Free Astronomy Magazine July-August 2020

18 JULY-AUGUST 2020 SPACE CHRONICLES A LMA observed hydrogen cyanide gas (HCN, left) and carbon monoxide gas (CO, right) coming out of interstellar comet 2I/Borisov. The ALMA images show that the comet contains an unusually large amount of CO gas. ALMA is the first telescope to measure the gases originating directly from the nucleus of an object that travelled to us from another planetary system. [ALMA (ESO/ NAOJ/NRAO), M. Cordiner & S. Milam; NRAO/AUI/NSF, S. Dagnello] the solar system a comet was formed; some has to do with how often a comet’s orbit brings it closer to the Sun and leads it to release its more easily evaporated ices. “If the gases we observed reflect the composition of 2I/Borisov’s birth- place, then it shows that it may have formed in a different way than our own solar system comets, in an extremely cold, outer region of a distant planetary system,” added Cordiner. This region can be com- pared to the cold region of icy bod- ies beyond Neptune, called the Kuiper Belt. The team can only speculate about the kind of star that hosted 2I/Borisov’s planetary system. “Most of the protoplanetary disks observed with ALMA are around younger versions of low-mass stars like the Sun,” said Cordiner. “Many of these disks extend well beyond the region where our own comets are believed to have formed, and contain large amounts of extremely cold gas and dust. It is possible that 2I/Borisov came from one of these larger disks.” Due to its high speed when it trav- eled through our solar system (33 km/s or 21 miles/s) astronomers sus- pect that 2I/Borisov was kicked out from its host system, probably by in- teracting with a passing star or giant planet. It then spent millions or billions of years on a cold, lonely voyage through interstellar space before it was discovered on 30 Au- gust 2019 by amateur astronomer Gennady Borisov. 2I/Borisov is only the second inter- stellar object to be detected in our solar system. The first – 1I/’Oumua- mua – was discovered in October 2017, at which point it was already on its way out, making it difficult to reveal details about whether it was a comet, asteroid, or something else. The presence of an active gas and dust coma surrounding 2I/Borisov made it the first confirmed interstel- lar comet. Until other interstellar comets are observed, the unusual composition of 2I/Borisov cannot easily be ex- plained and raises more questions than it answers. Is its composition typical of interstellar comets? Will we see more interstellar comets in the coming years with peculiar chemical compositions? What will they reveal about how planets form in other star systems? “2I/Borisov gave us the first glimpse into the chemistry that shaped another planetary system,” said Milam. “But only when we can com- pare the object to other interstel- lar comets, will we learn whether 2I/Borisov is a special case, or if every interstellar object has unusu- ally high levels of CO.” !