How many interstellar comets are out there?

For the second time in two years, astronomers have spotted an interstellar interloper heading into our Solar System.

The first, dubbed ‘Oumuamua, was spotted on 19 October 2017.

This one, named 2I/Borisov, was discovered on 30 August 2019 by amateur astronomer Gennady Borisov of Nauchnij, Crimea, using a home-built 65-centimeter telescope.

It was initially thought to be an ordinary comet, says Davide Farnocchia of NASA’s Center for Near-Earth Object Studies, Pasadena, California.

But a week of observations by amateur and professional astronomers revealed that it was on an orbit that must have originated outside the Solar System and is now in the process of slingshotting around the Sun before heading back into interstellar space. (The “2I” in its name means it is “interstellar object number 2.”)

Already, it is proving to be quite different from its predecessor. To begin with, it’s much larger. ‘Oumuamua (now officially called 1I/‘Oumuamua) was a cigar-shaped object, only 800 meters long. 2I/Borisov is probably several kilometers in diameter.

In part because of the difference in size, ‘Oumuamua wasn’t spotted until it was already well past its closest approach to the Sun, on its way back into interstellar space. 2I/Borisov won’t make its closed approach until 7 December, when it will be about 300 million kilometers from both the Sun and the Earth, meaning that astronomers have the next 10 weeks to study its inbound progress…with additional time to watch it recede.

But that’s just the beginning. 1I/‘Oumuamua appeared to have been the burned out core of a dead comet. 2I/Borisov is already beginning to develop a tail. 

At the moment, it’s not clear if that tail is gas or dust, “but if they detect gas, we’re going to learn about the system where it originated,” says Humberto Campins, a planetary scientist at Central Florida University, Orlando.

To date, he says, the only other interstellar messengers from which we have been able to learn that type of information are interstellar grains sometimes found in meteorites and interplanetary dust.

Among other things, Campins says, these have revealed that the nebula from which our solar system formed included dust particles that had been ejected by a number of other stars into the interstellar media. “Who knows what we’re going to learn from this guy?” he says.

Meanwhile, the discovery of two interstellar interlopers in such short succession raises the question of how many more are out there.

Two such discoveries in short succession might be a statistical fluke. But it’s also possible that a lot of other solar systems have outlying regions like our own system’s Kuiper Belt and Oort Cloud, from which comet-like bodies could be flung not only toward their own suns, but into interstellar space. 

“I think what this is telling us is that there must be lots of them,” Campins says, “and we are getting samples of them sent to our own Solar System. We get to study pieces of another stellar system that we would never be able to reach with our own probes – or at least not be able to reach for many years.”

Farnocchia agrees. “In the next few years we will have even better technology and we should discover even more [such objects],” he says.

Meanwhile, 2I/Borisov will be at its brightest in December and January, in the Southern Hemisphere sky. Will it be visible to backyard observers armed with binoculars? Who knows? But even if it’s just a dim blob observable only to small-telescope hobbyists, it should generate a good deal of excitement.