Why two-sun planets keep disappearing scientists blame Einstein

Among the more than 6,000 confirmed extrasolar planets, or exoplanets, discovered so far -- most by NASA's Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS) -- only 14 have been seen orbiting binary stars. Based on expectations, astronomers thought there should be hundreds. So where are the real-life versions of Star Wars' Tatooine?

Researchers at the University of California, Berkeley, and the American University of Beirut now suggest an answer, and it points to Einstein's general theory of relativity.

How Gravity Shapes Orbits in Binary Star Systems

In a typical binary system, two stars with slightly different masses circle each other along elongated, or elliptical, paths. A planet orbiting both stars experiences competing gravitational pulls, which cause its orbit to slowly rotate, or precess, much like a spinning top wobbling under gravity.

The stars themselves also undergo precession, but for a different reason. Their motion is influenced by general relativity. Over time, tidal forces between the two stars gradually pull them closer together. As their orbit shrinks, the stars' precession speeds up, while the planet's precession slows down.

Eventually, these two rates can line up in what scientists call a resonance. When that happens, the planet's orbit becomes stretched and unstable. It swings farther out at one point and dives much closer at another.

"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," 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 does not mean binary stars lack planets entirely. The ones that remain tend to orbit much farther away, making them difficult to detect with current transit methods used by Kepler and TESS.

"There are surely planets out there. It's just that they are difficult to detect with current instruments," said co-author Jihad Touma, a physics professor at the American University of Beirut.

The team reported their results in The Astrophysical Journal Letters.

A Planetary "Desert" Around Tight Binary Stars

Both Kepler and TESS detect planets by measuring small dips in starlight when a planet passes in front of its star. Kepler also identified around 3,000 eclipsing binary systems, where one star periodically passes in front of the other.

Since about 10% of Sun-like stars host large planets, scientists expected a similar fraction around binary stars, which would amount to roughly 300 systems. Instead, only 47 candidates have been found, and just 14 are confirmed as planets orbiting both stars.

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. "The overwhelming majority of eclipsing binaries are tight binaries and are precisely the systems around which we most expect to find transiting circumbinary planets."

Binary systems also contain what scientists call an instability zone, a region where planetary orbits cannot remain stable. In this zone, the combined gravitational effects of the two stars either fling planets out of the system or pull them inward until they are destroyed.

Interestingly, 12 of the 14 known circumbinary planets orbit just beyond this unstable region. This suggests they likely formed farther out and later migrated inward, since forming near the boundary would be extremely difficult.

"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," he said.

Einstein's Role in Clearing Out Planets

Touma had long suspected that general relativity could influence how planets behave in binary systems, though it was unclear how strong the effect might be. As binary stars slowly spiral closer over time, relativistic effects become more important.

Using detailed mathematical calculations and computer simulations, the researchers showed that these effects can dramatically reshape planetary systems. Their results indicate that about eight out of 10 planets around tight binary stars would be destabilized, and most of those would ultimately be destroyed.

The Physics Behind Orbital Precession

Proposed by Albert Einstein in 1915, general relativity describes gravity as the bending of spacetime by mass, similar to how a heavy object distorts a stretched surface. One of its earliest confirmations came from Mercury's orbit, which shifts slightly more than Newton's laws alone could explain.

A similar process occurs in binary stars. These systems often begin with stars far apart, but interactions with surrounding gas gradually draw them closer over tens of millions of years. Over billions of years, tidal forces continue to shrink their orbit.

As the stars move closer, their orbital motion changes more rapidly, including the position of their closest approach, known as periastron. Meanwhile, a planet orbiting both stars also experiences precession, though in this case it is driven by classical gravitational forces.

As the binary tightens, the planet's precession slows while the stars' precession speeds up. When the two rates match, resonance occurs, and the planet's orbit becomes increasingly stretched.

Once the closest point of the orbit enters the instability zone, the planet is either flung outward or pulled inward and destroyed. This process unfolds relatively quickly on cosmic timescales, which helps explain why planets around tight binaries are so rarely observed.

"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 said. "And on the route, it encounters that instability zone around binaries, where three-body effects kick into place and gravitationally clear out the zone."

"Just the natural way you form these tight binaries, these sub-seven-day binaries, you get rid of the planet naturally, without invoking additional disruption from a nearby star or other mechanisms," Farhat said.

A Broader Impact Across the Universe

According to Touma, these same processes may remove multiple planets from binary systems, especially those that would otherwise be detectable by missions like Kepler or TESS.

The team is now extending its models to explore how relativity affects star clusters around supermassive black hole pairs. They are also investigating whether similar mechanisms could help explain the lack of planets around binary pulsars, which are pairs of rapidly spinning neutron stars that emit regular radio pulses.

The findings highlight how Einstein's theory continues to shape our understanding of the universe, even in systems once thought to be well explained by classical physics.

"Interestingly enough, nearly a century following Einstein's calculations, computer simulations showed how relativistic effects may have saved Mercury from chaotic diffusion out of the solar system. Here we see related effects at work disrupting planetary systems," Touma said. "General relativity is stabilizing systems in some ways and disturbing them in other ways."

Farhat is supported by the Miller Institute for Basic Research in Science at UC Berkeley.