Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and therefore they inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles, organic material, and prebiotic chemicals around their host system (Rubin et al. 2019). In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions. However, knowledge of exocomets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars (Strom et al. 2020).
2I/Borisov is the first notably active interstellar comet discovered in our Solar System. We report on near-contemporaneous observations of 2I/Borisov with the Cosmic Origins Spectrograph onboard Hubble Space Telescope and the Ultraviolet Optical Telescope on the Neil Gehrels-Swift Observatory. Our observations indicate that the coma of 2I/Borisov contains substantially more CO than H2O gas, with abundance ratios of between 130–155% (Bodewits et al. 2020). This is much higher than previously measured for almost all comets in the inner (< 2.5 au) Solar System (CO/H2O typically between 0.2%–23%; Bockelee-Morvan & Biver 2017), with the exception of the strange comet C/2016 R2 (McKay et al. 2019).
CO ice is highly volatile and its abundance in a comet is very sensitive to both the local temperature of the region where the comet formed and its thermal history since then (Eistrup et al. 2019). As such, the observed high abundance of CO suggests that 2I/Borisov originated from a host system that was chemically distinct from our own, that it must have formed beyond a CO snowline of its host system, and that it must have formed in a system that experienced dynamical interactions sufficient to strip planetesimals beyond a CO snowline. These requirements could be met by an M-type star, the most common type of star in our galaxy.
Bockelée-Morvan, D. & Biver, N. Phil. Trans. R. Soc. A 375, 20160252-11 (2017).
Bodewits, D. et al. Nat. Astron. doi:10.1038/s41550-020-1095-2 (2020).
Eistrup, C., et al. A&AS 629, A84 (2019).
McKay, A., et al., AJ 158, 3, id. 128 (2019).
Rubin, M., et al. 2019. ACS Publications 3, 1792-1811 (2019).
Strom, P. et al. arXiv:2007.09155 (2020).