In a universe where over 4,500 stars are known to host planets, and many stars come in pairs, you'd think two-sun worlds like Star Wars' Tatooine would be everywhere. But out of more than 6,000 confirmed exoplanets - most discovered by NASA's Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS) - only 14 have been found orbiting binary stars. Based on expectations, astronomers thought there should be hundreds, so where did they all go?
Researchers at the University of California, Berkeley, and the American University of Beirut have an answer, and it points squarely at Einstein's general theory of relativity. In binary systems, two stars with slightly different masses orbit each other elliptically, and a planet around them experiences competing gravitational pulls. This causes the planet's orbit to precess, while the stars' precession is sped up by general relativistic effects as tidal forces pull them closer. When these precession rates sync up in resonance, the planet's orbit becomes unstable.
'Two things can happen: Either the planet gets very, very close to the binary, suffering tidal disruption or being engulfed by one of the stars, or its orbit gets significantly perturbed to be eventually ejected from the system,' said Mohammad Farhat, a Miller Postdoctoral Fellow at UC Berkeley and first author of the paper. 'In both cases, you get rid of the planet.'
This doesn't mean binary stars are planet-free; surviving planets tend to orbit farther out, making them hard to detect with current transit methods. 'There are surely planets out there. It's just that they are difficult to detect with current instruments,' added co-author Jihad Touma, a physics professor at the American University of Beirut.
The team reported their results in The Astrophysical Journal Letters, highlighting a planetary 'desert' around tight binary stars. Kepler and TESS detect planets by measuring dips in starlight during transits, and Kepler identified around 3,000 eclipsing binary systems. Given that about 10% of Sun-like stars host large planets, scientists expected roughly 300 systems with circumbinary planets. Instead, only 47 candidates have been found, with just 14 confirmed.
Notably, none of these confirmed planets orbit very close binary stars that complete an orbit in less than about seven days. 'You have a scarcity of circumbinary planets in general and you have an absolute desert around binaries with orbital periods of seven days or less,' Farhat said. Binary systems have an instability zone where planets can't remain stable, and interestingly, 12 of the 14 known circumbinary planets orbit just beyond this region, suggesting they formed farther out and migrated inward.