Of the more than 6,000…

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?

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.