First interstellar comet discovered

An amateur astronomer has discovered the first interstellar comet by focusing his telescope on a part of the sky that has so far been neglected by larger sky survey projects.

In August 2019, Gennadiy Borisov, an engineer, was surveying for comets along the very edge of the pre-dawn sky, using a self-built telescope, when he spotted a tiny blurry object in the edge of one frame. Over the next few nights, he was able to determine that it was moving along an unusual trajectory compared to other objects in the Solar system.

The discovery was later confirmed by other observatories and on 24 September, the International Astronomical Union designated the object as interstellar comet 2I/Borisov. It is only the second interstellar object to be discovered, the first being the asteroid 1I/Oumuamua, identified in October 2017.

2I/Borisov was unusual in that its orbital trajectory was highly hyperbolic, which means that having flown through the Solar System, it will continue on through the interstellar expanse. Only a body moving at an extremely high speed — 2I/Borisov’s speed of 30km/s on approach to the Sun — can have an orbit of this shape. “That’s why it was identified absolutely as an interstellar object, because no domestic objects could have such a high speed,” says Boris Shustov from the Institute of Astronomy at the Russian Academy of Sciences, who co-authored a Solar System Research paper with Borisov about the comet’s discovery.

Analysis of the comet’s light spectrum revealed much higher levels of frozen carbon monoxide relative to water than would be expected. Shustov, says the relative abundance of carbon monoxide was orders of magnitude higher in 2I/Borisov than is normally seen in comets in the Solar system.

Carbon monoxide turns from ice to gas at extremely low temperatures, -250 °C, so “it means that this comet formed and lived all the time in the area where the radiation of the host star was so diluted that the ice could survive,” says Shustov. “This indicates that the comet came to us after being ejected from the distant outskirts of some other planetary system,” he adds.

One of the reasons Borisov found the comet was that many large telescopes involved in sky surveys are not trained on the horizon when the sun rises. “Since the angle between the object and the Sun is small, observations have to be carried out in a bright sky. Borisov discovered the comet in this difficult area of the sky to observe. After this discovery many observatories are now revising their strategies to enable them to survey these areas as well,” Shustov says.