The solar system has another interstellar visitor, but there is no question of this being an alien spacecraft. A true comet and the first ever ever confirmed that comes from interstellar space, and the Hubble Space Telescope captured some amazing imagery of it. Good thing, too – because you never come back.
You probably remember 'Oumuamua as the interstellar object that launched a thousand headlines – mostly around the idea that it could be an alien ship of some kind. Needless to say, that hypothesis didn't really pan out, but honestly the object was interesting enough without being emissary from another world.
This new comet, called 2I / Borisov (not as catchy), was first identified in August by an amateur astronomer named Gennady Borisov, who lives in Crimea. Studies by other near-Earth object authorities observed its trajectory and concluded that it did indeed come from interstellar space.
How do they know? Well, for one thing, going 1
Basically the angle it's coming in, plus the speed at which it is traveling, means it cannot possibly be in even a super-wide orbit of the Sun. December just passing through – and early December will be less than 200 million miles from the Sun. It's not on track to hit anything, fortunately, that would be a truly cosmic coincidence, so in a couple of months it'll be gone again.
But its short visit is an ample opportunity to study its makeup, which appears to be very similar to our own “local” comets. Although it would be cool for 2I / Borisov to be super weird, its similarity is interesting in itself – it suggests that comet formation in other solar systems is not necessarily different.
It is, however, very different from 'Oumuamua, which appeared to be an inert, oblong rock. Interesting in its own way, but comets are dynamic: clouds of dust and ice surrounding a much smaller core. Very picturesque, even if the tails may not always point the way you think they should.
Note that these interstellar visitors are actually thought to be quite common, with perhaps thousands in the solar system at any given moment. But the few are large and bright enough to be detected and studied.
Hubble will continue observing 2I / Borisov through January and perhaps beyond. If you never go back, we want to collect as much data as possible while we can.