Why Planets Orbiting Two Stars Are So Rare: Einstein's Relativity Holds the Key
Astronomers have long been puzzled by the scarcity of planets orbiting binary star systems, reminiscent of the iconic Tatooine from Star Wars. Given that most stars form with planets and a significant fraction exist in pairs, one might expect these double-sun worlds to be abundant. However, they remain elusive, and new research from the University of Berkeley suggests that Einstein's general relativity is to blame, gradually pushing such planets into unstable orbits over millions of years.
The Gravitational Chaos of Binary Stars
Binary stars often orbit in close proximity, creating complex and dynamic gravitational fields. A planet orbiting both stars experiences constantly changing tugs, leading to a phenomenon known as orbital precession, where its path slowly rotates, akin to a wobbling spinning top. The stars themselves also undergo precession, with general relativity playing a critical role. Over time, tidal forces draw the stars closer together, accelerating their orbits while slowing the planet's motion.
Eventually, the rhythms of the stars and planet align, triggering chaos. When precession rates match, the planet's orbit becomes highly elliptical, swinging it far away at one point and dangerously close to the stars at another. This instability often leads to catastrophic outcomes.
The Fate of Planets in Binary Systems
"Either the planet gets too close, or it is eventually ejected," explains Mohammad Farhat, a postdoctoral researcher at UC Berkeley. "You end up losing the planet either way." Only planets situated at great distances from the binary stars manage to survive, which is why circumbinary planets are so rare. It is not that they do not exist; rather, they often evade detection by telescopes like Kepler or TESS because their distant orbits rarely cause transits in front of their host stars.
The Desert of Close Binary Stars
Observations from Kepler and TESS have yielded only 14 confirmed circumbinary planets, most orbiting stars that are not tightly bound. A notable desert exists for binaries with orbital periods of less than seven days, precisely where one would expect to find more planets. This gap is attributed to relativistic effects and orbital chaos, which clear out this region over billions of years.
The universe makes these double-sun worlds rare not due to formation challenges but because physics gently nudges them toward instability and often destruction. This process mirrors the relativistic precession observed in Mercury's orbit around the Sun, but for binary stars, the effects are magnified over cosmic timescales, warping planetary orbits until they are either flung into space or consumed.
This research highlights how fundamental laws of physics shape the architecture of planetary systems, offering insights into the dynamic and often violent nature of our cosmos.
