Launched in 2021, JWST has proven to be formidable in the characterization of already-known exoplanets. But this satellite, with its stunning images, doesn’t stop there—it also goes hunting for new exoplanets! An international team of researchers led by Chas Beichman (JPL and Caltech) and including astrophysicists from the Department of Astrophysics of Irfu (DAp) has just published two papers in Astrophysical Journal Letters. These provide strong evidence for the presence of a giant exoplanet located in the habitable zone of Alpha Centauri A—the closest Sun-like star to us. The observations were carried out using JWST’s MIRIm imager, developed primarily in France.
More than 6,000 exoplanets have been detected to date, so why focus on finding yet another one around Alpha Centauri A? Quite simply because Alpha Centauri A is the closest Sun-like star to us: only 4.37 light-years away. At that distance, it becomes possible to explore the presence of exoplanets in its so-called “habitable zone” using the technique of direct imaging. But simply pointing the telescope at the Alpha Centauri system isn’t enough to observe an exoplanet. Such a planet, being close to its star, has its faint glow completely drowned out by the overwhelming brightness of Alpha Centauri A.
These are extremely difficult observations—but not impossible—thanks to a device that blocks starlight. JWST’s MIRI (Mid-Infrared Instrument) is equipped with just such a device: a coronagraph. However, even with a coronagraph, residual starlight remains, which must be removed by observing a reference star. Observing the surroundings of Alpha Centauri A comes with additional challenges, such as light pollution from its companion star, Alpha Centauri B.
The first series of observations of the system took place in August 2024: an object over 10,000 times fainter than Alpha Centauri A, located at about twice the distance between the Sun and Earth, was detected. Two further observations were conducted in February 2025 and April 2025; these did not reveal any object similar to the one identified in August 2024. Far from negating the excitement of the first detection, these non-detections helped place constraints on the exoplanet’s orbit.
Taking into account the brightness of the potentially observed planet and orbital simulations, the favored scenario for now is the presence of an exoplanet with a mass similar to Saturn’s, orbiting Alpha Centauri A on an elliptical trajectory ranging between 1 and 2 times the distance from Earth to the Sun. In February and April, the exoplanet would have been too close to the star to be detected.
“Alpha Centauri A is already on the list of prime targets for observations starting in 2030 with the METIS instrument of ESO’s Extremely Large Telescope, currently under development. With a 39-meter primary mirror—compared to 6.5 meters for JWST—the starlight will be far more concentrated, and the planet should be clearly visible. Alpha Centauri hasn’t finished capturing astronomers’ interest!” exclaims Pierre-Olivier Lagage, co-author of the two publications, French scientific lead for the development of MIRI, and member of the METIS science team.
If the presence of this exoplanet is confirmed by future JWST observations, then this exoplanet—thanks to its great proximity, offering multiple possibilities for detailed characterization—will become a reference object for exoplanet science.
Contact : Pierre-Olivier Lagage
Références
- press release: https://webbtelescope.org/contents/news-releases/2025/news-2025-135
- Paper I : Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. 2 I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits
Charles Beichman, Aniket Sanghi et al. - Paper II : Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A.2 II. Binary Star Modeling, Planet and Exozodi Search, and Sensitivity Analysis
Aniket Sanghi,∗ Charles Beichman et al.