List of interstellar and circumstellar molecules

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

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 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 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, 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 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, 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 in 1937, through its strong electronic transition at 4300 angstroms. 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, 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 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.
One of the richest sources for detecting interstellar molecules is Sagittarius 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. Many of the largest molecules were first detected in another molecular cloud, TMC-1. A rich source of circumstellar molecules is CW Leonis, 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 inefficient. Many regions also have very low temperatures, 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, 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 are those used in the scientific literature describing the detection; if none was given that field is left empty. Mass is listed in daltons. Deuterated molecules, which contain at least one deuterium 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.

Diatomic (45)

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	—
NaS	Sodium sulfide	55	—
MgS	Magnesium sulfide	56	—
SiS	Silicon monosulfide	60	—
TiO	Titanium(II) oxide	64	—

Triatomic (45)

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	66	—
FeCN	Iron cyanide	82	—
—	Protonated molecular hydrogen	3
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
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	—
CaC₂	Calcium dicarbide	64	—
TiO₂	Titanium dioxide	79.9	—

Four atoms (31)

Molecule	Designation	Mass	Ions
CH₃	Methyl radical	15
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	—
HNSO	Thionylimide	63	—
SiC₃	Silicon tricarbide	64	—
HMgNC	Hydromagnesium isocyanide	51.3	—
HNO₂	Nitrous acid	47	—

Five atoms (21)

Molecule	Designation	Mass	Ions
—	Ammonium ion	18
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⁺
HCCN	Cyanoformaldehyde	55	—
C₅	Linear C₅	60	—
HCS₂H	dithioformic acid	78	—
SiC₄	Silicon-carbide cluster	92	—
SiH₄	Silane	32	—

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	—

Seven atoms (16)

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	—
CH₃CHS	Thioacetaldehyde	60	—
CH₃NCO	Methyl isocyanate	57	—
HOCH₂CN	Glycolonitrile	57	—
HC₃HCN	1-cyano propargyl radical	64	—
CH₂C₃N	3-cyano propargyl radical	64	—

Eight atoms (14)

Molecule	Designation	Mass
H₃CC₂CN	Methylcyanoacetylene	65
HC₃H₂CN	Propargyl cyanide	65
H₂COHCHO	Glycolaldehyde	60
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
₂CO	Urea	60

Nine atoms (11)

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	—
CH₃SCH₃	Dimethyl sulfide	62	—
CH₃NHCHO	N-methylformamide	59	—

Ten or more atoms (24)

Atoms	Molecule	Designation	Mass	Ions
10	₂CO	Acetone	58	—
10	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	₂CHCN	iso-Propyl cyanide	69	—
13	CH₃OCH₂CH₂OH	2-methoxyethanol	76	—
13		Benzonitrile	104	—
13	HC₁₀CN	Cyanopentaacetylene	147	—
17	C₉H₈	Indene	116	—
19	C₁₀H₇CN	1-cyanonaphthalene	153	—
19	C₁₀H₇CN	2-cyanonaphthalene	153	—
21	C₁₂H₇CN	1-cyanoacenaphtylene	177	—
21	C₁₂H₇CN	5-cyanoacenaphtylene	177	—
27	C₁₆H₉CN	1-cyanopyrene	227	—
27	C₁₆H₉CN	2-cyanopyrene	227	—
27	C₁₆H₉CN	4-cyanopyrene	227	—
37	C₂₄H₁₁CN	cyanocoronene	325	—
60	C₆₀	Buckminsterfullerene (C₆₀ fullerene)	720
70	C₇₀	C₇₀ fullerene	840	—

Deuterated molecules (22)

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

Atoms	Molecule	Designation
2	HD	Hydrogen deuteride
3	H₂D⁺,	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	; NHDCHO	Formamide
7	CH₂DCCH, CH₃CCD	Methylacetylene

Unconfirmed (16)

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
2	CaO	Calcium oxide
4	PH₃	Phosphine
4	MgCCH	Magnesium monoacetylide
4	NCCP	Cyanophosphaethyne
5	H₂NCO⁺	—
6	SiH₃CN	Silyl cyanide
10	H₂NCH₂COOH	Glycine
10	C₂H₅NH₂	Ethylamine
12	CO₂	Dihydroxyacetone
12	C₂H₅OCH₃	Ethyl methyl ether
18		Naphthalene cation
24	C₂₄	Graphene
24	C₁₄H₁₀	Anthracene
26	C₁₆H₁₀	Pyrene
27	C₁₁H₁₂N₂O₂	Tryptophan