Interactions among the small bodies of our Solar System are expected to hurl small objects out into interstellar space with some regularity, and the frequency was probably much higher early in the system’s history. Given that the same thing almost certainly happens at exosolar systems—and we now know there are a lot of those—it’s likely that the vast volume of interstellar space is lightly sprinkled with small objects, some of which may sporadically pass through our own Solar System. But up until very recently, we’d had no evidence of their existence.
That situation changed with the discovery of ‘Oumuamua, a strange, cigar-shaped body that was the first confirmed exosolar visitor. But ‘Oumuamua was so strange that it set some astronomers speculating that it could be an alien craft. Earlier this year, however, scientists spotted a second potential exosolar visitor, and this one looked a lot like a comet. Now, the first data on the object, 2I/Borisov, is in, and it’s clearly exosolar in origin but looks so much like our existing comets that we might not have realized where it was from if we didn’t have a good grip on its orbit.