Molecules in Space

Note: Deuterium isotopic species are not given separately anymore.

The documentations generally provide information on the detection of the respective molecules, including minor isotopic species, molecules in excited vibrational states, or different media in which the species has been detected as well as links to articles.

All molecules have been detected (also) by rotational spectroscopy in the radiofrequency to far-infrared regions unless indicated otherwise.

* indicates molecules that have been detected by their rotation-vibration spectrum,
** those detected by electronic spectroscopy only.

Transition metal molecules detected in atmospheres of stars by electronic spectroscopy are currently not included. We plan to create a new table in the not so distant future dealing with molecules in stellar atmospheres. In this context, the table of molecules detected in the interstellar medium (ISM) or circumstellar envelopes (CSE) of late type stars, will be split. In the case of some metal monoxides, emission features have been detected in optical or IR spectra. Most likely, this emission indicates a presence of these molecules in the circumstellar shell. Around one handful of molecules may have to be added.

Note: Questionable or rebuted detections have been omitted from the table. Some noteworthy cases are given below the table. Tentative detections, which have a reasonable chance to be correct, are indicated by “?”. Usually the number of observed transitions that are, at least, fairly free of overlap is small. Some detections that have been reported as secure ones are indicated by “(?)” because (partial) overlap of lines cannot be ruled at the moment or because the line list is somewhat small; obviously, there may be cases on the edge.

As the decision whether a molecule can be considered as detected frequently is a controversial one we refrain from stating an exact number of molecules as detected. As of November 2024, 327 molecules are listed in the section of interstellar or circumstellar molecules. Only few these are tentative or somewhat uncertain. On the other hand some molecules are still missing (mainly metal compounds outside stellar atmospheres) in the table. Thus, we recommend phrases such as “more than 320 molecules have been detected in the interstellar medium or circumstellar shells”. Two reported detections have been questioned seriously. Therefore, these are viewed currently as “probably not yet detected” – see below. They do not show up in the table anymore.

The number of molecules detected in extragalactic sources is 74 including three tentative detections. More may have to be added. The list includes protonated acetylene, C₂H₃⁺, the first molecule detected in an extragalactic source, but not in a galactic source. The first species detected in an extragalactic but not galactic source is ³⁴SH⁺ also in this peculiar foreground galaxy.

We welcome comments on this list!

The year of the detection is given for recent results — the past two to three years.

Currently, three molecules have been reported as detected whose detections have been questioned fairly convincingly in subsequent papers. These molecules are aminoacetic acid, H₂NCH₂COOH, aka glycine, and 1,3-dihydroxypropanone, aka dihydroxyacetone. We consider these molecules as not yet detected.

The detection of SH by rovibrational spectroscopy of the S-type star R Andromedae has been omitted from this table as the molecule is present in the stellar atmosphere. It is hoped that a table of molecules in stellar atmospheres will be available in the not too distant future. Note, however, that the SH radical has recently been detected unambiguously in the ISM.