Interstellar comets, generally believed to be very rare visitors to our Solar System, might be very much more common than normally assumed, two US astrophysicists have suggested. The scientists concerned are Amir Siraj and Avi Loeb of the Centre for Astrophysics/Harvard & Smithsonian.

The question of interstellar comets was first seriously raised when the first such body, dubbed the Borisov comet, was detected in 2019. It travelled through the Solar System at a speed of some 177 000 km/h, and remains the only such object so far discovered.

“Before the detection of the first interstellar comet, we had no idea how many interstellar objects there were in our Solar System, but theory on the formation of planetary systems suggests that there should be fewer visitors than permanent residents,” pointed out Siraj. “Now we’re finding that there could be substantially more visitors.”

The two researchers reached their conclusion from calculations they made, derived from observations of the Borisov comet. Published on Monday in the Monthly Notices of the Royal Astronomical Society, these calculations, although including “significant uncertainties” (in the words of the Centre’s press release), indicate that interstellar objects outnumber local ones in the Oort Cloud. (The Oort Cloud is a hollow spherical ‘shell’ of icy debris, known as planetesimals, very far out in and around the Solar System.)

Such interstellar objects have not previously been detected because they are so small and so far away, and the technology does not yet exist to see them. But next-generation instruments, such as the Vera C Rubin Observatory (currently being built in Chile) may be able to detect them, allowing the calculations of Siraj and Loeb to be tested.

“Our findings show that interstellar objects can place interesting constraints on planetary system formation processes, since their implied abundance requires a significant mass of material to be ejected in the form of planetesimals,” noted Siraj. “Together with observational studies of protoplanetary discs and computational approaches to planet formation, the study of interstellar objects could help us unlock the secrets of how our planetary system – and others – formed.”