A technique that planet hunters have never used at scale has found 27 candidate worlds orbiting binary star pairs — and exposed a structural gap in how astronomy has been counting planets for decades.
Astronomers at the University of New South Wales (UNSW) in Sydney published the discovery on May 4, 2026, in the Monthly Notices of the Royal Astronomical Society. The team, led by PhD candidate Margo Thornton and senior author Associate Professor Ben Montet, used a method called apsidal precession to search 1,590 eclipsing binary star systems in data from NASA's Transiting Exoplanet Survey Satellite (TESS). They found 27 strong circumbinary planet candidates — planets that orbit two stars simultaneously — more than doubling the catalog of 18 such worlds previously known. The result matters not just for the candidate count, but because the method does something the dominant transit approach cannot: it finds planets regardless of whether their orbits happen to face Earth.
Transit Method Works Only When Geometry Cooperates
For most of the history of exoplanet science, the dominant detection tool has been the transit method, which identifies planets by catching them as they pass in front of a host star and briefly dim its light. The technique is powerful but geometrically restricted: a planet's orbit must be aligned close to edge-on as seen from Earth. Miss that alignment, and the planet is invisible to transit surveys.
For single-star systems, that bias shapes the data but does not make detection impossible — there are many other angles to observe. For binary star systems, the constraint is far more severe. A circumbinary planet must cross in front of both stars in a way that registers from Earth's vantage point, demanding a precise coincidence of orbital planes. As a result, only planets with nearly coplanar orbits have been detectable by transit. "Most of our current knowledge on planets is biased, based on how we've looked for them," Thornton said. "We've mostly found the easiest ones to detect."
The oversight is not trivial: more than half of all stars in the Milky Way exist in binary or multiple systems. Any method limited to coplanar geometries is effectively blind to a substantial fraction of the galaxy's planetary population.
How Apsidal Precession Works as Planetary Radar
Apsidal precession is the slow rotation of a binary orbit's orientation over time — a well-known effect in stellar physics, caused by general relativity, tidal forces, and rotational distortions in the stars themselves. What Thornton and her colleagues recognized is that it can serve as a planetary signal detector.
Binary stars that eclipse one another produce predictable timing patterns as they orbit. If those eclipse timings shift in a systematic way that cannot be accounted for by relativistic or tidal effects alone, something else is exerting a gravitational pull on the system. A planet is the natural candidate. By isolating the "excess precession" — the portion of orbital drift left after known physical causes are subtracted — the team could infer the presence of an unseen gravitational perturber, constrain its possible mass range, and estimate its orbital distance.
The method had been used to characterize binary stars before, and in April 2025, a team at the University of Birmingham led by Thomas Baycroft used it to detect a polar circumbinary exoplanet — one orbiting nearly perpendicular to its host binary's plane — publishing the result in Science Advances. That polar orbit would have produced no measurable transit from Earth and would have been completely missed by conventional surveys. The UNSW study is the first to deploy apsidal precession as a large-scale planetary search tool, applying it systematically to nearly 1,600 systems from the Gaia DR3 catalog of eclipsing binary candidates.
27 Candidates, Scattered Across Both Hemispheres
The 27 candidate planets range in distance from approximately 650 light-years to roughly 18,000 light-years from Earth. Because the team searched both northern and southern hemisphere systems from the TESS all-sky catalog, at least one of the candidate systems is above the horizon for any observer with a telescope at any time of year, according to Montet.
The team's analysis also identified six additional companion candidates with higher inferred minimum masses, which the paper categorizes separately from the planetary candidates.
There is an important caveat. The precession signal is inherently ambiguous: the same degree of orbital drift can be produced by a low-mass planet at a small orbital separation or by a more massive object on a wider orbit. The method cannot, on its own, distinguish between the two scenarios. Radial velocity measurements — tracking the line-of-sight velocity wobble of each star — are the standard way to break this degeneracy and confirm whether each perturbing body falls within planetary mass range. "Now we get to start the really fun project of figuring out which ones are real planets," Montet said.
Occurrence Rate Points to Larger Hidden Population
Among the 1,590 binary systems the team examined, the 27 candidates represent roughly a 2 percent occurrence rate. Applied across the full population of binary systems in the Milky Way, that figure implies a planetary population of circumbinary worlds far larger than current confirmed counts suggest.
The TESS archive is only one starting point. Montet identified the Vera C. Rubin Observatory's ongoing Legacy Survey of Space and Time as the next logical resource. The Rubin Observatory, which achieved first light in June 2025 at its site on Cerro Pachón in Chile, is conducting a 10-year all-sky photometric survey of the southern hemisphere. Montet estimated the technique could yield thousands, or even tens of thousands, of additional circumbinary planet candidates when applied to that survey's data. "It's a really exciting first step — and it also shows that there's going to be a lot of work to do over the next few years," he said.
Circumbinary Habitability and Life Beyond Single-Star Systems
The habitable zones of circumbinary systems behave differently from those around single stars. A planet orbiting two suns receives combined light from both, and the relative contribution shifts as the stellar pair orbits a shared center of mass. Research has characterized Earth-like planets in circumbinary habitable zones as generally resilient to the temperature swings driven by shifting stellar configurations, suggesting that life-supporting conditions are physically plausible in such systems.
Montet put the stakes plainly. "If circumbinary planets do turn out to be habitable, that means life could be anywhere. Life could be everywhere," he said. "The sheer numbers are really exciting."
More details on the study are available in Universe Today's coverage published May 19, 2026. The study was co-authored by Thornton, Montet, Riley White, Arden Shao, and Diya T. Kumar — all part of the UNSW and California Institute of Technology research group that carried out the pilot survey.
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