When worlds collide in a binary star system. Credit: NASA/JPL-Caltech.

About half the points of light you can see in the night sky are binary star systems: two stars (sometimes more) orbiting each other. Sometimes, these stars are far apart, and planets can orbit just one of them with no problem, but are there any planets that orbit around both stars in a binary system?

Well, PSR 1620-26 b does, but pulsar planets are weird and don’t really count the same way other planets do. For a while, astronomers thought we might not find very many of these “circumbinary planets” that orbit around both stars. That’s because they looked at a certain type of binary stars called RS Canum Venaticorum variables and found that some of them have big, thick disks of dust around them, much bigger than our own asteroid belt. It’s believed that this happens because the gravity of the two stars makes the orbits of their planets unstable, causing them to collide. These stars are called “destroyers of worlds“.

But a year later, Kepler-16b was discovered. This is a Saturn-sized and decidedly not destroyed circumbinary planet orbiting both stars of the Kepler-16 system. We’ve discovered at least 16 of these circumbinary planets, now. This fits well with the theoretical estimate that a planet’s orbit will be stable if it is at least five times larger than the distance between the stars (or five times smaller if it orbits just one star). It looks like half the stars in the sky being binaries won’t cut down on the planets much at all.