Einstein’s Relativity Explains Rare Tatooine-Like Planets

Astronomers have found a reason why planets that circle pairs of stars are so hard to spot. These worlds, known as circumbinary planets and made famous by Tatooine in Star Wars, should be common since most stars form in pairs and many have planets. But out of more than 6,000 confirmed exoplanets, only 14 orbit both stars in a binary system. A team from the University of California, Berkeley, and the American University of Beirut says Albert Einstein's general theory of relativity is behind this gap. Their work shows how relativity disrupts planet orbits in close binary systems, causing most to get destroyed or thrown out.

Background

Stars often come in pairs, called binary stars. They form together from the same cloud of gas and dust. Observations show that half of all stars in our galaxy are part of such pairs. Planets are also common, with nearly every star expected to have at least one. NASA's Kepler Space Telescope and Transiting Exoplanet Survey Satellite, or TESS, have spotted thousands of exoplanets over the past decade. Yet planets that loop around both stars in a pair remain elusive.

This puzzle has lasted for years. Earlier ideas pointed to problems in planet formation around binaries or limits in telescope detection. But those did not fully explain the numbers. In tight binary systems, where stars orbit each other in days or less than a week, no such planets show up at all. Kepler alone found hundreds of these close pairs, but zero planets circling them.

The new study changes the picture. It suggests planets do form around these binaries, but the systems do not stay stable. Over time, forces at work alter the orbits until planets can no longer hold position. This points to basic physics, not just formation issues or blind spots in observations.

Key Details

In a typical binary system, the two stars have masses that are close but not the same. They follow oval paths around each other, pulled by gravity. A planet orbiting both feels tugs from each star. These pulls make the planet's path wobble or precess, much like a spinning top tilts as it slows under Earth's gravity.

The stars' orbit precesses too, but for a different reason. General relativity steps in here. Einstein's theory says massive objects warp space and time. In close binaries, this warping causes the stars' orbit to rotate faster than expected from simple gravity alone.

How Orbits Change Over Time

As binary stars age, tides between them shrink their orbit. Material around them adds to this pull, drawing the stars closer. This shrinking speeds up the stars' precession from relativity. Meanwhile, from farther out, the planet sees the pair more like one object. Its own precession slows down.

Eventually, the two rates line up in a resonance. This match stretches the planet's orbit into a long, thin oval. The planet swings far out, then dives close to the stars. Each pass brings it nearer until chaos takes over.

"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. In both cases, you get rid of the planet." – Mohammad Farhat, lead author and postdoctoral researcher at UC Berkeley

Computer models back this up. They show the process wipes out about 80 percent of planets around tight binaries. Of those lost, 75 percent meet a violent end by crashing into a star or getting torn apart by tides. The rest get flung into space. The few survivors sit just beyond the danger zone, where orbits hold steady enough for detection.

This matches what telescopes see. The 14 known circumbinary planets orbit wider binaries, with stars farther apart. No planets appear around the closest pairs, right where the models predict total loss.

What This Means

The discovery solves a long-standing mismatch in exoplanet counts. It means the universe has plenty of Tatooine-like planets at first. But physics clears them out from the inner zones we can observe. Farther out, planets may still circle binaries, but current tools struggle to find them. Kepler and TESS rely on transits, where planets dim starlight as they pass. Distant worlds do this rarely or not at all.

General relativity now explains everyday astronomy, not just black holes or fast orbits like Mercury's. Over a century after Einstein fixed Mercury's path puzzle, his ideas strike again. This work shows relativity matters even where Newton's laws seemed enough.

Researchers plan to test the model further. They want to see how it affects star clusters around pairs of supermassive black holes. It might also explain why binary pulsars, spinning neutron stars in tight orbits, lack planets. Pulsars beam radio signals we can time precisely, but few show planetary signs.

Future telescopes could spot more distant circumbinary worlds. The James Webb Space Telescope peers deeper into other systems. Better models might predict where to look. For now, the study fills a key gap. It reminds us that Einstein's gravity rules more of the cosmos than we once thought.

The findings appear in The Astrophysical Journal Letters. Mohammad Farhat led the work with help from Jihad Touma, a physicist at the American University of Beirut. Their simulations ran thousands of scenarios, tracking orbits over millions of years.