Due to complicated gravitational interactions from planets and other bodies, it’s expected that our Solar System has ejected various small bodies like comets and asteroids. Since exosolar systems are likely to do the same, it’s thought that the vast distances of interstellar space are sparsely populated by these small bodies. As such, we should expect one of these objects to wander through our Solar System, an expectation that was confirmed in 2017 with the arrival of ‘Oumuamua, an odd, cigar-shaped object that shot through the Solar System at an extreme angle.

Now, just two years later, we seem to have our second. Officially termed C/2019 Q4 (Borisov), the comet is approaching the inner Solar System at an angle that almost certainly indicates it didn’t originate here.

Hyperbolic orbits

Right now, there’s not much public information about C/2019 Q4 (Borisov). A press release from the Jet Propulsion Lab provides some basic details. Discovered on August 30, it takes its name from Gennady Borisov, who spotted it from an observatory in the Crimea. Since then, observations have firmed up its orbit, indicating that it will make its closest approach to the Sun in December, passing no closer than Mars’ orbit.

The release notes that indications of its non-solar origin include a high speed and its orbit’s very large angle relative to the plane of the Solar System that the planets orbit in.

To find out more about the object, we talked with University of Maryland astronomer YE Quanzhi, who studies small bodies in the Solar System. He said one of Borisov’s critical contributions to this discovery was reporting the find to the International Astronomical Union’s Minor Planet Center, which has portals for both asteroids and comets and a software system that can rapidly determine if an object is likely to be something we’ve already seen. “Astronomers anywhere in the world, if they’re interested, or they study this kind of stuff, they will check this page very often and see if there is anything that they need to follow up,” Quanzhi told Ars.

In this case, C/2019 Q4 (Borisov) attracted follow-up observations from the Canary Islands, China, Hawaii, and Italy, among other places. With about 100 observations, Quanzhi said that we now have a pretty good sense of C/2019 Q4 (Borisov)’s orbit. And that orbit is strange. “This first came to my attention in on Sunday morning,” Quanzhi said. “Somebody posted on a mailing list saying, ‘Hey, guys, this looks really weird because the orbit seems hyperbolic.'”

An eccentric visitor

Hyperbolic, he went on to explain, refers to the eccentricity of the orbit. If an orbit is perfectly circular, it would have an eccentricity of zero (the Earth’s eccentricity is just above that). Eccentricity goes up as you have orbits that are more off-center and stretched out along one axis and/or more elliptical. At an eccentricity of one, an orbit is parabolic, and the object will make a single pass through the Solar System and not return.

An eccentricity of one, however, is not enough to indicate extrasolar origins. If the eccentricity is slightly above one, “we tend to think that they’re still Solar System comets, but their orbit was slightly modified by the gravity of major planets.”

“But if we have something that’s much larger than one, that is when things start becoming interesting,” Quanzhi said. “‘Oumuamua, the first extrasolar object that we ever found, has an eccentricity of 1.2. And we considered that very high. And this is three. It is, you know, much more clear, much more crazy.”

He went on to say that we’re lucky to have spotted this on its approach to the Solar System. ‘Oumuamua was identified when it was already on its way out, and it quickly dimmed, limiting the observations we obtained before it faded from sight. C/2019 Q4 (Borisov), in contrast, will be visible for nearly a year. In addition, astronomers are probably better prepared for its appearance. “After ‘Oumuamua, there have had been a lot of discussions on how we can best respond to the next interstellar object because these objects are so rare,” Quanzhi told Ars. “They carry precious information from other solar systems that we otherwise don’t have the technology to get.”

Right now, however, we know very little. Quanzhi said that because it has a fuzzy appearance indicating a coma and is trailed by a small tail, everyone agrees this is a comet (there’s still debate about ‘Oumuamua’s identity). But if this were ejected from its system of origin without having passed close to a star, then it should retain many of the volatile materials that can only condense into solids in the colder areas far from stars. If that’s the case here, we may see a lot more activity as it warms up in our Solar System.

“This is the first interstellar comet that we ever found. We don’t have a clue what it’s going to do,” Quanzhi said. “Maybe it’ll just behave like a very quiet comet for one year; maybe it’ll just all burst out for a month. Who knows?” If we do see outbursts, then we’ll be able to sample both the coma and some sub-surface materials as they’re ejected into space, obtaining spectra that will allow us to study the chemical composition of the comet.

The second of many?

The fact that we’ve spotted a second extrasolar object just two years after the first raises the question of whether the galaxy is teeming with them and we just haven’t noticed. But Quanzhi sounded a note of caution here. “Sky surveys had been doing their jobs for the past… I would say, like, 20 to 30 years, and before 2017, nobody found anything,” he told Ars. “There are some astronomers doing very sophisticated modeling, trying to figure out how many interstellar objects we should be able to see and getting some weird numbers. It’s either from once in every 10,000 years to one object every one year. So you know, nobody knows the true rate.”

For now, scientists are just thinking about how to get the most out of their observation time with C/2019 Q4 (Borisov). For Quanzhi, the excitement of the opportunities has been given an extra boost from the fact that Gennady Borisov, despite having access to an observatory, is an amateur astronomer. “Before I start doing my PhD and the work I do now, I myself was an amateur astronomer and was fascinated by stars. And I’m pretty happy that this was found by an amateur.”

https://arstechnica.com/?p=1568385