We’ve been waiting for extraterrestrial visitors our whole lives – but in reality, they’re already here and have been with us for a long time. No, I don’t mean “Men in Black.” But it’s also not science fiction, it’s the truth. When astronomers discovered 2017 1I/ʻOumuamua, their surprise was enormous: we’d never seen an interstellar object inside our Solar System before. Or had we?
We had. For some time, astronomers have known about asteroids that don’t orbit the Sun in the same plane as the planets (the ecliptic), but instead on orbits that are at a greater or smaller angle to this plane. We know that the whole Solar System was made from a flat disk of dust and gas. Therefore, every object formed from that disk should also move within the plane of that flat disk. Unless, of course, some collision or gravitational interaction changed an object’s orbit at some point during the history of the Solar System.