Key Takeaways:
- Using the James Webb Space Telescope (JWST), astronomers detected a potential gas giant exoplanet, designated S1, orbiting Alpha Centauri A, a star in the closest stellar system to Earth.
- The detection utilized JWST's MIRI instrument and a coronagraph to mitigate the overwhelming light from Alpha Centauri A and its binary companion, Alpha Centauri B, revealing a faint point source after advanced image processing.
- Follow-up observations failed to detect S1, but orbital modeling suggests a 52% probability of this being due to the planet's orbital motion, consistent with a Saturn-mass gas giant on an elliptical orbit between 1 and 2 AU from Alpha Centauri A.
- Confirmation of S1 as a planet requires further observations to definitively rule out alternative explanations and track its orbital motion; its existence would significantly advance exoplanet imaging and challenge current planetary formation models.
A whole new world next door
A team of astronomers has found evidence of a candidate gas giant planet orbiting Alpha Centauri A, a Sun-like star in the closest stellar system to our own. Initial observations hinting at the planet’s existence were made in August 2024 using data from the James Webb Space Telescope (JWST). The Alpha Centauri system is located just 4 light-years from Earth.
The findings, detailed in two papers accepted for publication in The Astrophysical Journal Letters, could represent a major milestone. As Charles Beichman of NASA’s Jet Propulsion Laboratory noted in a JWST news release, “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.”
The Alpha Centauri system
The Alpha Centauri system is a fascinating trio of stars. It consists of Alpha Centauri A and Alpha Centauri B, two Sun-like stars that form a binary pair, and the more distant and faint red dwarf star, Proxima Centauri.
While the smaller star in the system, Proxima Centauri, is known to host three planets, its larger companions, Alpha Centauri A and B, have proven to be far more elusive targets for planet hunters. The primary obstacle is the intense, overlapping glare of the two main stars. Even when the light from one star is masked, the brilliance of its binary partner is enough to completely obscure the faint light of a potential planet.
Blocking out the light
So, how did JWST succeed? The telescope’s powerful Mid-Infrared Instrument (MIRI) was pointed at Alpha Centauri A. These were “incredibly challenging observations to make,” according to Beichman, “because these stars are bright, close, and move across the sky quickly.” He explained that the operations team “had to come up with a custom observing sequence just for this target, and their extra effort paid off spectacularly.”
To achieve this, the team employed a coronagraph, a specialized instrument within MIRI that acts like an artificial eclipse, using a physical mask to block the overwhelming glare from Alpha Centauri A. This process revealed a faint “point source, called S1” — an object too distant to be resolved as anything more than a dot of light.
Simply blocking the star isn’t enough, as residual starlight still scatters around the mask and within the telescope’s optics. To account for this, the team performed advanced image processing, creating a model of this contaminating light from both Alpha Centauri A and its companion star B. By subtracting this model from the original image, they were able to perform a digital cleanup that revealed the faint object.
Rigorous tests were conducted to ensure S1 wasn’t an instrument glitch or a distant background galaxy masquerading as a planet. Despite this, the team emphasized in their conclusions that with only a single robust sighting, the candidate could not be “unambiguously confirmed as a bona fide planet” without follow-up observations to track its motion.
To search for a planet orbiting Alpha Centauri A, the nearest Sun-like planet to our own solar system, researchers had to mask the light from the star and from its binary partner, Alpha Centauri B. Credit: NASA, ESA, CSA, Aniket Sanghi (Caltech), Chas Beichman (NExScI, NASA/JPL-Caltech), Dimitri Mawet (Caltech)
The case of the ‘disappearing planet’
The story took a mysterious turn when follow-up observations in February and April 2025 failed to spot the object again. “We are faced with the case of a disappearing planet!” Aniket Sanghi, a Ph.D. student at Caltech and co-first author on the papers, said in the news release. To solve this, the team “used computer models to simulate millions of potential orbits.” These models revealed that 52% of the stable orbits were consistent with a nondetection in early 2025, indicating the planet was “likely missed in both follow-up observations due to orbital motion,” the authors noted in their paper.
What we know so far
Based on the initial JWST observations and the orbital models, a picture of this potential new world has started to form. It is likely a gas giant with a mass similar to Saturn, following an elliptical path around Alpha Centauri A that varies between one and two times the distance from the Earth to the Sun. While this orbit places it within the star’s habitable zone, scientists say that as a gas giant, it would not support life as we know it. If confirmed, Sanghi stated, it “would mark a new milestone for exoplanet imaging efforts.” He added that 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.” For now, the discovery highlights the complexities of exoplanet hunting, and as the research team concluded, additional observations are necessary to confirm its nature as a planet.
