Why two-sun planets keep disappearing scientists blame Einstein
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Why two-sun planets keep disappearing scientists blame Einstein
Einstein’s relativity may be quietly erasing most “two-sun” planets from the Universe.
Date:
April 19, 2026
Source:
University of California - Berkeley
Summary:
Astronomers have long been puzzled by a cosmic mystery: planets orbiting two stars—like Star Wars’ Tatooine—are surprisingly rare, even though they should be common. New research suggests the culprit is none other than Einstein’s theory of general relativity.
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Planets with two suns are mysteriously scarce, and scientists now think Einstein’s relativity is to blame. Credit: AI/ScienceDaily.com
Out of more than 4,500 known planet-hosting stars, one surprising pattern stands out. While planets are expected to form around most stars and many stars exist in pairs, worlds that orbit both stars are unusually uncommon.
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 lo