Planets With Two Suns Are Almost Impossible To Find — General Relativity May Be Why

Planets with two suns have long captured the imagination of science fiction fans, thanks in large part to Tatooine, the iconic desert world of Star Wars. Given what astronomers know about star and planet formation, these worlds should be common. Most stars are born with planets, and many stars form in pairs. Yet when scientists look at the sky, planets orbiting two stars at once are surprisingly rare.

Of the more than 6,000 exoplanets discovered so far – largely thanks to NASA’s Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS) – only 14 are known to orbit binary star systems, which is far fewer than expected. Now astrophysicists write Letters from the astrophysical journal believe they know why: the vanished planets could be the victims of a subtle but powerful effect predicted by Albert Einstein more than a century ago.

“There is a shortage of circumbinary planets in general and there is an absolute desert around binary planets,” first author Mohammad Farhat said in a press release.

Why planets like Tatooine are so rare

Binary star systems are chaotic places. In most cases, the two stars have slightly different masses and follow elongated egg-shaped orbits around each other. Any planet that circles both stars – a configuration astronomers call a circumbinary orbit – must deal with competing gravitational forces that constantly throw it off balance.

There is also a danger zone around binary stars, where planetary orbits are fundamentally unstable. In this region, complex three-body interactions can either fling a planet into interstellar space or bring it close enough to be shredded or swallowed by one of the stars. Observations suggest that most known circumbinary planets lie just outside this zone, implying that they likely formed farther away and migrated inward.

“Planets form from the bottom up, by gluing small-scale planetesimals together. But forming a planet at the edge of the instability zone would be like trying to glue snowflakes together in a hurricane,” Farhat added.

Learn more: 5 of the strangest and most dangerous exoplanets ever discovered

Binary stars and planetary orbits

The problem doesn’t stop at instability alone. The planet’s orbit and that of the two stars rotate slowly over time in a process called precession, similar to the way a top wobbles when it slows down. The planet’s orbit precesses due to the gravitational attraction of stars, while the orbit of binary stars precesses largely because of relativistic effects.

As tidal interactions gradually shrink the orbits of stars, the rate of star precession speeds up, while that of the planet slows down. When these rates match, the system enters a resonance that expands the planet’s orbit until it forms an extreme oval.

This resonance usually results in one of two outcomes, as Farhat explains: “Either the planet comes very, very close to the binary, experiencing a tidal disruption or being swallowed up by one of the stars, or its orbit is significantly disrupted by the binary and ultimately gets ejected from the system. In either case, you get rid of the planet.”

How general relativity tips the scales

The key idea of ​​this new research is the role played by Einstein’s theory of general relativity. Proposed in 1915, the theory describes gravity not as a force, but as a curvature of space-time itself, often compared to the way a heavy object deforms a trampoline.

In tight binary systems, this relativistic deformation accelerates orbital precession sufficiently to trigger destructive resonances. Computer models suggest that the effects of general relativism eliminate about eight out of ten circumbinary planets around nearby binaries, with most of these planets ultimately destroyed.

The result is a universe in which Tatooine-like worlds can exist, but mostly far from their suns and largely beyond the reach of current telescopes.

Learn more: Astronomers discover a third planet in this strange “Tatooine” star system

Article sources

Our Discovermagazine.com editors use peer-reviewed research and high-quality sources for our articles, and our editors review the articles for scientific accuracy and editorial standards. See the sources used below for this article: