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Astronomers find a planet around two stars by first modeling the binary system’s own eclipses, then looking for additional, precisely timed changes in the combined starlight. They confirm the interpretation by showing that the extra events repeat in a dynamically consistent way, are not instrumental artifacts or stellar eclipses, and agree with independent evidence such as eclipse-timing changes or radial-velocity measurements.
This is harder than finding a planet around one star: the two stars move while the planet travels, so a circumbinary transit can change in timing, depth, and duration from one passage to the next.
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What “circumbinary planet” means
A circumbinary planet orbits both members of a binary pair, rather than circling just one star. From far enough away, the two stars act as a moving gravitational system and the planet follows an orbit around their common center of mass. The term describes the orbit, not the planet’s size, temperature, or habitability.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →For an observer on Earth, the main clues come from light and motion. A planet may cross in front of one of the stars and block a small amount of light (a transit). The stars also pass in front of each other and produce much larger, repeating eclipses. Astronomers must model the binary first so they can distinguish the planet’s signal from the stars’ signal.
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NASA’s Kepler-16 analysis illustrates the basic logic: Kepler saw extra dimming when the stars were not eclipsing. The events returned at irregular intervals as the binary occupied different orbital positions, indicating that a third body moved around both stars rather than orbiting only one. Changes in the binary eclipses’ timing also constrained the third body’s mass. NASA Jet Propulsion Laboratory’s Kepler-16 report called it the first unambiguous circumbinary planet discovery, announced September 15, 2011.
How do astronomers find planets around two stars?
1. Record the binary’s changing brightness
Transit searches begin with photometry: repeated measurements of how bright a target appears. NASA’s Transiting Exoplanet Survey Satellite (TESS) takes full-frame images of sky sectors. Scientists turn those images into time series, or light curves, showing brightness against time. In a binary, the light curve normally contains the two stars’ mutual eclipses. Those recurring events establish the binary’s period, relative timing and approximate geometry.
A planet crossing a star creates an additional dip. Because the planet is much smaller than either star, its dip is usually shallower than a stellar eclipse. The combined light of both stars also dilutes the signal, making the planet harder to see than it would be around a single star.
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2. Search for extra dips between stellar eclipses
Once the binary model is in place, researchers inspect the residual light curve for events that the model cannot explain. They ask whether each candidate dip has a plausible duration and depth, whether it occurs when a planet could cross a star, and whether later events follow an orbit rather than random noise.
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TESS’s TOI 1338 b discovery shows why this search cannot rely on a simple fixed-period signal. The reported transits appeared every 93 to 95 days, but their depths and durations varied. Transits across the larger star were detectable; crossings of the smaller star were too faint for TESS to detect. NASA’s account of TOI 1338 b describes the first circumbinary planet found by TESS, reported January 7, 2020.
3. Use the geometry to predict where a transit can occur
For a single star, a planet on a stable orbit usually transits at nearly regular intervals. Around a binary, the stars’ positions shift between successive planetary passages. A planet that misses both stars on one orbit may cross one star on the next, or cross a different part of the stellar disk. The search therefore uses a dynamical model of the binary and planet, not a single repeating interval.
How can they tell a planet transit from a binary-star eclipse?
Both phenomena are brightness dips, so classification depends on context rather than dip shape alone.
| Clue | Stellar eclipse | Planetary transit |
|---|---|---|
| Objects involved | One star passes in front of the other. | A planet passes in front of one star. |
| Typical depth | Often large because a star blocks a substantial fraction of the system’s light. | Usually shallow and diluted by light from both stars. |
| Timing | Tracks the binary’s established orbital period. | Can occur at irregular intervals as the binary moves. |
| Duration and shape | Predicted by the two stars’ sizes and binary orbit. | Can change from event to event as the crossing geometry changes. |
| Independent check | Should fit the binary model without requiring a third body. | Should fit a planet-and-binary dynamical model and, ideally, another measurement such as eclipse timing or radial velocity. |
In the TOI 1338 b investigation, an initial signal looked like the smaller star passing in front of the larger one. Its timing did not fit the expected stellar eclipse, so the team treated it as a possible planetary transit instead. Researchers then used the eleanor software package to verify that the dips were present in the TESS data and not produced by the instrument or data processing.
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Human inspection remains useful because the pattern is not strictly periodic. Veselin Kostov of the SETI Institute and NASA Goddard said, “These are the types of signals that algorithms really struggle with,” and noted that “the human eye is extremely good at finding patterns in data, especially non-periodic patterns like those we see in transits from these systems.” The practical approach combines automated searches with visual and physical checks.
Why are circumbinary planet transits irregular?
The irregularity is a geometric consequence of two moving stars. The planet follows its own orbit while the stars revolve around each other. At the moment of a possible crossing, one star may be to the left, right, above or below the planet’s projected path. That changes four observable quantities:
- Interval: the time between visible transits can vary because a crossing occurs only when a star is in the right place.
- Depth: a crossing of the larger star blocks more combined light than a crossing of the smaller star; the star’s surface brightness and the amount of disk covered also matter.
- Duration: relative speeds and the chord across the stellar disk change as the binary moves.
- Visibility: some planetary passages miss both stars, and some crossings are too shallow to rise above the instrument’s noise.
TOI 1338 b demonstrates all of these complications. Its observed transits were separated by roughly 93 to 95 days, but the spacing was not a perfectly repeating planetary period, and the smaller star’s transits were below TESS’s detection threshold. An apparent absence of a transit therefore does not automatically disprove the planet.
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How binary eclipse timing reveals a planet’s gravity
A planet does not simply orbit a fixed pair of stars. Its gravity makes the binary and planet orbit their shared center of mass. As a result, the stars’ mutual eclipses can arrive slightly earlier or later than predicted by a two-star model.
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Astronomers measure the times of many primary and secondary stellar eclipses, calculate a timing residual for each one, and test whether the pattern is consistent with an additional orbiting body. The effect is small, but the binary’s eclipses provide a natural clock that can be monitored over many cycles.
For Kepler-16 b, the extra transits proved that a body passed in front of a star, while the eclipse-time variations supplied a gravitational constraint on the third body’s mass. The planet’s reported orbit around the binary was 229 days; NASA/JPL described it as roughly Saturn-sized and cold, not as an Earth-like world. The combined transit and timing evidence made the case unambiguous in the agency’s 2011 report.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How radial velocity adds a second line of evidence
Radial velocity measures motion toward or away from Earth through shifts in stellar spectral lines. A planet’s gravitational pull makes its host stars move slightly along the line of sight. Repeated velocity changes can therefore support a planetary interpretation and help estimate the orbit and mass-related parameters.
The TOI 1338 b team analyzed archived ground-based radial-velocity observations after identifying the transit signal. This did not replace the transit evidence: it tested whether the system’s measured motion was compatible with the proposed third body.
Radial velocity can also reveal a candidate when transits are unavailable. NASA’s analysis of the 2M1510 system used radial-velocity measurements and a subtle alteration to the 21-day mutual orbit of two brown dwarfs. The agency’s page, updated December 16, 2025, calls 2M1510 b a possible perpendicular, or polar-orbiting, planet and explicitly treats it as a candidate pending further measurements. A radial-velocity candidate should not be described as equivalent to a confirmed transiting planet.
The confirmation workflow, step by step
- Characterize the binary. Measure the stars’ eclipse period, depths and durations, and establish which events belong to the binary itself.
- Identify unexplained signals. Search the light curve for additional dips that occur outside the predicted stellar eclipses.
- Fit a three-body geometry. Test whether the candidate’s times, depths, durations and missed events are physically consistent with a planet orbiting the binary.
- Reject instrumental causes. Check the original image data and independent light-curve reductions. For TOI 1338 b,
eleanorwas used to verify that the events were not processing artifacts. - Reject stellar explanations. Recalculate whether the event could be an unmodeled stellar eclipse, a blended background eclipsing binary or another source of variable light.
- Measure dynamics independently. Compare the proposed orbit with binary eclipse-timing variations, radial velocity or both.
- Check consistency over time. New observations should land where the dynamical model allows a transit or velocity signal, including predicted non-transits caused by the moving geometry.
- Assign the evidence status carefully. A named NASA report may call an object confirmed, while another object remains a candidate. The label reflects the evidence available for that system, not a universal single test.
This workflow is why one attractive dip is not enough. A genuine planet must explain the pattern in the light curve and the system’s motion while surviving plausible stellar and instrumental alternatives.
What the landmark systems show
| System | Primary signal | What made it informative | Status in the cited report |
|---|---|---|---|
| Kepler-16 b | Planetary transits plus binary eclipse timing. | Extra dips occurred between stellar eclipses at irregular times, and the binary’s eclipse clock shifted under the planet’s gravity. | NASA/JPL’s first unambiguous circumbinary planet discovery, announced September 15, 2011; reported orbital period 229 days. |
| Kepler-47 | Multiple transiting planets. | Showed that more than one planet can form and remain in orbit around a binary. | NASA described it in August 2012 as the first transiting circumbinary system with multiple planets. NASA’s Kepler-47 report. |
| TOI 1338 b | TESS transit photometry, checked with eleanor, plus archived radial velocity. |
Transit intervals, depths and durations varied; crossings of the smaller star were too faint for TESS. | NASA’s first TESS circumbinary planet, reported January 7, 2020. NASA’s report. |
| 2M1510 b | Radial velocity and a change in the brown-dwarf pair’s orbit. | Suggests a planet on a possible polar or perpendicular orbit around two brown dwarfs. | Candidate pending further measurements, according to NASA Science’s page updated December 16, 2025. NASA Science’s report. |
The cases use different discovery paths. Transit photometry can show the planet crossing a star directly; eclipse timing and radial velocity test the gravitational consequences. The geometry may permit transits in one system but not another, so no single observing technique is required for every circumbinary planet.
How many circumbinary planets are known?
Counts depend on the date and catalog definition. NASA’s TOI 1338 article reported 12 circumbinary planets in 10 systems from Kepler and K2 as of 2020. A NASA Science page updated December 16, 2025 stated that 16 circumbinary planets had been identified among more than 5,800 confirmed exoplanets. The latter is a dated agency figure, not a live 2026 census; catalogs can change as candidates are confirmed, reclassified or added.
These numbers also mix evidence histories. Some objects are confirmed through transits and dynamical checks, while others remain candidates while teams gather additional measurements. A careful account should always name the source and date when quoting a total.
What observers should remember
- “Circumbinary” means the planet orbits both stars in the pair.
- Researchers model the stars’ eclipses before searching for planetary transits.
- Irregular timing, depth and duration are expected because the stars move during the planet’s orbit.
- Confirmation requires more than a dip: teams test stellar and instrumental explanations and seek dynamical or spectroscopic support.
- A confirmed transiting planet and a radial-velocity candidate represent different evidence states; 2M1510 b remains a candidate in NASA’s December 2025 description.
Kepler-16 b’s discovery captures the central idea: extra transits identify a body crossing a star, while the binary’s changing eclipse times show that the body is gravitationally part of the same two-star system. That combination—light blocked, motion measured and alternative explanations checked—is how astronomers turn a complicated pattern into evidence for a planet around two stars.
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