Astronomers May Have Found 27 Planets Hiding Around Twin Stars

Hidden Planet Candidates Around Twin Stars Spotted Using Clever Gravitational Trick

Astronomers have identified 27 possible planets that may be orbiting pairs of stars, not by watching them cross in front of their suns, but by detecting a subtle wobble in the stars’ orbits that shouldn’t be there.

Since the first confirmed discovery just 15 years ago, scientists have spotted only 14 planets orbiting two stars at once by watching for the telltale dimming that occurs when a planet passes in front of its host stars. A team of researchers has now taken a completely different approach, one that doesn’t require a planet to cross our line of sight at all.

Traditional detection only works when a planet’s orbit is nearly perfectly aligned with our view from Earth. Even a tilt of just a degree or two can make a planet invisible. That means the handful of two-star planets found so far almost certainly represent a skewed sample of reality, like trying to understand all ocean life by only studying what washes up on one beach.

A Gravitational Wobble That Shouldn’t Be There

A new study, published in Monthly Notices of the Royal Astronomical Society and led by Margo Thornton at the University of New South Wales and the SETI Institute, takes advantage of a slow rotation in a binary star system’s oval-shaped orbit. When two stars circle each other in an ellipse, the long axis of that ellipse can gradually spin over time. When an unseen planet tugs on the pair, it causes this spinning to speed up beyond what physics would otherwise predict. By measuring the timing of eclipses between the two stars and watching those timings drift apart, the researchers could infer that something massive but invisible was pulling on the system from outside.

How Astronomers Mined Five Years of Space Telescope Data for Hidden Planet Candidates

To pull this off, the team started with the European Space Agency’s Gaia catalogue of roughly 2 million eclipsing binary candidates, then narrowed it to systems with usable data from NASA’s TESS space telescope, which launched in 2018 and repeatedly monitors large swaths of the sky. After filtering for proximity, brightness, observation length, and excluding tight pairs where gravitational stretching can mimic the signal, 1,590 binary systems remained for inspection.

For each one, the team independently measured the timing of two types of eclipses: when the first star passes in front of the second, and when the second passes in front of the first. In a system with no orbital rotation, both sets should tick along like clockwork. In a rotating system, one set gradually starts arriving late while the other arrives early, and that clock drift is the fingerprint of something pulling on the pair from outside.

How Researchers Ruled Out False Alarms to Find Hidden Two-Star Planet Candidates

Finding that orbital rotation was only half the battle. Binary stars can wobble for reasons that have nothing to do with hidden planets. Einstein’s general relativity predicts that any massive object in an oval-shaped orbit will experience some amount of this effect, the same physics that explained a long-standing mystery about Mercury’s orbit over a century ago. The stars’ own shapes matter too: gravitational stretching and the spin of the stars themselves can warp their gravitational fields, adding to the signal.

All of those non-planetary contributions can be calculated when the masses, sizes, and orbital characteristics of the stars are known. The team computed the expected wobble from each source, then subtracted those values from the total observed signal. Whatever remained pointed to something else tugging on the stars from outside.

Out of 71 binaries initially flagged as potentially wobbling, 36 showed excess signals that couldn’t be explained by known physics alone. After further vetting, including removing systems whose signals fell below a confidence threshold and systems showing signs of a more massive stellar companion rather than a planet, the team arrived at 27 candidate planets orbiting twin stars.

What These Hidden Planet Candidates Around Twin Stars Might Look Like

More than half of the 27 candidates allow for masses smaller than Jupiter’s, which would put them in planetary territory if follow-up observations confirm those estimates. However, the wobble signal alone can’t tell scientists exactly where the planet is or how heavy it is. A small planet in a tight orbit can produce the same wobble as a larger planet farther out.

Most known two-star planets found by transits sit just outside the closest distance at which a planet can maintain a stable orbit without being flung from the system. If these new candidates follow the same pattern, many could be genuine planets rather than heavier objects like brown dwarfs or small stars. Radial velocity follow-up, tracking the back-and-forth motion of the stars caused by a planet’s gravitational pull, could break that ambiguity. Several of the systems identified are bright enough to be observed with existing ground-based telescopes.

Despite finding only two planets orbiting twin stars through transits across its entire sky survey, TESS may have been quietly collecting evidence of dozens more all along. It just took a different way of reading the data to find them, and that method could work on thousands of star systems still waiting to be checked.

---