Alpha Centauri has gripped the imaginations of sci-fi aficionados for decades. Only four light-years from Earth, the three-star system inspires fictional alien worlds and journeys through interstellar space.
Its proximity also makes it alluring for astronomers, including a team that, on Thursday, provided the strongest evidence yet that a planet circles one of Alpha Centauri’s sunlike stars. The world is at the very edge of the region around the star where liquid water can exist, known as the habitable zone.
Because it is made of gas, the planet itself would not support life as we know it. But the possible planet, discovered through observations from NASA’s powerful James Webb Space Telescope, would be the closest ever found orbiting within a sunlike star’s habitable zone. However, further observations are needed to confirm it is indeed a planet.
The find was announced in two papers that have been accepted for publication in The Astrophysical Journal Letters. It is a potential preview of the types of discoveries that will be possible in the future as astronomers’ tools for hunting exoplanets — particularly ones like our Earth — evolve.
Three stars make up Alpha Centauri. But only two, Alpha Centauri A and Alpha Centauri B, are like our sun. They are locked in close orbit around each other. Circling this pair from farther away is a faint red dwarf known as Proxima Centauri.
The stars themselves are “pretty run-of-the mill,” said Charles Beichman, an astronomer at NASA’s Jet Propulsion Laboratory and the California Institute of Technology. But the system is a touchstone for investigations of stars like our sun, Dr. Beichman added, because cosmically speaking, “it’s right next door.”
So far, only Proxima Centauri has known planets. Astronomers discovered those worlds indirectly, by measuring the way their gravity tugs on their star.
Snapping an image of a planet is a more direct method, but also more difficult. Astronomers must isolate the faint light emitted by the planet from the more vibrant glow of its host star. According to Mr. Sanghi, direct imaging so far has best been used to find young, massive planets, because they are hotter and burn more brightly.
With the Webb telescope, that trend may change.
Launched in 2021, scientists designed the space telescope to peer into the far reaches of the cosmos. But it trained its eyes much closer to home last August, when astronomers pointed it at Alpha Centauri.
A coronagraph on the telescope blocked most of the glow from Alpha Centauri A. That, combined with clever image processing to remove the glare from Alpha Centauri B, was enough for the astronomers to make out a faint speck of light — a possible planet.
Further analysis ruled out the speck as a photo-bombing asteroid, a background galaxy or an image artifact.
“We spent over the past year trying to kill this object in our images, but we haven’t been successful in doing so,” said Aniket Sanghi, a graduate student at Caltech involved in the discovery.
The planet, if it is one, is about the size of Jupiter and has about the mass of Saturn. It orbits Alpha Centauri A every two Earth-years at roughly one to two times the distance between our sun and Earth. It has a temperature of about minus 55 degrees Fahrenheit.
To claim a discovery, astronomers will need to find the planet again, either with the Webb telescope, with another observatory on the ground or in space. The Extremely Large Telescope, a European observatory under construction in Chile, could help. So could NASA’s Roman Space Telescope, scheduled to launch by May 2027.
Direct imaging of planets is difficult, even more so when there are multiple sources of light close by, as in Alpha Centauri. But the technique has high reward: It yields a wealth of information about the planets, including their size, mass, temperature and distance from host star.
“This is really the only technique that we’ll ever be able to use to look for biosignatures, or any sort of signatures of habitability,” Dr. Beichman said.
The Habitable Worlds Observatory, a space telescope proposed as NASA’s next flagship mission, will use direct imaging to hunt for Earthlike exoplanets around stars dozens of light-years away. According to Dr. Beichman, discoveries like a giant gas planet much closer to home are headway toward that goal.
“This is the first step,” he said. “We’re doing it with Webb.”
Katrina Miller is a science reporter for The Times based in Chicago. She earned a Ph.D. in physics from the University of Chicago.
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