Located just 4 light-years from the Sun, Alpha Centauri is a triple star system that has long been a prime target in the search for worlds beyond our solar system. It contains two Sun-like stars, Alpha Centauri A and Alpha Centauri B, as well as a red dwarf star, Proxima Centauri, which is known to host three planets on its own.
Until now, it has been difficult to detect the presence of other worlds around Alpha Centauri A and Alpha Centauri B, as these stars are too bright for large professional telescopes and are very close to each other in the sky.
“Preparing for observation with the Webb Space Telescope required very precise determination of the position of the star Alpha Centauri A, which was actually quite a complex undertaking,” explains Pierre Kervella, astronomer at the Paris Observatory – PSL and co-author of the study, adding : "This required a range of complementary ground-based observations : using the ALMA radio telescope, spectroscopic measurements of radial velocity, and adaptive optics with the VLT. The star Alpha Centauri is not included in the Gaia catalog because it saturates the detectors, and determining its position with an accuracy of a few milliseconds of arc is a real challenge."
On the Webb telescope, the first observations of the system took place in August 2024, using the coronagraphic mask of MIRI (Mid-Infrared Instrument) – manufactured in part at the Paris Observatory - PSL – to block the light from Alpha Centauri A. Although the additional brightness of the neighboring companion star, Alpha Centauri B, complicated the analysis, the scientific team managed to subtract the light from both stars to reveal an object more than 10,000 times fainter than Alpha Centauri A, separated from the star by a distance almost twice that of the Sun from Earth. However, this initial detection required more data in order to receive definitive confirmation.
However, additional observations of the system, conducted in February 2025 and then in April 2025, revealed no objects similar to the one identified in August 2024. However, these non-detections remain consistent with the orbit of a planet that may have come too close to the star to be detectable.
If this candidate planet is confirmed, it would be the closest to Earth to orbit in the habitable zone of a star similar to the Sun. However, as it is a gas giant, scientists confirm that it could not itself support life as we know it.
“The object remains a candidate that needs to be confirmed.”
“If this is the case, it will be the third exoplanet to have been discovered since the start of the Webb telescope mission, using the MIRI coronagraph, a unique instrument for detecting this type of ‘cold’ object,” says Anthony Boccaletti, astrophysicist at the Paris Observatory – PSL and associate researcher on this project.
He concludes : “We need to be able to observe this object again—which means obtaining telescope time—either using the same observation mode or with the MIRI spectrograph.”
The LIRA team at the Paris Observatory – PSL will be involved in these future analyses. To be continued.