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Astronomers have utilized NASA’s James Webb Space Telescope to uncover substantial evidence of a planet revolving around a star in the triplet system nearest to our Sun. Located merely 4 light-years distant from Earth, the Alpha Centauri star system has consistently been an appealing target in the quest for celestial bodies beyond our solar system, known as exoplanets.

The system consists of a close binary of orbiting stars, Alpha Centauri A and Alpha Centauri B, the two nearest Sun-like stars to Earth, alongside the dim red dwarf star Proxima Centauri. Although there are three verified planets circling Proxima Centauri, confirming the existence of additional worlds around the Sun-like duo of Alpha Centauri A and Alpha Centauri B has proven difficult.

At present, Webb’s observations from its Mid-Infrared Instrument (MIRI) are producing the most compelling evidence to date of a gas giant planet traveling within the habitable zone of Alpha Centauri A. (The MIRI instrument was partly developed by the Jet Propulsion Laboratory [JPL], which is operated by Caltech for NASA). The habitable zone represents the area around a star where temperatures could allow liquid water to accumulate on a planet’s surface.

If verified, this planet would be the nearest to Earth that orbits within the habitable zone of a Sun-like star. Nevertheless, since the candidate planet is a gas giant, scientists assert it would not be capable of supporting life as we understand it.

The findings are documented in two papers accepted for publication in The Astrophysical Journal Letters. The research was co-directed by Aniket Sanghi, a Caltech graduate student, and Charles (Chas) Beichman, the executive director of the NASA Exoplanet Science Institute at Caltech’s IPAC astronomy center and a senior scientist at JPL. Sanghi collaborated on the project with his advisor and study co-author Dimitri Mawet, David Morrisroe Professor of Astronomy at Caltech and a senior research scientist at JPL.

“Given that this system is so near to us, any discovered exoplanets would provide our best opportunity to gather data regarding planetary systems outside our own. However, these observations are exceptionally challenging to conduct, even with the world’s most powerful space telescope, due to the brightness, closeness, and rapid movement of these stars across the sky,” remarks Beichman.

The team’s initial observations of the system occurred in August 2024 utilizing a mechanism aboard MIRI known as a coronagraphic mask. These masks block the star’s brightness to facilitate direct imaging of fainter planets orbiting around them. The closer a planet is to its star, the more challenging it is to image. While a few dozen planets have been directly imaged previously, this candidate planet, if confirmed, would be the closest to its host star ever imaged, separated by two astronomical units, or twice the distance from the Sun to Earth.

Moreover, the candidate planet would be the first imaged around a star that matches our Sun in both age and temperature.

Another obstacle was the proximity of Alpha Centauri A’s companion, Alpha Centauri B, whose brightness can also obscure the faint signals of orbiting planets.

Eventually, the team successfully subtracted the light from both stars to reveal a candidate planet that is over 10,000 times dimmer than Alpha Centauri A, positioned within the habitable zone of Alpha Centauri A.

“The observations were quite demanding, but because this is the nearest star system to Earth, imaging a planet this close to its star is more feasible,” remarks Beichman. “The next nearest Sun-like star, Tau Ceti [around 12 light-years from Earth], will be significantly more challenging, even with Webb.”

Based on the brightness of the planet in the mid-infrared observations, researchers suggest it could be a gas giant with a mass similar to Saturn’s, orbiting Alpha Centauri A in an elliptical trajectory. Beichman states that “the elliptical orbit of the candidate giant planet traverses much of Alpha Centauri A’s habitable zone, making it improbable for smaller rocky planets to endure.”

Follow-up Webb observations did not indicate signs of the planet; computer simulations by the team imply it was probably too close to the star in its orbit to be detected in those subsequent images. Future observations with Webb, along with NASA’s forthcoming Nancy Grace Roman Space Telescope, which is slated for launch by May 2027, should provide more insights about the planet candidate.

“If validated, the potential planet captured in the Webb image of Alpha Centauri A would signify a new landmark for exoplanet imaging endeavors,” says Sanghi. “Out of all the directly imaged planets, this would be the closest to its star seen to date. It’s also the most akin in temperature and age to the giant planets in our solar system and nearer to our home, Earth,” he adds. “Its presence in a system of two closely spaced stars would challenge our understanding of how planets form, survive, and evolve within chaotic surroundings.”

Read the complete story on the James Webb Space Telescope website.

This study was supported by the US National Science Foundation and NASA.

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