New frontier for science as astronomers detect gas molecules in comet from another star

An international team of astronomers have made a historic discovery, detecting gas molecules in a comet which has tumbled into our solar system from another star.

It is the first time that astronomers have been able to detect this type of material in an interstellar object. The discovery marks an important step forward, as it will now allow scientists to begin deciphering exactly what these objects are made of and how our home solar system compares with others in our galaxy.

Comet Borisov was discovered by Crimean amateur astronomer Gennady Borisov in August. Observations over the following 12 days showed that it was not orbiting the Sun, but was just passing through the Solar system on its own path around our galaxy. By 24 September it had been renamed 2I/Borisov, the second interstellar object ever discovered by astronomers. Unlike the first such object discovered two years ago, 1I/'Oumuamua, this object appeared as a faint comet, with a surrounding atmosphere of dust particles, and a short tail.

A team of colleagues, including Dr Colin Snodgrass from the School’s Institute for Astronomy, used the William Herschel Telescope on La Palma in the Canary Islands to detect the gas in the comet.

Astronomers at the observatory pointed the giant telescope at the comet low down in the morning sky between 6am and 7am last Friday. Passing the faint comet light into a spectrograph enabled the astronomers to measure how much light the comet was emitting as a function of wavelength, or colour. The spectrum allows them to detect individual types of gas by their spectral fingerprints. The data was received at midday, and by 5pm that evening they knew they had successfully detected gas for the first time. The gas detected was cyanogen, made of a carbon atom and a nitrogen atom bonded together. It is a toxic gas if inhaled, but it is relatively common in comets.

Combining these spectra with filtered images of the comet obtained with the TRAPPIST-North telescope in Morocco, the team also measured the amount of dust being ejected by the comet, and placed limits on the size of the central nucleus. Preliminary analysis based on the amount of gas seen coming off the nucleus suggests that it is likely that much of the surface is active, in contrast to typical short period comets.

The team concluded that the most remarkable thing about the comet is that it appears ordinary in terms of the gas and dust it is emitting. It looks like it was born 4.6 billion years ago with the other comets in our Solar system, yet has come from an - as yet - unidentified star system.

As the comet approaches the Sun it will become brighter and more visible to astronomers. The team will follow 2I's evolution as it travels through our Solar System. In comparison, astronomers have a shorter period to study other comets, as was the case with 'Oumuamua, before it became too faint.

The team consists of Alan Fitzsimmons (Queen's University Belfast), Olivier Hainaut, Cyrielle Opitom, Youssef Moulane and Bin Yang (European Southern Observatory), Karen Meech, Jacqueline V. Keane and Jan T. Kleyna (University of Hawai'i), Emmanuel Jehin (Universite de Liege), Marco Micheli (European Space Agency), and Colin Snodgrass (University of Edinburgh).

The European Space Agency approved a space mission earlier this year that may visit a future interstellar visitor. Dr Colin Snodgrass in the team is also Deputy Principal Investigator on the ESA Comet Interceptor, due to be launched in 2028.

Colin Snodgrass commented:

This new discovery is very exciting for Comet Interceptor. Although we can't reach comet Borisov, it shows that interstellar comets can be discovered on their way into the solar system, which gives us more hope that we could visit one.