If detecting exoplanets was easy, then we should have a complete understanding of the planetary system around our nearest stellar neighbour, Alpha Centauri. But we don’t, because it’s not easy. Alpha Centauri is a triple star system about 4.25 light-years away. The primary star is called Alpha Centauri A, a Sun-like star, and it’s in a binary relationship with Alpha Centauri B, another Sun-like star. The third star is a red dwarf named Proxima Centauri, and it’s the closest one to us.
Astronomers know of three confirmed exoplanets around Proxima Centauri, and there have been hints of other planets in the system orbiting the other stars. Confirming any of these planets has proven difficult. Now the JWST has found additional evidence of a gas giant orbiting Alpha Centauri A.
Two companion papers present the discovery. One is “Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of Alpha Cen A.
I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits.” The co-lead authors are Charles Beichman from NASA’s Exoplanet Science Institute and JPL, and Aniket Singh from the California Institute of Technology and JPL.
The second paper is “Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of Alpha Cen A. II. Binary Star Modeling, Planet and Exozodi Search, and Sensitivity Analysis.” It shares the same co-lead authors.
“If confirmed, the potential planet seen in the Webb image of Alpha Centauri A would mark a new milestone for exoplanet imaging efforts.” – Aniket Sanghi, California Institute of Technology/JPL.
The planet is referred to as S1. It’s existence is difficult to conclusively prove because of noise from the three stars, zodiacal dust, and background sources. The astronomers used the JWST’s Mid-Infrared Instrument (MIRI) and its coronagraphic mask to detect it, and the detection took two years and required help from other telescopes.
“With this system being so close to us, any exoplanets found would offer our best opportunity to collect data on planetary systems other than our own. Yet, these are incredibly challenging observations to make, even with the world’s most powerful space telescope, because these stars are so bright, close, and move across the sky quickly,” said co-first author Beichman. “Webb was designed and optimized to find the most distant galaxies in the universe. The operations team at the Space Telescope Science Institute had to come up with a custom observing sequence just for this target, and their extra effort paid off spectacularly.”
Three views of Alpha Centauri from three different telescopes. The left image is from the ground based Digital Sky Survey and shows the triple star system as a single point of light. The middle image is from the Hubble Space Telescope and shows A Cent A and A Cent B separately. The third image is from the JWST’s MIRI and its coronagraph, and shows A Cent A and the candidate planet. Image Credit: NASA, ESA, CSA, STScI, DSS, A. Sanghi (Caltech), C. Beichman (JPL), D. Mawet (Caltech), J. DePasquale (STScI). LICENCE: CC BY 4.0 INT or ESA Standard Licence
Searching for a planet around Alpha Centauri A is extremely complicated. Astronomers have to account for the star’s proper motion and the light from the star’s binary companion. They needed to use reference stars to calibrate their observations, but that’s complicated by the fact that they have to be bright enough compared to Alpha Centauri A. The reference stars also have to have similar photospheric properties to the coronagraphic filter being used, in this case, F1550C. Due to filtering and stellar brightness, the astronomers also had to use blind offset stars to track Alpha Centauri A. They used Epsilon Muscae (e Mus) as a blind offset star, and also used a guide star to acquire it. On top of that, they had to use computer simulations of its orbit. That’s a dictionary definition of complicated.
These images illustrate some of the difficulty in searching for exoplanets around Alpha Centauri A. The image on the left shows Alpha Centauri AB, the binary star, and Gaia stars in green boxes. Red boxes are MIRI point source detections. The stars labelled G0 and G5 were used for target acquisition. The right panel shows the bind offset star Epsilon Muscae, and a guide star labelled G9 used to acquire Eps Muscae. Image Credit: Beichman, Sanghi, et al. 2025. ApJL
If the planet can be confirmed, it will be a noteworthy discovery. It would be the closest habitable zone planet orbiting a Sun-like star. Since it’s gas giant, it is not habitable, but its proximity still makes it a scientifically valuable observational target.
The first JWST observations are from August 2024. These were tricky, because although MIRI has a coronagraph, there are multiple stars to contend with. The coronagraph blocked out the light from Alpha Centauri A, but bright light from its companion Alpha Centauri B complicated the observations. The researchers were eventually able to block out Alpha Centauri B’s light. That revealed the presence of an object 10,000 times dimmer than Alpha Centauri A. It’s separated from the star by about 2 au.
With this initial detection of an exoplanet candidate, excitement built. But these observations alone weren’t enough to confirm it. The team conducted more observations with the JWST’s Director’s Discretionary Time in February and April of 2025, and those proved inconclusive.
This is JWST’s view of the Alpha Centauri AB system. The candidate planet is seen in the images from August 2024, but not in subsequent images. Image Credit: Beichman, Sanghi, et al. 2025. ApJL
JWST observing time isn’t handed out like candy, and requests for more were not in the cards. Instead, the researchers worked with the observational data they’d already acquired and turned to computer models to take the next step.
“We are faced with the case of a disappearing planet! To investigate this mystery, we used computer models to simulate millions of potential orbits, incorporating the knowledge gained when we saw the planet, as well as when we did not,” said PhD student Aniket Sanghi of the California Institute of Technology in Pasadena, California. Sanghi is also a co-first author on the two papers covering the team’s research.
Here’s where another potential exoplanet enters the picture. In 2021, astronomers working with the VLT detected a candidate planet around Alpha Centauri A referred to as C1. When considering potential orbits for S1, the team also had to consider C1.
“With only a single JWST/MIRI sighting (and non-detections at two other epochs), it is challenging to uniquely constrain the orbit of S1,” the authors explain. To make progress, they decided to consider C1 as an earlier detection of the newly-detected S1, which they refer to as the S1 + C1 candidate.
This figure shows 100 randomly selected stable planetary orbits fitting the S1+C1 astrometry (marked as green points) and consistent with the February and April 2025 non-detections, for each orbital family. Image Credit: Beichman, Sanghi, et al. 2025. ApJL
“We find that 52% of the stable orbits that fit the S1 + C1 astrometry are also consistent with non-detections in both February and April 2025,” the authors write. “There is, thus, an a priori significant chance that, if real, the planet candidate could have been missed in both follow-up observation epochs.”
“We found that in half of the possible orbits simulated, the planet moved too close to the star and wouldn’t have been visible to Webb in both February and April 2025,” said Sanghi.
In the end, the researchers think they’ve discovered a Saturn-mass gas giant orbiting Alpha Centauri A. They say it follows an eccentric orbit that moves within 1 to 2 au of the star. The planet is a bit brighter than expected for its type, so they say zodiacal dust could be contributing. The planet could also be rotating rapidly, if being viewed from the pole, could show more surface area. Or it could have rings like Saturn does.
When the JWST was being designed and then launched, scientists knew how powerful it would be. The telescope excels at looking back in time at extremely distant galaxies and supermassive black holes, but that same power can be used on our closest stellar neighbour. Directly imaging a nearby exoplanet is a stunning achievement. According to the researchers, these are some of the most complex and demanding observations performed with the space telescope.
“These are some of the most demanding observations we’ve done so far with MIRI’s coronagraph,” said Pierre-Olivier Lagage, of CEA, France, who is a co-author on the papers and was the French lead for the development of MIRI. “When we were developing the instrument we were eager to see what we might find around Alpha Centauri, and I’m looking forward to what it will reveal to us next!”
“If confirmed, the potential planet seen in the Webb image of Alpha Centauri A would mark a new milestone for exoplanet imaging efforts,” Sanghi says. “Of all the directly imaged planets, this would be the closest to its star seen so far. It’s also the most similar in temperature and age to the giant planets in our solar system, and nearest to our home, Earth,” he says. “Its very existence in a system of two closely separated stars would challenge our understanding of how planets form, survive, and evolve in chaotic environments.”