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Quick answer: Astronomers first map the binary pair from its repeating stellar eclipses, then search the combined brightness record for extra, planet-sized dips. They test whether those events recur with a dynamically coherent orbit, are not instrumental artifacts or ordinary stellar eclipses, and agree with independent evidence such as eclipse-timing changes or radial-velocity measurements. A planet orbiting both members of the pair is called a circumbinary planet.
This is harder than finding a planet around one star because the two stars keep moving while the planet travels. As a result, a circumbinary transit can occur at irregular intervals and can change in depth and duration from one crossing to the next. The most convincing discoveries combine transit photometry with dynamical or spectroscopic checks rather than relying on one brightness dip.
What “circumbinary” means
A circumbinary planet orbits both stars in a gravitationally bound binary system. It is not a planet that circles one star while the other happens to be nearby. The binary stars orbit their common center of mass, and the planet follows a wider orbit around that pair.
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From Earth, both stars contribute to one blended light signal. A stellar eclipse occurs when one star passes in front of the other. A planetary transit occurs when the planet crosses the visible disk of one star. Both events lower the measured brightness, so the first task is to understand the binary’s own eclipse pattern before looking for a third body.
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NASA’s Kepler-16 account illustrates the basic logic: Kepler saw extra dimming when the stars were not eclipsing. The extra events returned at irregular times as the binary occupied different orbital positions, indicating that a third body was passing in front of the stars and orbiting both of them.
How astronomers search for a circumbinary planet
1. Identify the binary from its stellar eclipses
Long, repeated measurements of the system’s brightness reveal when one star blocks the other. These deep, recurring eclipses establish that the target is a binary and provide a timing model for the stars’ orbit. Researchers can predict when a normal stellar eclipse should occur and how much light it should remove.
That baseline matters because any unexplained dip must be compared with the binary model. A dip at the expected time and depth may simply be a stellar eclipse. A shallower event between the known eclipses is a stronger planet-transit candidate, but it still requires additional tests.
2. Turn the light curve into a transit search
Transit photometry measures the system’s brightness over time. NASA’s Transiting Exoplanet Survey Satellite (TESS) records full-frame images of successive sky patches; scientists process those images into time series showing how each target brightens and dims. They then search the time series for decreases that are too small, too late, or otherwise inconsistent with the modeled stellar eclipses.
The first TESS circumbinary discovery, TOI 1338 b, shows why the search cannot assume a fixed period. NASA reported transits roughly every 93 to 95 days in the 2020 data, but the intervals were not perfectly regular. The transits also changed in depth and duration because the two stars were moving. Crossings of the larger star were visible; crossings of the smaller star were too faint for TESS to detect. Details are in NASA’s TOI 1338 b report.
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3. Subtract the binary model and inspect the residual events
Researchers model the binary eclipses and remove their expected contribution from the light curve. The remaining data, often called residuals, are inspected for additional dips. In a circumbinary system, a planet may cross either star, or miss both stars on a particular orbit, so the residual events do not form the neat, evenly spaced sequence familiar from many single-star transit searches.
Automated algorithms are useful for scanning large data sets, but these signals are difficult for periodic-search software. Veselin Kostov of the SETI Institute and NASA Goddard described circumbinary signals as “the types of signals that algorithms really struggle with.” He also noted that the human eye is good at recognizing the non-periodic patterns in these transits. Human inspection, specialist modeling and repeated observations therefore remain important parts of the process.
Why circumbinary transits are irregular
The planet’s orbit and the binary’s orbit are superimposed. Every time the planet approaches the system, the stars have moved to new positions around their own orbit. That changes three observable properties:
- Timing: the planet may line up with a star earlier or later than a simple fixed-period prediction suggests.
- Depth: a crossing of the larger or brighter star blocks a different fraction of the combined light than a crossing of the smaller or dimmer star. Some crossings can be too shallow to detect.
- Duration: the relative speed and crossing angle change as the planet and the selected star move, so one transit can last longer or shorter than another.
These variations are not automatically evidence of a planet. They are a physical consequence of the geometry that a valid model must reproduce. A candidate becomes persuasive when the sequence of changing events fits one orbit around the binary rather than a collection of unrelated eclipses or noise.
How can astronomers tell a planet transit from a binary-star eclipse?
- Predict the stellar eclipses. Use the established binary orbit to calculate when one star should cross the other and what the resulting dip should look like.
- Flag events outside that pattern. An additional dip while the stars are not eclipsing is a potential planetary transit. Timing that fails to match the expected stellar eclipse is another warning that the event has a different cause.
- Check the changing shape. A planet crossing different stars can naturally produce variable depth and duration. The changes must be consistent with the stars’ sizes, brightnesses and positions, not random from event to event.
- Require recurrence or a coherent orbit. Several events need not be evenly spaced, but they should be explainable by one planet moving on a stable path around the binary.
- Rule out other explanations. Teams test whether the feature is a stellar eclipse, a background eclipsing system blended into the aperture, a processing problem or an instrumental artifact.
In the TOI 1338 b analysis, an initial signal resembled the smaller star passing in front of the larger star, but its timing did not fit the expected eclipse. After identifying the candidate, the team used the eleanor software package to verify that the transits were present in the data rather than created by the instrument or reduction process.
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Independent evidence that strengthens a discovery
Binary eclipse timing: using the stars as dynamical tracers
A planet’s gravity pulls on the binary as the three bodies orbit. The stars therefore do not return to exactly the same eclipse configuration at perfectly uniform times. Measuring small shifts in the timing of the stellar eclipses can reveal the gravitational influence of a third body.
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For Kepler-16, the eclipse-time changes supplied a dynamical constraint on the third body’s mass, while the extra transits showed that the object passed in front of a star. The two measurements answer different questions: photometry establishes a crossing geometry, and eclipse timing tests whether the proposed object has the expected gravitational effect.
Radial velocity: measuring motion along our line of sight
As a planet tugs on its host stars, the stars move slightly toward and away from Earth. Their spectral lines shift correspondingly. Measuring those shifts over time produces a radial-velocity signal that can support the presence of a third body and constrain its orbit and mass.
The TOI 1338 b team analyzed archived ground-based radial-velocity observations after finding the transit signal. Radial velocity is especially valuable when it agrees with the transit timing and the binary model, because it is an independent type of measurement rather than another reduction of the same light curve.
Radial velocity can also identify a candidate that does not transit. NASA’s discussion of the 2M1510 system describes a subtle alteration to the 21-day mutual orbit of two brown dwarfs, together with radial-velocity evidence, as support for a possible planet on a perpendicular or polar orbit. NASA’s page, updated December 16, 2025, calls 2M1510 b a candidate pending further measurements; it should not be described as a confirmed planet.
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What “confirmation” means in practice
There is no single observation that proves every circumbinary planet. NASA’s case studies show a layered process:
- Signal reality: the dip appears in the underlying observations and survives checks for detector or processing artifacts.
- Geometric explanation: the timing, depth and duration fit a planet crossing one of the binary stars rather than an ordinary stellar eclipse.
- Orbital coherence: the irregular sequence can be modeled as one body orbiting the binary.
- Dynamical support: eclipse-timing changes or radial-velocity measurements show the expected gravitational influence.
- False-positive rejection: alternative stellar and instrumental explanations are tested and found inadequate.
The exact balance of evidence depends on the system. A transiting planet may have several photometric events plus dynamical timing data. A non-transiting object may rely more heavily on spectroscopy and changes in the binary orbit. “Confirmed” should therefore be read in the context of the named NASA report or study, not as a universal checklist applied identically to every system.
Four systems that show the different pathways
| System | Signal and geometry | Independent support | Status and dated facts |
|---|---|---|---|
| Kepler-16 b | Extra transits appeared between the binary stars’ own eclipses and returned at irregular times as the stars changed position. | Changes in the binary stars’ eclipse times constrained the third body’s mass. | NASA/JPL announced it on September 15, 2011, calling it the first unambiguous circumbinary planet. Its orbit around the pair is 229 days; it is about Saturn-sized and cold. |
| Kepler-47 | A transiting circumbinary system with more than one planet. | The multiple-planet configuration demonstrated that planets can form and remain stable around a binary. | NASA described it in August 2012 as the first transiting circumbinary system with multiple planets. |
| TOI 1338 b | TESS detected variable transits, reported roughly every 93 to 95 days. Events crossing the smaller star were too faint for TESS. | The team checked the signal with eleanor and analyzed archived ground-based radial-velocity data. |
NASA reported it in January 2020 as TESS’s first circumbinary planet. |
| 2M1510 b | Evidence for a possible planet on a perpendicular or polar orbit around two brown dwarfs; the object is not presented as a routine transiting planet. | Radial velocity and a subtle change in the brown dwarfs’ 21-day mutual orbit. | NASA’s page updated December 16, 2025 labels it a possible planet and candidate pending further measurements. |
Case study: why Kepler-16 b was decisive
Kepler-16 b is the clearest early example of combining photometric and dynamical evidence. Kepler first identified the binary eclipses. It then recorded additional brightness dips at times when the stars were not eclipsing. Because the events recurred at changing intervals corresponding to different binary positions, the simplest explanation was a third body orbiting both stars.
The binary’s eclipse timing shifted as well. That gravitational tug supplied a mass constraint, helping distinguish a planet from an unrelated source of light. NASA/JPL reported a 229-day planetary year and described the world as roughly Saturn-sized and cold. The famous two-sun imagery is a useful visualization, but the evidence establishes an orbit and physical properties—not an Earth-like environment.
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Case study: multiple planets can survive around a binary
NASA’s Kepler-47 report, published in August 2012, described the first transiting circumbinary system with multiple planets. The significance is architectural: a binary does not automatically prevent a planetary system from forming or remaining dynamically organized. Multiple transiting worlds also provide more than one set of events with which to test the binary-plus-planets model.
Case study: TOI 1338 b and the limits of periodic searches
TOI 1338 b demonstrates the observational difficulty in a modern survey. TESS’s full-frame data produced a brightness time series, but the transit pattern did not repeat like a standard single-star planet. NASA reported intervals of about 93 to 95 days, changing depths and changing durations. Only crossings of the larger star were detectable; a crossing of the smaller star removed too little of the blended light.
The team had to distinguish the candidate from a possible stellar eclipse whose timing did not fit the binary model. It then checked the light curve with eleanor and used archived radial-velocity measurements. This combination—specialized photometric validation plus an independent spectroscopic data set—made the interpretation stronger than a single anomalous dip.
Case study: 2M1510 b is a candidate, not a settled detection
2M1510 is a pair of brown dwarfs with a 21-day mutual orbit. NASA describes a possible third body whose orbit may be perpendicular or polar relative to that pair. The evidence comes from radial-velocity measurements and a subtle alteration in the brown dwarfs’ mutual motion, not from the familiar sequence of transits used for Kepler-16 b or TOI 1338 b.
The distinction in wording matters. The NASA Science page updated December 16, 2025 calls 2M1510 b a possible planet and says further measurements are needed. It is an example of how astronomers can report compelling dynamical evidence while keeping the object’s status at candidate until the remaining uncertainties are resolved. Read the details in NASA’s discovery alert.
How many circumbinary planets are known?
Counts depend on the date and on the catalog definition. NASA’s 2020 TOI 1338 article referred to 12 circumbinary planets in 10 systems discovered by Kepler and K2 at that mission-era point. A NASA Science page updated December 16, 2025 stated that 16 circumbinary planets were among more than 5,800 confirmed exoplanets. The latter is a dated figure from that page, not a live 2026 census; catalogs can change as candidates are confirmed, reclassified or added.
These numbers also should not be used to estimate the true frequency of circumbinary planets. The sources describe discoveries and mission counts, not a formal population study. Circumbinary systems are particularly difficult to find because many planetary orbits never produce a transit from Earth, and even transits that do occur can be too shallow or irregular for an automated search.
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- Does the report define the object as orbiting both stars, rather than one star in a binary system?
- Were the binary eclipses modeled first, so ordinary stellar eclipses have a known explanation?
- Are the extra events observed when the stars are not eclipsing, and do their changing timings, depths and durations fit the system geometry?
- Did the investigators check the original data for instrumental or reduction artifacts?
- Is there independent evidence, such as eclipse-timing variation or radial velocity?
- Does the source call the object confirmed, possible or a candidate, and is that status tied to a date?
The central idea is simple but demanding in execution: identify the two-star clock, find brightness changes that do not belong to that clock, and then show that the changes and the system’s dynamics are all explained by one body circling both stars. Transit photometry reveals alignments, eclipse timing measures the binary’s response to gravity, and radial velocity tracks motion through shifted spectral lines. Used together, these observations let astronomers separate a genuine circumbinary planet from an eclipse, a blend or a detector artifact.
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