Why Are Tatooine Planets Rare? Blame General Relativity

hubie writes:

Astronomers have found thousands of exoplanets around single stars, but few around binary stars - even though both types of stars are equally common. Physicists can now explain the dearth:

Of the more than 4,500 stars known to have planets, one puzzling statistic stands out. Even though nearly all stars are expected to have planets and most stars form in pairs, planets that orbit both stars in a pair are rare.

Of the more than 6,000 extrasolar planets, or exoplanets, confirmed to date - most of them found by NASA's Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS) - only 14 are observed to orbit binary stars. There should be hundreds. Where are all the planets with two suns, like Tatooine in Star Wars?

Astrophysicists at the University of California, Berkeley, and the American University of Beirut have now proposed a reason for this dearth of circumbinary exoplanets - and Einstein's general theory of relativity is to blame.

In most binary star systems, the stars have similar but not identical masses and orbit one another in an egg-shaped or elliptical orbit. If a planet is orbiting the pair of stars, the gravitational tugs from the stars make the planet's orbit precess, meaning the orbital axis rotates similar to the way the axis of a spinning top rotates or precesses in Earth's gravity.

The orbit of the binary stars also precesses, but mainly because of general relativity. Over time, tidal interactions between the binary pair shrink the orbit, which has two effects: The precession rate of the stars increases, but the precession rate of the planet slows. When the two precession rates match, or resonate, the planet's orbit becomes wildly elongated, taking it farther from the star but also nearer at its closest approach.

"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."

That doesn't mean that binary stars don't have planets, he cautioned. But the only ones that survive this process are too far from the stars for us to detect with transit techniques 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.

[...] Farhat points out that binaries have an instability zone around them in which no planet can survive. Within that zone, the three-body interactions between the two stars and the planet either expel the planet from the system or pull it close enough to merge with or be shredded by the stars. Peculiarly, 12 of the 14 known transiting exoplanets around tight binaries are just beyond the edge of the instability zone, where they apparently migrated from farther away, since planets would have a hard time forming there.

"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.

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