyanodiacetylene	75	—
HC4NC	Isocyanodiacetylene	75	—
HC5O	—	77	—
CH3CHO	Acetaldehyde	44	—
CH3NCO	Methyl isocyanate	57	—
HOCH2CN	Glycolonitrile	57	—

The radio signature of acetic acid, a compound found in vinegar, was confirmed in 1997.

Eight atoms (14)

Molecule	Designation	Mass
H3CC2CN	Methylcyanoacetylene	65
HC3H2CN	Propargyl cyanide	65
H2COHCHO	Glycolaldehyde	60
(CHOH)2	1,2-ethenediol	60
HCOOCH3	Methyl formate	60
CH3COOH	Acetic acid	60
H2C6	Hexapentaenylidene	74
CH2CHCHO	Propenal	56
CH2CCHCN	Cyanoallene	65
CH3CHNH	Ethanimine	43
C2H3NH2	Vinylamine	43
C7H	Heptatrienyl radical	85
NH2CH2CN	Aminoacetonitrile	56
(NH2)2CO	Urea	60

Nine atoms (10)

Molecule	Designation	Mass	Ions
CH3C4H	Methyldiacetylene	64	—
CH3OCH3	Dimethyl ether	46	—
CH3CH2CN	Propionitrile	55	—
CH3CONH2	Acetamide	59	—
CH3CH2OH	Ethanol	46	—
C8H	Octatetraynyl radical	97	C8H−
HC7N	Cyanohexatriyne or Cyanotriacetylene	99	—
CH3CHCH2	Propylene (propene)	42	—
CH3CH2SH	Ethyl mercaptan	62	—
CH3NHCHO	N-methylformamide

Ten or more atoms (23)

Atoms	Molecule	Designation	Mass	Ions
10	(CH3)2CO	Acetone	58	—
10	(CH2OH)2	Ethylene glycol	62	—
10	CH3CH2CHO	Propanal	58	—
10	CH3OCH2OH	Methoxymethanol	62	—
10	CH3C5N	Methylcyanodiacetylene	89	—
10	CH3CHCH2O	Propylene oxide	58	—
11	NH2CH2CH2OH	Ethanolamine	61	—
11	HC8CN	Cyanotetraacetylene	123	—
11	C2H5OCHO	Ethyl formate	74	—
11	CH3COOCH3	Methyl acetate	74	—
11	CH3C6H	Methyltriacetylene	88	—
12	C6H6	Benzene	78	—
12	C3H7CN	n-Propyl cyanide	69	—
12	(CH3)2CHCN	iso-Propyl cyanide	69	—
13	CH3OCH2CH2OH	2-methoxyethanol	76	—
13	Template:Chem	Benzonitrile	104	—
13	HC10CN	Cyanopentaacetylene	147	—
17	C9H8	Indene	116	—
19	C10H7CN	1-cyanonaphthalene	153	—
19	C10H7CN	2-cyanonaphthalene	153	—
60	C60	Buckminsterfullerene (C60 fullerene)	720	Template:Chem
70	C70	C70 fullerene	840	—

Deuterated molecules (22)

These molecules all contain one or more deuterium atoms, a heavier isotope of hydrogen. Template:Sticky header

Atoms	Molecule	Designation
2	HD	Hydrogen deuteride
3	H2D+, Template:Chem	Trihydrogen cation
3	HDO, D2O	Heavy water
3	DCN	Hydrogen cyanide
3	DCO	Formyl radical
3	DNC	Hydrogen isocyanide
3	N2D+	—
3	NHD, ND2	Amidogen
4	NH2D, NHD2, ND3	Ammonia
4	HDCO, D2CO	Formaldehyde
4	DNCO	Isocyanic acid
5	NH3D+	Ammonium ion
6	Template:Chem; NHDCHO	Formamide
7	CH2DCCH, CH3CCD	Methylacetylene

Unconfirmed (15)

Evidence for the existence of the following molecules has been reported in the scientific literature, but the detections either are described as tentative by the authors, or have been challenged by other researchers. They await independent confirmation.

Atoms	Molecule	Designation
2	SiH	Silylidine
4	PH3	Phosphine
4	MgCCH	Magnesium monoacetylide
4	NCCP	Cyanophosphaethyne
5	H2NCO+	—
6	SiH3CN	Silyl cyanide
10	H2NCH2COOH	Glycine
10	C2H5NH2	Ethylamine
12	CO(CH2OH)2	Dihydroxyacetone
12	C2H5OCH3	Ethyl methyl ether
18	Template:Chem	Naphthalene cation
24	C24	Graphene
24	C14H10	Anthracene
26	C16H10	Pyrene
27	C11H12N2O2	Tryptophan

See also

• Astrochemistry
• Cosmic dust
• Diffuse interstellar band
• Lists of molecules
• Molecular astrophysics
• Molecular spectroscopy
• Molecules in stars
• Polycyclic aromatic hydrocarbon (PAH)
• Tholin

External links

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Template:Molecules detected in outer space