Comets, asteroids, and interstellar objects: oh my. This week, astronomers have observed the second interstellar object on record to enter our solar system. This interstellar object is a comet called Borisov, Gizmodo reports, and astronomers are hoping to track its progress through our solar system to help learn about life outside our ring of planets.
Technically designated Comet C/2019 Q4, the comet has been named for Gennady Borisov, who first spotted the object in late August. Astronomers are now comparing Borisov (the comet, not the human) to the only other interstellar object on record, a cigar-shaped asteroid or tailless comet dubbed ‘Oumuamua. This first interstellar object sparked many speculations of potential alien origins because of its long, thin, ovular shape.
Lacking the tail that can be observed in association with most comets, 'Oumuamua is tailless because it lost about 10% of its mass on its way to our solar system, according to a 2019 paper published in the journal Astrophysics. This mass loss, combined with its chemical composition, indicates that 'Oumuamua probably flew here after being slung out of an planetesimal disk (in non-nerd English: the disk of dust, rocks, and other space debris surrounding a small, could-be planet). And now, after noticing Borisov speeding into our solar system, scientists are asking if it came from somewhere similar to 'Oumuamua, or somewhere entirely different.
So far, there are several detectable differences between Borisov and 'Oumuamua, the first of which is that Borisov has a tail that you typically expect to see with a comet. The tails of comets are not literal, of course: rather, cometary tails are actually temporary atmospheres, called coma, that are created when the sun heats the comet and sparks the release of gas and dust from the comet. And, because it is traveling through space, this gas and dust winds up looking like they're trailing behind the rock, like a tail.
And Borisov has got one of these. This makes a lot of sense, because astronomers expect interplanetary objects to be full of ice. And ice, upon being heated by the sun, makes for some pretty spectacular cometary tails. Because Borisov has such an icy tail, it makes sense that it comes from outside of our solar system. Why? Because the farther a big hunk of rock (AKA, a comet) is from the sun, or from any star, the colder it gets. The colder it gets, the more its gasses or liquids tend to solidify into ice. Sling the interstellar object toward our sun for a little reheating, and you've got yourself a cometary tail like Borisov's.
But that's not the only reason scientists can tell that Borisov isn't from around here. The shape of Borisov's orbit, too, indicates to astronomers that it's not local. It has what astronomers call a hyperbolic orbit, which in this case means that Borisov has been traveling fast enough to escape the solar system's gravitational pull. The gravitational pull is what keeps planets, including Earth, orbiting the sun. It's what gives Earth's oceans its tides and what literally creates night and day. And Borisov's hyperbolic orbit, combined with its speed, means that it must have come from outside this solar system.
So why does any of this matter? As scientists get the opportunity to study objects like Borisov more in depth, they can learn more about the formation of planets and planetary systems like ours. And this kind of research on planetary formation can yield important insights into everything from Earth's climate patterns to what life could be like in other solar systems. And really, how cool is that? Almost as cool as the ice making a fancy tail in Borisov's wake.