Researchers have pinpointed why planets orbiting two stars, like Tatooine in Star Wars, are rarer than expected. Einstein's general theory of relativity causes orbital instabilities that eject or destroy most such planets around tight binary stars. Only 14 confirmed circumbinary exoplanets exist among over 6,000 discovered.
Astronomers have long noted the scarcity of circumbinary planets, which orbit pairs of stars. Despite expectations of hundreds based on the frequency of binary stars and planet formation, NASA's Kepler and TESS telescopes have confirmed just 14 among more than 6,000 exoplanets. None orbit tight binaries with periods under seven days, creating a planetary 'desert' in those systems, said Mohammad Farhat, a Miller Postdoctoral Fellow at the University of California, Berkeley, and lead author of the study published in The Astrophysical Journal Letters. 12 of the known planets lie just beyond an instability zone where gravitational forces disrupt orbits. Farhat and co-author Jihad Touma, a physics professor at the American University of Beirut, used calculations and simulations to show how general relativity drives precession in binary stars' orbits as they shrink due to tidal forces. This creates a resonance with planets' slower precession, stretching their orbits until they enter the instability zone and get ejected or destroyed. '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 by the binary to be eventually ejected from the system,' Farhat said. Their models predict eight out of 10 planets around tight binaries are destabilized. Surviving planets orbit farther out, evading detection by transit methods. 'There are surely planets out there. It's just that they are difficult to detect with current instruments,' Touma said. The findings, based on materials from University of California, Berkeley, highlight relativity's role in shaping planetary systems.