Einstein's General Relativity Is Destroying Planets Around Binary Stars, Scientists Reveal

The Tatooine Problem: Where Are All the Two-Sun Planets?

For decades, astronomers have wrestled with a baffling cosmic gap. Planets should form readily around stars — including the many stars that exist in pairs — yet worlds orbiting two stars simultaneously are extraordinarily scarce. Of more than 6,000 confirmed exoplanets catalogued to date, a mere 14 have been verified orbiting both stars in a binary system. Scientists expected hundreds. New research now points to a surprising culprit: Albert Einstein's general theory of relativity.

The study, conducted by researchers at the University of California, Berkeley, and the American University of Beirut, was published in The Astrophysical Journal Letters. Its findings reframe how we understand planetary survival in complex star systems — and explain why so-called circumbinary planets (those orbiting both stars in a pair) are vanishingly rare.

What Happens Inside a Binary Star System

In a typical binary system, two stars of slightly different masses orbit one another along elongated, elliptical paths. Any planet circling both stars must navigate the competing gravitational pulls of each, causing its orbital path to slowly rotate over time — a phenomenon known as precession, similar to a spinning top gradually wobbling off-axis.

The two stars themselves also precess, but for a distinct reason rooted in Einstein's general relativity. Over time, tidal forces between the stars pull them gradually closer together. As their shared orbit shrinks, the stars' precession rate accelerates — while the planet's precession rate simultaneously slows.

At a certain point, these two precession rates align, creating what physicists call a resonance. The consequences for any nearby planet are severe.

Resonance: A Death Sentence for Planets

Once resonance takes hold, the planet's orbit becomes progressively more distorted and unstable. It swings to extreme distances on one end of its path and plunges dangerously close to the binary stars on the other.

"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," explained Mohammad Farhat, a Miller Postdoctoral Fellow at UC Berkeley and lead author of the study. "In both cases, you get rid of the planet."

The team's mathematical models and computer simulations indicate that roughly eight out of every ten planets around tight binary star systems would be destabilized through this mechanism — with most ultimately being destroyed.

A Planetary Desert Around Close Binary Pairs

NASA's Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS) detect planets by recording tiny dips in a star's brightness as a planet passes in front of it. Kepler alone identified around 3,000 eclipsing binary systems. Given that approximately 10% of Sun-like stars host large planets, astronomers anticipated finding a comparable proportion among binary stars — roughly 300 circumbinary systems. Instead, only 47 candidates have been identified, with just 14 confirmed.

Strikingly, not a single confirmed circumbinary planet orbits a binary pair whose stars complete a mutual orbit in fewer than 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," said Farhat. "The overwhelming majority of eclipsing binaries are tight binaries and are precisely the systems around which we most expect to find transiting circumbinary planets."

The Instability Zone

Binary star systems contain what researchers call an instability zone — a region surrounding the stellar pair where planetary orbits simply cannot be maintained. Within this boundary, the combined gravitational influence of two stars either ejects planets outward or drags them inward to their destruction.

Notably, 12 of the 14 known circumbinary planets sit just beyond this unstable region. This suggests they likely formed at greater distances and migrated inward over time — because forming a planet at the edge of the instability zone would be nearly impossible.

"Planets form from the bottom up, by sticking small-scale planetesimals together. But forming a planet at the edge of the instability zone would be like trying to stick snowflakes together in a hurricane," Farhat noted.

How Einstein's Theory Reshapes Entire Planetary Systems

First proposed by Albert Einstein in 1915, general relativity describes gravity not as an invisible force but as the curvature of spacetime caused by mass. One of the theory's earliest observational confirmations was the subtle shift in Mercury's orbital path — a drift that Newton's classical laws alone could not account for.

A parallel process unfolds in binary star systems. Stars that begin their lives relatively far apart are gradually drawn closer through interactions with surrounding gas clouds, a process that can span tens of millions of years. Tidal forces continue tightening their orbit over billions of years more. As this happens, relativistic effects on the stars' orbital precession grow increasingly significant.

Co-author Jihad Touma, a physics professor at the American University of Beirut, had long suspected general relativity played a role in shaping planetary fates around binary stars — but the magnitude of the effect was previously unclear.

"A planet caught in resonance finds its orbit deformed to higher and higher eccentricities, precessing faster and faster while staying in tune with the orbit of the binary, which is shrinking," Touma explained. "And on the route, it encounters that instability zone around binaries, where three-body effects kick into place and gravitationally clear out the zone."

Surviving Planets Are Simply Too Far to Detect

This does not mean binary stars are entirely without planets. Those that survive the resonance-driven purge tend to reside at much greater orbital distances — too far out to be detected by current transit-based observatories like Kepler and TESS, which are best suited to finding planets in relatively close orbits.

"There are surely planets out there. It's just that they are difficult to detect with current instruments," said Touma.

What Comes Next

The research team is now expanding their models beyond ordinary binary stars. Their next focus includes studying how relativistic effects influence star clusters surrounding pairs of supermassive black holes, as well as investigating whether similar mechanisms could help explain the scarcity of planets around binary pulsars — rapidly rotating neutron star pairs that emit precisely timed radio pulses.

The implications extend well beyond Tatooine-style fantasy. Understanding how general relativity shapes the architecture of planetary systems challenges long-held assumptions about where habitable worlds might exist — and where we should be looking for them.