First image of a planet so close to binary stars

19 décembre 2025 Par Raphaël de Assis Peralta First image of a planet so close to binary stars

With the latest Avatar film, set on Pandora, a moon of the gas planet Polyphemus located in the Alpha Centauri star system, the closest multiple star system to us, now in cinemas, it is legitimate to wonder whether such worlds can really exist... The answer is yes ! Recently, a research team, including scientists from the Paris Observatory – LIRA, announced the discovery of a new exoplanet orbiting a double star system. Named HD 143811 b, this giant planet was detected by direct imaging using the SPHERE instrument on the Very Large Telescope (VLT) and the GPI instrument on the Gemini South telescope. Becoming the seventh circumbinary planet ever imaged, HD 143811 b stands out for its proximity to its host system and for being the least massive known in this category.

The rarity of circumbinary planets

Tatooine, la planète aux deux étoiles, imaginées par George Lucas dans Star Wars.
Crédit : Star Wars/Disney

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, known as circumbinary formation.

However, detections of planets in these systems — sometimes referred to as Tatooine-type planets in reference to Star Wars — remain exceptionally rare. Today, we know of nearly 800 binary systems hosting exoplanets. A recent study conducted by Philippe Thébault, a researcher at LIRA, shows that the presence of a second star disrupts planetary formation, even when the two stars are very far apart — sometimes at distances several hundred times greater than that between the Earth and the Sun. In fact, only 22% of stars with planets have a stellar companion, while 46% of stars in our Galaxy are part of a binary system.

Among this set of known binary systems, only six planets have been detected by direct imaging, i.e. observed directly on images. This is far too limited a sample, even though this technique provides crucial information about the physical properties of planets and a more solid understanding of their formation mechanisms, compared to those at work around single stars.

A planet hidden in archives

L’instrument SPHERE, dont le LIRA est fortement impliqué, équipe un des quatre télescopes géants du Very large telescope (VLT) au Chili, qui a révélé des exoplanètes par imagerie directe.
Crédit : Claude DELHAYE/ESO/CNRS Photothèque

As part of the ERC COBREX project, which aims to systematically reanalyse thousands of archival observations using advanced tools that significantly improve planet detection, the team of scientists identified several candidates that had escaped initial analyses of data from the Gemini Planet Imager (GPI) instrument on the Gemini South telescope. One of them orbited HD 143811, a young binary system, approximately 15 million years old, located 137 parsecs away in the Scorpius-Centaurus association, within 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 the VLT’s SPHERE instrument in July 2025 settled the matter : the companion was shining exactly where expected for an orbiting planet. Thanks to this effort, the team confirmed the existence of HD 143811 b, a planet orbiting two binary stars. It thus becomes the seventh circumbinary planet to be imaged and joins the very select group of around 50 planets directly photographed over the last 20 years.

This discovery was also independently corroborated by an American team, which simultaneously detected the planet using data from the Keck telescope (Jones et al., 2025), further strengthening the robustness of the result.

HD 143811 b, the circumbinary planet with particularly interesting properties

Vidéo accélérée de l’exoplanète HD 143811 b orbitant autour de ses deux étoiles.
Ce timelapse, interpolé informatiquement à partir de 2 observations des instrument GPI et SPHERE (VLT), prises sur une période de 9 années d’observations. Une orbite complète de la planète autour de ses deux étoiles nécessite environ 320 années terrestres, soit légèrement plus que la durée de l’orbite de Pluton autour du Soleil. Afin de rendre la planète visible, les astronomes ont masquée la lumière des étoiles au centre par coronographie. Deux icônes en forme d’étoile indiquent leur position.
Crédit : Jason Wang (Northwestern)/Nathalie Jones (Northwestern)/Vito Squicciarini(LIRA - Paris Observatory - PSL / Exeter)

HD 143811 b differs from the six other circumbinary planets imaged to date. It is the closest to its system, at only 60 astronomical units, a distance more comparable to that observed in our Solar System, whereas other planets of this type are generally located several hundred astronomical units away. It is also the least massive in this category.

Nine years of observations have shown that it orbits its two stars in approximately 320 years on a nearly circular trajectory, seen almost face-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.

Next observations

Image en noir et blanc des quatre lasers pointés vers la cible d’observation du Very Large Telescope Interferometer (VLTI) de l’ESO avec l’instrument GRAVITY+.
Crédit : ESO

In the coming months, this team plans to use the GRAVITY+ instruments on the VLT and the MIRI coronagraph on the James Webb Space Telescope, two instruments with significant LIRA contributions, to refine its orbit and characterise its atmospheric composition in detail. This discovery also illustrates the potential of reanalysing archives with modern algorithms. The precise characterisation of this new circumbinary planet and comparison with other young planets will be crucial to understanding the formation mechanisms of these fascinating worlds.

Contacts : Johan Mazoyer & Anne-Marie Lagrange

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