Newly discovered comet may be yet another interstellar visitor

The astronomical community is abuzz with the prospect that a recently discovered comet may actually have come from beyond our solar system.

This would make it only the second known interstellar interloper. The first — officially named 1I/'Oumuamua — was discovered in 2017.

But this visitor is very different.

"This one is actually nothing like 'Oumuamua," said Canadian astronomer Robert Weryk, who identified 'Oumuamua as the first interstellar object while working at the University of Hawaii's Institute for Astronomy.

"We were pretty sure right off the bat we had something pretty special. It was just the case that we'd never seen something like this before."

Astronomers had long believed that an interstellar visitor would be icy and comet-like, with gas and ice being released as it neared the sun. However, 'Oumuamua didn't do that. In fact, there was debate as to whether it was an asteroid — which has little to no water or ice — or a comet. In the end, scientists found evidence it was a comet.

"We were stunned, we were like, 'What did we get wrong?" said Olivier Hainaut, an astronomer with the European Southern Observatory.

But C/2019 Q4, on the other hand, is behaving just as astronomers originally believed an interstellar object would.

"It's a comet, there is no question," said Hainaut. "Assuming that it's interstellar … we now have a second interstellar object, and it does not look at all like the first one. Isn't that amazing? I don't know what would have been more exciting: to get a second one that looked exactly like 'Oumuamua or to have one that is completely different."

What we know

While 'Oumuamua was discovered by Weryk using the Pan-STARRS telescope in Hawaii, which is dedicated to searching for asteroids and comets, the discovery of C/2019 was made by astronomer Gennady Borisov on Aug. 30 in Nauchnij, Crimea.

The comet, which has been designated C/2019 Q4 (Borisov), has a hyperbolic orbit, meaning that it isn't captured by the gravity of the sun. This is why astronomers are fairly confident that C/2019 Q4 isn't from our solar system.

The International Astronomical Union's Minor Planet Center officially names solar system objects. The C in the name designates C/2019 Q4 as a comet, which means it's not yet confirmed that it's interstellar. More observations are needed in order to better refine its orbit. If it is confirmed it will be designated 2I plus a name (the "I" stands for interstellar).

Weryk is being cautious about calling it an interstellar object.

"We need to observe this one for a lot more and be cautious before making any significant claims, because it is very different," Weryk said. "And that is part of science: it being so different is what makes it so exciting."

But the good news is that astronomers have a lot of time.

In 2017, 'Oumuamua was found on its way out of our solar system, while this comet is on its way in. This means that astronomers will have about a year to further study the object.

So far, astronomers have determined that C/2019 Q4 is fairly large, much larger than 'Oumuamua was. It's also icy, which means it's fairly bright and will get brighter as it approaches the sun and more of the ice sublimates, or changes directly from a solid to a gas.

Right now C/2019 Q4 is roughly 420 million kilometres from the sun, travelling about 150,000 km/h. It will reach its closest point to our star — called perihelion — on Dec. 8, at a distance of roughly 300 million kilometres, taking it outside the orbit of Mars.

"We won't have a super-close approach, but we can observe it for months and months and months," said Hainaut.

"So that means that there are many things that we will be able to [learn] with 2I that we were not able to with 1I. Like what is its evolution? What is the non-gravitational force? Is it tumbling like 1I or does it have a stable rotation? What's its shape? Is it a funky, elongated shape like 1I?"

Hainaut said that he's ready to do followup observations with large, ground-based telescopes, and many astronomers will be vying for time on telescopes to do the same. 

In the meantime, Weryk and Hainaut said it will be interesting to see what unfolds as the comet gets closer to the sun. Will it get a lot brighter? Could it break apart?

"It's really one of these cases where we just need to wait," Hainaut said. "We might have surprises."