Understanding how planets form sheds light on the birth of our solar system and our own origins. Unlike the Sun, the only star in the solar system, most stars are born in pairs or groups in multiple systems. It is therefore crucial to estimate the effect of this binary nature on planetary formation. However, detections of planets in these systems—sometimes referred to as Tatooine-type planets, in reference to Star Wars—remain exceptionally rare.
This is particularly true for those whose light can be photographed directly, a technique that allows the composition of their atmosphere to be analyzed. To date, only six circumbinary planets had been detected by direct imaging, a sample far too small to understand their formation mechanisms in comparison to single star systems.
This discovery is the result of the ERC COBREX project, which systematically reanalyzes thousands of archival observations with advanced tools that significantly improve planet detection. Using data from the Gemini Planet Imager, the team identified candidates that had escaped initial analysis. One of them orbited HD 143811, a young (15 million years old) binary system located 137 parsecs away in the Scorpius-Centaurus association, the closest star nursery to us.
Observed in 2016 and 2019 with GPI, this candidate appeared to accompany the stars in their movement, but its weak signal in 2019 left some doubt. A new observation with SPHERE in July 2025 settled the matter : the companion was shining exactly where expected for an orbiting planet. Thanks to this effort, the team confirmed HD 143811 b, which joins the small group of about 50 planets directly photographed over the past 20 years.
HD 143811 b is remarkable in several ways. It becomes the seventh circumbinary planet ever imaged and, above all, the closest to its stars : it orbits at only 60 astronomical units, a distance close to our solar system, while other known circumbinary planets are generally hundreds of astronomical units away. It is also one of the least massive in this category. Due to its proximity and moderate mass, it is a unique reference point for studying planetary formation in binary systems.
Nine years of observations have made it possible to reconstruct its orbit : it revolves around its two suns in approximately 320 years on a nearly circular trajectory, seen almost head-on from Earth. Analysis of its light has revealed a surface temperature of 1000 Kelvin, corresponding to a gas planet 6.1 times more massive than Jupiter, but only 40% larger. The planet still appears hot and inflated, like all gas giants in their youth.
In the coming months, the GRAVITY instruments on the VLT and MIRI on the James Webb Space Telescope will refine its orbit and characterize the composition of its atmosphere in detail. This discovery also illustrates the potential of reanalyzing archives with modern algorithms. The precise characterization of this new circumbinary planet and comparison with other young planets will be crucial to understanding the formation mechanisms of these fascinating worlds.
Reference
V. Squicciarini et al., "GPI+SPHERE detection of a 6.1MJup circumbinary planet around HD 143811", Astronomy & Astrophysics, septembre 2025
https://www.aanda.org/articles/aa/full_html/2025/10/aa57104-25/aa57104-25.html
French laboratories involved
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