Relativity Could Explain Why Circumbinary Planets Are So Rare

Astronomers expected hundreds of planets orbiting binary stars, but only 14 have been found. New work in The Astrophysical Journal Letters suggests Einstein’s general relativity drives precession of the stars’ orbits that resonates with a planet’s orbit, destabilizing it. In tight binaries, this leads to a large instability zone and an “80% survival gap” in which planets are likely ejected or swallowed by the stars. As a result, there’s a desert of circumbinary planets for systems with orbital periods under about seven days, and most observed planets sit just outside the unstable region—likely formed farther out and migrated inward to the stability edge.