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

Abstract

Since the discovery of exoplanets around pulsars, there has been a debate on their origin. Popular scenarios include in situ formation or the dynamical capture of a planet in a dense stellar system. The possibility of a planet surviving its host star's supernova is often neglected, because a planet in orbit around a single exploding star is not expected to survive the supernova. A circum-binary planet, however, may stand a chance in staying bound when one of the binary components explodes. We investigate the latter and constrain the distribution of post-supernova orbital parameters of circum-binary planets. This is done by performing population synthesis calculations of binary stars until the first supernova. Just before the supernova, we add a planet in orbit around the binary to study its survivability. In our supernova model, the exploding star's mass is assumed to change instantaneously, and we apply a velocity kick to the newly formed remnant. The mass loss and velocity kick affect the orbits of the two stars and the planet. Only 2 • 10 −3 of systems survive the supernova while keeping the circum-binary planet bound. The surviving planetary orbits are wide (a > ∼ 10 au) and eccentric (e > ∼ 0.3). It turns out much more likely (3 • 10 −2 system fraction) that the newly formed compact object is ejected from the system, leaving the planet bound to its companion star in a highly eccentric orbit (typically > ∼ 0.9). We expect that the Milky way Galaxy hosts at most 10 x-ray binaries that are still orbited by a planet, and < ∼ 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.