List of interstellar and circumstellar molecules

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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.

Detection

The molecules listed below were detected by spectroscopy. Their spectral features are generated by transitions of component electrons between different energy levels, or by rotational or vibrational spectra. Detection usually occurs in radio, microwave, or infrared portions of the spectrum.^[1]
Interstellar molecules are formed by chemical reactions within very sparse interstellar or circumstellar clouds of dust and gas. Usually this occurs when a molecule becomes ionized, often as the result of an interaction with a cosmic ray. This positively charged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower.^[2] The dust plays a critical role of shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars.^[3]

History

The first carbon-containing molecule detected in the interstellar medium was the methylidyne radical (CH) in 1937.^[4] From the early 1970s it was becoming evident that interstellar dust consisted of a large component of more complex organic molecules, probably polymers. Chandra Wickramasinghe proposed the existence of polymeric composition based on the molecule formaldehyde (H₂CO).^[5] Fred Hoyle and Chandra Wickramasinghe later proposed the identification of bicyclic aromatic compounds from an analysis of the ultraviolet extinction absorption at 2175A.,^[6] thus demonstrating the existence of polycyclic aromatic hydrocarbon molecules in space.

In 2004, scientists reported^[7] detecting the spectral signatures of anthracene and pyrene in the ultraviolet light emitted by the Red Rectangle nebula (no other such complex molecules had ever been found before in outer space). This discovery was considered a confirmation of a hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's core to get caught in stellar winds, and radiate outward.^[8] As they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms. The scientists inferred^[7] that since they discovered polycyclic aromatic hydrocarbons (PAHs) — which may have been vital in the formation of early life on Earth — in a nebula, by necessity they must originate in nebulae.^[8]

In 2010, fullerenes (or "buckyballs") were detected in nebulae.^[9] Fullerenes have been implicated in the origin of life; according to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth."^[10]

In October 2011, scientists found using spectroscopy that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.^[11][12][13] The compounds are so complex that their chemical structures resemble the makeup of coal and petroleum; such chemical complexity was previously thought to arise only from living organisms.^[11] These observations suggest that organic compounds introduced on Earth by interstellar dust particles could serve as basic ingredients for life due to their surface-catalytic activities.^[14][15] One of the scientists 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."^[11]

In August 2012, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellarbinary IRAS 16293-2422, which is located 400 light years from Earth.^[16][17] Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.^[18]

In September 2012, NASA scientists reported that PAHs, subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation, and hydroxylation, to more complex organics — "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively".^[19][20] Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."^[19][20]

In June 2013, PAHs were detected in the upper atmosphere of Titan, the largest moon of the planet Saturn.^[21]

In 2013, Dwayne Heard at the University of Leeds suggested^[22] that quantum mechanical tunneling could explain a reaction his group observed taking place, at a significantly higher than expected rate, between cold (around 63 Kelvin) hydroxyl and methanol molecules, apparently bypassing intramolecular energy barriers which would have to be overcome by thermal energy or ionization events for the same rate to exist at warmer temperatures. The proposed tunneling mechanism may help explain the common observation of fairly complex molecules (up to tens of atoms) in interstellar space.

A particularly large and rich region for detecting interstellar molecules is Sagittarius B2 (Sgr B2). This giant molecular cloud lies near the center of the Milky Way galaxy and is a frequent target for new searches. About half of the molecules listed below were first found near Sgr B2, and nearly every other molecule has since been detected in this feature.^[23] A rich source of investigation for circumstellar molecules is the relatively nearby star CW Leonis (IRC +10216), where about 50 compounds have been identified.^[24]

Molecules

The following tables list molecules that have been detected in the interstellar medium, grouped by the number of component atoms. If there is no entry in the Molecule column, only the ionized form has been detected. For molecules where no designation was given in the scientific literature, that field is left empty. Mass is given in Atomic mass units. The total number of unique species, including distinct ionization states, is listed in parentheses in each section header.

Most of the molecules detected so far are organic. Only one inorganic species has been observed in molecules which contain at least five atoms, SiH₄.^[25] Larger molecules have so far all had at least one carbon atom, with no N-N or O-O bonds.^[25]

Carbon monoxide is frequently used to trace the distribution of mass in molecular clouds.^[26]

Diatomic (43)

Molecule	Designation	Mass	Ions
AlCl	Aluminium monochloride[27][28]	62.5	—
AlF	Aluminium monofluoride[27][29]	46	—
AlO	Aluminium monoxide[30]	43	—
—	Argon hydride[31][32]	41	ArH+
C2	Diatomic carbon[33][34]	24	—
—	Fluoromethylidynium	31	CF+[35]
CH	Methylidyne radical[36]	13	CH+[37]
CN	Cyanogen radical[27][36][38][39]	26	CN+,[40] CN-[41]
CO	Carbon monoxide[27][42][43]	28	CO+[44]
CP	Carbon monophosphide[39]	43	—
CS	Carbon monosulfide[27]	44	—
FeO	Iron(II) oxide[45]	82	—
H2	Molecular hydrogen[46]	2	—
HCl	Hydrogen chloride[47]	36.5	HCl+[48]
HF	Hydrogen fluoride[49]	20	—
HO	Hydroxyl radical[27]	17	OH+[50]
KCl	Potassium chloride[27][28]	75.5	—
NH	Nitrogen monohydride[51][52]	15	—
N2	Molecular nitrogen[53][54]	28	—
NO	Nitric oxide[55]	30	NO+[40]
NS	Nitrogen sulfide[27]	46	—
NaCl	Sodium chloride[27][28]	58.5	—
—	Magnesium monohydride cation	25.3	MgH+[40]
NaI	Sodium iodide[56]	150	—
O2	Molecular oxygen[57]	32	—
PN	Phosphorus nitride[58]	45	—
PO	Phosphorus monoxide[59]	47	—
SH	Sulfur monohydride[60]	33	SH+[61]
SO	Sulfur monoxide[27]	48	SO+[37]
SiC	Carborundum[27][62]	40	—
SiN	Silicon mononitride[27]	42	—
SiO	Silicon monoxide[27]	44	—
SiS	Silicon monosulfide[27]	60	—
TiO	Titanium oxide[63]	63.9	—

The H₃⁺ cation is one of the most abundant ions in the universe. It was first detected in 1993.^[2][64]

Triatomic (41)

Molecule	Designation	Mass	Ions
AlNC	Aluminium isocyanide[27]	53	—
AlOH	Aluminium hydroxide[65]	44	—
C3	Tricarbon[34]	36	—
C2H	Ethynyl radical[27][38]	25	—
C2O	Dicarbon monoxide[66]	40	—
C2S	Thioxoethenylidene[67]	56	—
C2P	—[68]	55	—
CO2	Carbon dioxide[69]	44	—
FeCN	Iron cyanide[70]	82	—
—	Protonated molecular hydrogen	3	H3+[2][64]
H2C	Methylene radical[33]	14	—
—	Chloronium	37.5	H2Cl+[71]
H2O	Water[72]	18	H2O+[73]
HO2	Hydroperoxyl[74]	33	—
H2S	Hydrogen sulfide[27]	34	—
HCN	Hydrogen cyanide[27][38][75]	27	—
HNC	Hydrogen isocyanide[76]	27	—
HCO	Formyl radical[77]	29	HCO+[37][77][78]
HCP	Phosphaethyne[79]	44	—
—	Thioformyl	45	HCS+[37][78]
HNC	Hydrogen isocyanide[80]	27	—
—	Diazenylium	29	HN2+[78]
HNO	Nitroxyl[81]	31	—
—	Isoformyl	29	HOC+[38]
KCN	Potassium cyanide[27]	65	—
MgCN	Magnesium cyanide[27]	50	—
MgNC	Magnesium isocyanide[27]	50	—
NH2	Amino radical[82]	16	—
—	—	29	N2H+[37][83]
N2O	Nitrous oxide[84]	44	—
NaCN	Sodium cyanide[27]	49	—
NaOH	Sodium hydroxide[85]	40	—
OCS	Carbonyl sulfide[86]	60	—
O3	Ozone[87]	48	—
SO2	Sulfur dioxide[27][88]	64	—
c-SiC2	c-Silicon dicarbide[27][62]	52	—
SiCN	Silicon carbonitride[89]	54	—
SiNC	[90]	54	—
TiO2	Titanium dioxide[63]	79.9	—

Formaldehyde is an organic molecule that is widely distributed in the interstellar medium.^[91]

Four atoms (26)

Molecule	Designation	Mass	Ions
CH3	Methyl radical[92]	15	—
l-C3H	Propynylidyne[27][93]	37	l-C3H+[94]
c-C3H	Cyclopropynylidyne[95]	37	—
C3N	Cyanoethynyl[33]	50	C3N−[68]
C3O	Tricarbon monoxide[93]	52	—
C3S	Tricarbon sulfide[27][67]	68	—
—	Hydronium	19	H3O+[96]
C2H2	Acetylene[97]	26	—
H2CN	methylene amidogen[98]	28	H2CN+[37]
H2CO	Formaldehyde[99]	30	—
H2CS	Thioformaldehyde[100]	46	—
HCCN	—[101]	39	—
—	Protonated hydrogen cyanide	28	HCNH+[78]
—	Protonated carbon dioxide	45	HOCO+[102]
HCNO	Fulminic acid[103]	43	—
HOCN	Cyanic acid[104]	43	—
HOOH	Hydrogen peroxide[105]	34	—
HNCO	Isocyanic acid[88]	43	—
HNCS	Isothiocyanic acid[106]	59	—
NH3	Ammonia[27][107]	17	—
HSCN	Thiocyanic acid[108]	59	—
SiC3	Silicon tricarbide[27]	64	—
HMgNC	Hydromagnesium isocyanide[109]	51.3	—

Methane, the primary component of natural gas, has also been detected on comets and in the atmosphere of several planets in the Solar System.^[110]

Five atoms (19)

Molecule	Designation	Mass	Ions
C5	Linear C5[34]	60	—
—	Ammonium Ion[111][112]	18	NH4+
CH4	Methane[51][97]	16	—
CH3O	Methoxy radical[113]	31	—
c-C3H2	Cyclopropenylidene[38][114][115]	38	—
l-H2C3	Propadienylidene[115]	38	—
H2CCN	Cyanomethyl[citation needed]	40	—
H2C2O	Ketene[88]	42	—
H2CNH	Methylenimine[116]	29	—
HNCNH	Carbodiimide[117]	42	—
—	Protonated formaldehyde	31	H2COH+[118]
C4H	Butadiynyl[27]	49	C4H–[119]
HC3N	Cyanoacetylene[27][38][78][115][120]	51	—
HCC-NC	Isocyanoacetylene[121]	51	—
HCOOH	Formic acid[115]	46	—
NH2CN	Cyanamide[122]	42	—
HC(O)CN	Cyanoformaldehyde[123]	55	—
SiC4	Silicon-carbide cluster[62]	92	—
SiH4	Silane[124]	32	—

In the ISM, formamide (above) can combine with methylene to form acetamide.^[125]

Six atoms (15)

Molecule	Designation	Mass	Ions
c-H2C3O	Cyclopropenone[125]	54	—
E-HNCHCN	E-Cyanomethanimine[126]	54	—
C2H4	Ethylene[97]	28	—
CH3CN	Acetonitrile[88][127]	40	—
CH3NC	Methyl isocyanide[127]	40	—
CH3OH	Methanol[88]	32	—
CH3SH	Methanethiol[106]	48	—
l-H2C4	Diacetylene[27][128]	50	—
—	Protonated cyanoacetylene	52	HC3NH+[78]
HCONH2	Formamide[125]	44	—
C5H	Pentynylidyne[27][67]	61	—
C5N	Cyanobutadiynyl radical[129]	74	—
HC2CHO	Propynal[130]	54	—
HC4N	—[27]	63	—
CH2CNH	Ketenimine[114]	40	—

Acetaldehyde (above) and its isomers vinyl alcohol and ethylene oxide have all been detected in interstellar space.^[131]

Seven atoms (9)

Molecule	Designation	Mass	Ions
c-C2H4O	Ethylene oxide[132]	44	—
CH3C2H	Methylacetylene[38]	40	—
H3CNH2	Methylamine[133]	31	—
CH2CHCN	Acrylonitrile[88][127]	53	—
H2CHCOH	Vinyl alcohol[131]	44	—
C6H	Hexatriynyl radical[27][67]	73	C6H–[115][134]
HC4CN	Cyanodiacetylene[88][120][127]	75	—
CH3CHO	Acetaldehyde[27][132]	44	—

The radio signature of acetic acid, a compound found in vinegar, was confirmed in 1997.^[135]

Eight atoms (11)

Molecule	Designation	Mass
H3CC2CN	Methylcyanoacetylene[136]	65
H2COHCHO	Glycolaldehyde[137]	60
HCOOCH3	Methyl formate[88][115][137]	60
CH3COOH	Acetic acid[135]	60
H2C6	Hexapentaenylidene[27][128]	74
CH2CHCHO	Propenal[114]	56
CH2CCHCN	Cyanoallene[114][136]	65
CH3CHNH	Ethanimine[138]	43
C7H	Heptatrienyl radical[139]	85
NH2CH2CN	Aminoacetonitrile[140]	56
(NH2)2CO	Urea[141]	60

Nine atoms (10)

Molecule	Designation	Mass	Ions
CH3C4H	Methyldiacetylene[142]	64	—
CH3OCH3	Dimethyl Ether[143]	46	—
CH3CH2CN	Propionitrile[27][88][115][127]	55	—
CH3CONH2	Acetamide[114][125]	59	—
CH3CH2OH	Ethyl Alcohol[144]	46	—
C8H	Octatetraynyl radical[145]	97	C8H–[146]
HC7N	Cyanohexatriyne or Cyanotriacetylene[27][107][147][148]	99	—
CH3CHCH2	Propylene (propene)[149]	42	—
CH3CH2SH	Ethyl mercaptan[150]	62	—

A number of polyyne-derived chemicals are among the heaviest molecules found in the interstellar medium.

Ten or more atoms (15)

Atoms	Molecule	Designation	Mass	Ions
10	(CH3)2CO	Acetone[88][151]	58	—
10	(CH2OH)2	Ethylene glycol[152][153]	62	—
10	CH3CH2CHO	Propanal[114]	58	—
10	CH3C5N	Methyl-cyano-diacetylene[114]	89	—
10	(CH3)2CHCN	Isopropyl cyanide[154][155]	69	—
11	HC8CN	Cyanotetra-acetylene[27][147]	123	—
11	C2H5OCHO	Ethyl formate[156]	74	—
11	CH3COOCH3	Methyl acetate[157]	74	—
11	CH3C6H	Methyltriacetylene[114][142]	88	—
12	C6H6	Benzene[128]	78	—
12	C3H7CN	n-Propyl cyanide[156]	69	—
13	HC10CN	Cyanodecapentayne[147]	147	—
13	HC11N	Cyanopentaacetylene[147]	159	—
60	C60	Buckminsterfullerene (C60 fullerene)[158]	720	C60+[159][160]
70	C70	C70 fullerene[158]	840	—

Deuterated molecules (17)

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

Atoms	Molecule	Designation
2	HD	Hydrogen deuteride[161][162]
3	H2D+, HD2+	Trihydrogen cation[161][162]
3	HDO, D2O	Heavy water[163][164]
3	DCN	Hydrogen cyanide[165]
3	DCO	Formyl radical[165]
3	DNC	Hydrogen isocyanide[165]
3	N2D+	—[165]
4	NH2D, NHD2, ND3	Ammonia[162][166][167]
4	HDCO, D2CO	Formaldehyde[162][168]
5	NH3D+	Ammonium ion[169][170]
7	CH2DCCH, CH3CCD	Methylacetylene[171][172]

Unconfirmed (10)

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

Atoms	Molecule	Designation
2	SiH	Silylidine[76]
4	PH3	Phosphine[173]
5	H2NCO+	-[174]
10	H2NH2CCOOH	Glycine[38][175]
12	CO(CH2OH)2	Dihydroxyacetone[176]
12	C2H5OCH3	Ethyl methyl ether[177]
18	C10H8+	Naphthalene cation[178]
24	C24	Graphene[179]
24	C14H10	Anthracene[7][180]
26	C16H10	Pyrene[7]