Two articles – available here and here – accepted for publication in “The Astrophysical Journal Letters” report different aspects of the study that led to the possible discovery of a gas giant planet orbiting the star Alpha Centauri A. Astronomers used observations with the James Webb Space Telescope’s MIRI instrument to identify a light source whose characteristics make it a candidate exoplanet. The information gathered suggests it may be a gas planet similar in size to Jupiter, although its mass could be close to Saturn’s. If confirmed, it would be the closest exoplanet orbiting a Sun-like star in its system’s habitable zone.
The images (NASA, ESA, CSA, STScI, A. Sanghi (Caltech), C. Beichman (JPL), D. Mawet (Caltech), J. DePasquale (STScI)) show the Alpha Centauri system as seen by the James Webb Space Telescope on the left, a zoom of the two main stars in the center, and a further zoom on the right indicating the exoplanet candidate designated as S1.
The search for planets around the stars of the Alpha Centauri pair has been ongoing for quite some time. One might think that the stars closest to the Sun would be easy to investigate, but the brightness of two Sun-like stars poses a problem for telescopes that must detect extremely faint lights in their vicinity.
A candidate exoplanet around Alpha Centauri A was already announced in an article published in February 2021 in the journal “Nature Communications” and designated as C1 (Candidate 1). In that case, the observations were conducted with the VISIR instrument mounted on ESO’s Very Large Telescope (VLT) in Chile within the NEAR (New Earths in Alpha Centauri Region) project, part of the larger Breakthrough Watch program, which aims to search for potentially habitable planets orbiting nearby stars.
Between August 2024 and April 2025, the James Webb Space Telescope’s Mid-Infrared Instrument (MIRI) was used on three occasions to observe the area around Alpha Centauri A. Webb’s coronagraph was used to block the star’s light, but the light from its companion, Alpha Centauri B, complicated the study, requiring specific analysis to eliminate its light.
The result is that in the first observation, a light source over 10,000 times fainter than that of Alpha Centauri A was identified at a distance about twice that of the Earth from the Sun, which was named S1. The problem is that the source no longer appeared in the observations from February and April 2025.
Simulations suggest that S1 could be a gas giant exoplanet with a mass similar to Saturn’s in a highly elliptical orbit at a distance between one and two times that of Earth from the Sun. The C1 announced in 2021 could be the same object.
Further studies are needed to determine if the exoplanet candidate S1 actually exists and if it’s C1. It may be necessary to wait until the launch of NASA’s Nancy Grace Roman Space Telescope, and therefore at least until 2027, to obtain better images of the area around Alpha Centauri A.
The simulations of the possible orbit of the exoplanet candidate S1 will aid the attempts to detect it in the near future with the available instruments. It would orbit in its system’s habitable zone, but if it were indeed a gas giant, it wouldn’t be suitable for supporting life forms similar to those on Earth. It could, however, be of interest for other reasons. First of all, planets discovered in binary systems help us understand their evolution in these chaotic systems. If S1 had a large moon with an atmosphere, it could be habitable, but that’s just speculation. For now, the priority is to understand if this exoplanet actually exists.
