Lucky planets: how circum-binary planets survive the supernova in one of the inner-binary components

Abstract

A planet hardly ever survives the supernova of the host star in a bound orbit, because mass loss in the supernova and the natal kick imparted to the newly formed compact object cause the planet to be ejected. A planet in orbit around a binary has a considerably higher probability to survive the supernova explosion of one of the inner binary stars. In those cases, the planet most likely remains bound to the companion of the exploding star, whereas the compact object is ejected. We estimate this to happen to $\sim 1/33$ the circum-binary planetary systems. These planetary orbits tend to be highly eccentric ($e \apgt 0.9$), and $\sim 20$\,\% of these planets have retrograde orbits compared to their former binary. The probability that the planet as well as the binary (now with a compact object) remains bound is about ten times smaller ($\sim 3\cdot 10^{-3}$). We then expect the Milky way Galaxy to host $\aplt 10$ x-ray binaries that are still orbited by a planet, and $\aplt 150$ planets that survived in orbit around the compact object's companion. These numbers should be convolved with the fraction of massive binaries that is orbited by a planet.