The Sun is single, a solo star traveling around the galaxy.
But… did it once orbit another star? In the distant past, could it have temporarily had a traveling companion, been part of a binary system?
That's not a silly idea. Half the stars in the galaxy belong to binary or multiple systems, so a star is just as likely to be in one as not. This idea of the Sun once being in a binary has been around a long time, but a new paper takes a look at the possibility of the Sun having a companion for a while shortly after it formed as a way to explain a couple of odd things about our solar system, including the presence of Planet Nine, a theorized ninth planet orbiting the Sun far out past Neptune.
Astronomers think Planet Nine (or just P9) exists due to an alignment of the orbits of several smaller bodies also very far out from the Sun. But it's weird; it would have to be much more massive than Earth, and it's not easy to form such a planet that far out (it would orbit the Sun on an elliptical path about 75 billion km ...