Comet from another star system is on its way in, and astronomers say to expect more

For the second time in just under two years, astronomers have confirmed that we are being buzzed by an interstellar visitor, and some experts estimate that we'll be seeing more of them. 

In late August, amateur astronomer Gennady Borisov discovered a comet using a homemade 0.65-metre telescope. Early observations by professional astronomers suggested the orbit of the object took it outside our solar system, though further observations were needed to confirm.

Last week, the International Astronomical Union's (IAU) Minor Planet Center (MPC) was able to calculate a preliminary orbit, which verified that the comet had a hyperbolic orbit — meaning that it is orbiting in a way that prevents it from being captured by the sun's gravity. 

The comet was officially named 2I/Borisov by the MPC on Sept. 24. Comets are usually named after their discoverer. The 2I in the name designates it as the second interstellar object detected.

It will make its closest approach to the sun — called perihelion —on Dec. 7. It will be roughly 300 million kilometres from both the sun and Earth. It's even anticipated that amateur astronomers will be able to see it through large telescopes.

Astronomers are excited by the new find as it comes hot on the heels of the first discovery of an interstellar object: 1I/'Oumuamua, discovered in October 2017. But they believe these are the first of many.

"I think what this is telling us is that probably these are more common than we thought," said Peter Brown, professor of physics and astronomy at Western University in London, Ont.

It's not that our solar system has suddenly become a shooting gallery for interstellar objects: it's just likely that we're getting better at finding them.

Brown believes that one of the reasons for this is the increase in sky surveys, which use telescopes that constantly scan the sky specifically searching for asteroids and comets. Pan-STARRS is one such survey, and detected 1I/'Oumuamua, which was then confirmed by Canadian Robert Weryk, who is working out of the University of Hawaii's Institute for Astronomy.

And there are more sky surveys to come.

"I think when the [Large Synoptic Survey Telescope] comes online in a few years, this will be once a year we'll be finding something," Brown said.

That device, or LSST, is an 8.4-metre telescope in Chile scheduled to be fully operational in 2023. Among its objectives is to find hazardous asteroids.

But the thing is, Comet 2I/Borisov wasn't found by a sky survey at all, but rather by an amateur astronomer using a fairly large telescope.

Amateur contributions

"Amateurs do great work," said Norm Murray, a professor at the University of Toronto's department of astronomy and astrophysics and at the Canadian Institute for Theoretical Astrophysics. "And a lot of them are pretty technically sophisticated. And it's true that up until fairly recently most comets were found by amateurs. So that part is not surprising."

But what is surprising is that they're discovering something of this magnitude, something that Brown also appreciates.

"I think it just shows there's still a niche for amateurs; it's just fantastic," Brown said. "I think we're going to start seeing a lot of things we haven't seen before."

Differences in the interstellar objects

The interesting thing about the two interstellar objects is just how different they are. 

'Oumuamua, as the body is known in astronomers' shorthand, shows characteristics of both an asteroid and a comet. For one, orbital data suggested that it was outgassing, a process where ice turns from a solid to a gas, skipping the water stage. This is what produces the tails we most associate with comets. 

However, the issue was, it was invisible. The only reason astronomers could tell that it was outgassing was that something was nudging it slightly along its trajectory, which they concluded was due to escaping gases. But optically, it still remained as a point in the sky, missing the characteristic comet tail.

And 'Oumuamua was small, roughly 100 metres, and shaped more like a cigar. 

On the other hand, Borisov has already shown signs of a cometary tail, and it's estimated to be a few kilometres in diameter with a more traditional round shape.

These findings suggest that the two were formed differently. 

"One came from a system like our sun with comets around it, and the other one came from somewhere else," Murray said. 

What's most exciting for astronomers is that, while 'Oumuamua was discovered as it was on its way out of the solar system, Borisov was discovered on its way in, which means, instead of just a few weeks of observations, they will be able to study it far more in depth than they were able to with 'Oumuamua.

Unfortunately, it's likely that we won't know from which star system Borisov originated. Its size suggests that it's quite old, which makes it more difficult to trace back in time, over perhaps tens of millions of years.

Murray said that what is exciting is that astronomers have been looking for these objects for some time, but had theorized that, using sky surveys like the upcoming LSST, we could only expect to find one every hundred years. Instead, here we have two: one discovered by a sky survey, the other by an amateur astronomer.

So what does that tell us about our expectations?

"Expectations are dominated by things we already know," said Murray. "We have no imagination. That's what it tells you."