Why don't more Tatooine-like exoplanets exist in our Milky Way galaxy? Astronomers might have an answer

Astronomers have uncovered a potential explanation for the scarcity of planets orbiting binary star systems, often likened to the fictional "Star Wars" planet Tatooine. Despite the Milky Way containing thousands of binary star systems, only 14 confirmed exoplanets have been found orbiting two suns, far fewer than the roughly 10% occurrence rate observed around single stars. Researchers from the University of California, Berkeley, and the American University of Beirut attribute this rarity to the effects of general relativity on the orbital dynamics within these systems. In binary star systems, the two stars orbit each other in elliptical paths, exerting complex gravitational forces on any orbiting planets. This interaction causes the planet’s orbital orientation to slowly rotate, a phenomenon known as precession. Simultaneously, the binary stars’ orbits also precess due to relativistic effects described by Einstein’s theory of general relativity. Over time, tidal forces between the stars can draw them closer, accelerating their orbital precession while slowing the planet’s precession. When the precession rates of the stars and the planet synchronize, a resonance occurs that stretches the planet’s orbit significantly. This resonance can destabilize the planet’s orbit, leading to two possible outcomes: the planet either moves too close to the stars and is destroyed by tidal forces or its orbit becomes so perturbed that it is ejected from the system entirely. The research, led by Mohammad Farhat, highlights how these relativistic and tidal interactions create a hostile environment for stable planetary orbits around binary stars, explaining the observed scarcity of Tatooine-like exoplanets. The findings provide crucial insights into planetary formation and survival in complex stellar environments, refining our understanding of where habitable worlds might exist. This work also underscores the importance of incorporating general relativity into models of exoplanet dynamics, especially in multi-star systems, and may guide future searches for planets in binary star systems by identifying regions where stable orbits are more likely.