Astronomers using the NASA/ESA/CSA James Webb Space Telescope have found strong evidence of a giant planet orbiting a star in the stellar system closest to our own Sun. At just four light-years away from Earth, the Alpha Centauri triple star system has long been a target in the search for worlds beyond our solar system.
Visible only from the Southern hemisphere, the system is made up of the binary Alpha Centauri A and Alpha Centauri B, both Sun-like stars, and the faint red dwarf star Proxima Centauri. Alpha Centauri A is the third brightest star in the night sky.
While there are three confirmed planets orbiting Proxima Centauri, the presence of other worlds surrounding Alpha Centauri A and Alpha Centauri B has proved difficult to confirm, because the stars are so bright, close, and move across the sky quickly.
Now, observations from Webb's Mid-Infrared Instrument (MIRI) are providing the strongest evidence to date of a gas giant orbiting Alpha Centauri A. The results, from an international team including researchers from the University of Cambridge, have been accepted for publication in two papers in The Astrophysical Journal Letters.
If confirmed, the planet would be the closest to Earth that orbits in the habitable zone of a Sun-like star. However, because the planet candidate is a gas giant, scientists say it would not support life as we know it.
Several rounds of observations by Webb, analysis by the research team, and computer modelling helped determine that the source seen in Webb's image is likely to be a planet, and not a background object (like a galaxy), a foreground object (a passing asteroid), or another image artefact.
"Webb was designed and optimised to find the most distant galaxies in the universe. The team had to come up with a custom observing sequence just for this target, and their extra effort paid off spectacularly," said Charles Beichman, NASA's Jet Propulsion Laboratory and the NASA Exoplanet Science Institute at Caltech, co-first author on the new papers.
The first observations of the system took place in August 2024. While extra brightness from the nearby companion star Alpha Centauri B complicated the analysis, the team was able to subtract out the light from both stars to reveal an object over 10,000 times fainter than Alpha Centauri A, separated from the star by about two times the distance between the Sun and Earth.
While the initial detection was exciting, the research team needed more data to come to a firm conclusion. However, additional observations of the system in February 2025 and April 2025 did not reveal any objects like the one identified in August 2024.
"We were 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 co-first author Aniket Sanghi of the California Institute of Technology.
In these simulations, the team took into account both the 2019 sighting of a potential exoplanet candidate by the European Southern Observatory's Very Large Telescope, the new data from Webb, and considered orbits that would be gravitationally stable in the presence of Alpha Centauri B, meaning the planet wouldn't get flung out of the system.
The researchers say a non-detection in the second and third round of observations with Webb wasn't surprising.
"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 addition to these simulations, the Cambridge members of the research team analysed the Webb data to search for any signs of a type of cosmic dust, known as exozodiacal dust, around Alpha Centauri A. This cloud of dust, produced by objects such as comets and asteroids breaking apart, forms a faint, glowing disc around a star.
"Exozodiacal dust helps us learn about the architecture and evolution of planetary systems," said co-author Professor Mark Wyatt from Cambridge's Institute of Astronomy. "But it's also important when searching for rocky planets, since dust in the habitable zone of a star can obscure or mimic planetary signals."
No dust was detected in these observations, however, the team showed they were sensitive to dust levels an order of magnitude lower than any previous measurement, which could be valuable for future planet searches around this star.
"This observation shows how deeply Webb can probe the dust environment of the nearest Sun-like stars," said co-author Dr Max Sommer, also from Cambridge's Institute of Astronomy. "We can now explore exozodiacal dust at levels not much higher than those in our own Solar System, tapping into a whole new way of looking at other star systems."
Based on the brightness of the planet in the mid-infrared observations and the orbit simulations, the researchers say it could be a gas giant approximately the mass of Saturn orbiting Alpha Centauri A in an elliptical path varying between one to two times the distance between Sun and Earth.
If confirmed, the potential planet seen in the Webb image of Alpha Centauri A would mark a new milestone for exoplanet imaging efforts. Of all the directly imaged exoplanets, 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 the nearest to Earth.
"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," said Sanghi.
The James Webb Space Telescope is an international programme led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
Adapted from a NASA press release.
