Exoplanets orbiting two suns have so far been a rarity. But now astronomers have used a new method to track down 27 such planet candidates around binary stars – that’s a surprising amount. These circumbinary exoplanets were discovered through subtle shifts in the orbits of their host stars. Based on these findings, the team suspects that there could still be many undiscovered planets of this type in the Milky Way – real models for the fictional Star Wars planet Tatooine.
The fictional Star Wars planet Tatooine not only has two suns in the sky, its orbit takes the planet around both central stars. But whether such circumbinary exoplanets actually exist was controversial for a long time. Astronomers suspected that stable planets could not form in their surroundings due to the gravitational turbulence of the two orbiting star partners. It was only in 2011 that the first circumbinary exoplanet, Kepler-16b, was discovered. Since then, 17 more have been added – with more than 6,000 known exoplanets, that’s not a lot.
(Video: University of New South Wales)
Why the search is difficult
But whether astronomers find an exoplanet depends largely on the search method. Almost all previously known “Tatooine” planets were detected using the transit method – the planet gave itself away because it passed directly in front of its stars, dimming their light a little. “Their characteristic transit signatures make these very reliable evidence,” explain Margo Thornton from the University of New South Wales in Australia and her colleagues.
The problem: The transit method only works if the orbits of the planet and stars are on the same plane and in our line of sight. However, if the exoplanet does not pass directly in front of the stars, it remains invisible. “This leaves a potentially large population of circumbinary planets in wide or non-coplanar orbits undetected,” explain the astronomers.
How the new search method works
That’s why Thronton and her team have now searched for circumbinary exoplanets using a different method: the so-called apsidial precession, also known as apsid rotation. It occurs when the axis of an elliptical orbit of one celestial body around another gradually shifts over time. This usually happens due to the gravitational influence of a third celestial body. In the case of circumbinary planets, its presence influences the orbits of the two central stars around each other.
In observational data, this effect can be seen in slight temporal shifts in the stellar eclipses in the binary star system – the changes in the light curve that occur when one star passes in front of the other. “Until now, this technique has been difficult to use because precise, continuous measurements of eclipse timing over sufficiently long periods of time have been rarely available,” explain Thronton and her team.
However, with NASA’s TESS space telescope, this has changed. Since 21018, the telescope has repeatedly focused on tens of thousands of binary star systems over the years and recorded highly precise transit data. For their search, the astronomers analyzed the TESS transit data for 1,590 binary stars and looked for evidence of apsidial precession.
27 planet candidates – from sub-Neptune to super Jupiter
With success: “I would never have expected that we would find 27 planet candidates with this pilot study,” says co-author Benjamin Montet from the University of New South Wales. The 27 newly discovered circumbinary candidates lie between 650 and 18,000 light-years from Earth. “They are distributed across the entire northern and southern sky. “No matter where you look and at what time of year, one of these binary star systems is actually always visible in the sky – even if only with telescopes.”
Additional observations still need to confirm that the 27 circumbinary objects are actually planets. However, their masses speak for this, as astronomers explain. According to their analyses, some planet candidates are about as massive as Neptune, others have ten times the mass of Jupiter. The distance between these exoplanets and their central stars is similarly diverse: “Our closest system has an orbital period of 4.28 days, the longest requires a good 129 days to orbit its central stars,” report the astronomers. These systems are therefore far more variable than the 18 previously known circumbinary exoplanets.
“An entire population of previously hidden planets”
According to astronomers, these findings demonstrate the great potential of the new method for exoplanet detection. “I was surprised at how well this method was able to identify even very small signals in the TESS data,” says Thornton. “This suggests that in the future we could even use it to detect exoplanets the size of Earth. There could still be many real Tatooine planets out there.”
The detection of 27 circumbinary planet candidates in 1,590 binary star systems examined corresponds to a proportion of such systems of two percent – this is far more than previous findings had suggested. “Our method could help detect an entire population of previously hidden planets,” says Montet. The astronomers are already planning to use their search method for further binary star data from the TESS telescope. At the same time, they want to use model simulations to investigate how the circumbinary planet candidates that have now been discovered could have formed.
“As we learn more about the diversity of planets, we also learn more about how planets form and evolve, especially in the complex environment of two stars,” says Thronton. “This is also a first step towards understanding whether there is other life out there – whether we are alone in the universe or not.”
Source: Margo Thornton University of New South Wales, Kensington, Australia) et al., Monthly Notices of the Royal Astronomical Society, 2026; doi: 10.1093/mnras/stag515