Hubble Gets Its Closest Look Yet at Approaching Interstellar Comet

Astronomers have released new photos of comet 2I/Borisov, the second known interstellar visitor to our solar system after ’Oumuamua passed through in 2017. Discovered by amateur astronomer Gennady Borisov, the comet made its closest approach to the Sun on December 8, 2019.

NASA has released a pair of photos taken when the comet was approaching the Sun. The first image shows a distant background galaxy and is from November 16, while the second was taken on December 9 and shows Borisov as it rounded the sun. Hubble’s images do not resolve the nucleus of the comet, but they set an upper bound on its absolute size — about 1km in diameter.

Unlike ‘Oumuamua, which looked radically different than expected interstellar debris in terms of its shape and lack of activity, Borisov has been a much more conventional visitor. Put simply, Borisov looks like the sort of comet we see in our own solar system. That’s not a bad thing — one of the ways we test our predictions of how solar systems form is by examining the rest of the Universe to see if what we find out there looks at all like what we’ve got around here.

The mediocrity principle is an important underlying philosophical notion that guides a great deal of scientific exploration. The idea, at its most basic, is that the rest of the universe is built from the same collections of atoms and molecules that we are, and the rules that govern the assembly of these ingredients operate more or less the same way, with some allowance for unusual local conditions or rare events.

The trick to applying this principle in astronomy, of course, is that we don’t always know all the ways in which the Earth might legitimately be an unusual or unique system. Two decades ago, the existence of millions of planets in the Milky Way was thought to be likely but still theoretical based on the number of planets we’d detected outside our own solar system. Today, we know for a fact that many stars have planets, but the solar systems we’ve found often don’t look quite the way we expected they would — hot Jupiters (Jupiter-class planets orbiting very near their host stars) appear to be quite common in the galaxy, even though there’s no such planet in our own solar system.

The Moon is another potentially unique feature — no other planet in our solar system has a moon like Earth’s, and scientists estimate that they’re rare even on galactic terms — but whether this fundamentally changed our planet’s ability to support life is unknown. We’re still hashing out exactly which factors are likely to matter and which aren’t.

In this context, the fact that 2I/Borisov came back as a bog-standard comet is actually a good thing. Our theories for how planets and comets form rely on the interaction between gas and dust clouds as a protoplanetary disk of material collapses and a star ignites. 2I/Borisov resembles the Oort Cloud comets that form in the farthest frozen reaches of the solar system, with detected rates of cyanide outgassing, the presence of diatomic carbon, and even atomic oxygen at rates similar to those found in comets from the outer reaches of our solar system.

2I/Borisov is not tied to the sun and will not return to our solar system in the future. If you want to dive down a Wikipedia rabbit hole, check the page on so-called “Great Comets” — exceptionally bright comets, visible to the naked eye, that have visited our planet throughout history. If you’re familiar with the trope of a comet being treated as a harbinger of doom or a portent of great events, these are the comets that created those tropes in the first place.

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