Closest Exoplanet Yet? JWST Reveals Neighboring Planet Candidate

Astronomers using the James Webb Space Telescope (JWST) have reported a potential detection of a Saturn-like exoplanet orbiting Alpha Centauri A: the closest Sun-like star to Earth, located just over four light-years away. If confirmed, this would mark a major milestone in exoplanet science, as the new planet – though likely a gas giant – would be the closest to orbit in the habitable zone of its star. 

The discussion featured astronomer and instrument scientist Dr. Julien Girard of the Space Telescope Science Institute (STScI), interviewed by SETI Institute senior astronomer Dr. Franck Marchis.

The Discovery and Observational Approach

The potential exoplanet was detected using JWST’s Mid-Infrared Instrument (MIRI) at a wavelength of 15 microns, part of the thermal infrared spectrum that is inaccessible from ground-based telescopes due to atmospheric water vapor absorption. To isolate the planet’s faint signal from its bright host star, the team employed MIRI’s coronagraph, a mask that blocks direct starlight and allows detection of objects several orders of magnitude fainter.

Alpha Centauri is a triple star system comprising Alpha Centauri A, Alpha Centauri B, and Proxima Centauri, which hosts three planets of its own. This candidate planet appears to orbit Alpha Centauri A at approximately 2 astronomical units (AU), twice the Earth-Sun distance, with an estimated temperature of about 225 Kelvin (−50 °C). Based on its observed brightness, the object is consistent with a gas giant between the sizes of Saturn and Jupiter.

Why It Remains a Candidate

High-contrast imaging of exoplanets is inherently challenging. Planetary signals are often at the limits of instrument sensitivity and can be confused with optical or detector artifacts. To confirm a detection, astronomers seek to:

• Re-image the object at multiple roll angles of the telescope to verify its position relative to the star
• Observe it at different epochs (times) to track orbital motion or rule out background stars and galaxies

In this case, the JWST team obtained three observation sets between 2024 and 2025. The planet candidate was detected once but not recovered in the other two datasets, possibly because it moved too close to the coronagraph’s inner working angle. The result gains credibility from earlier Very Large Telescope mid-infrared observations that recorded a similar source at a compatible position and brightness, suggesting both detections may be of the same object.

Scientific Implications

If confirmed, the planet’s existence would inform models of planetary stability in binary systems. Alpha Centauri A and B are separated by 11-35 AU, close enough to limit where stable planetary orbits can exist. A gas giant at 2 AU could also influence the potential habitability of inner, smaller worlds by affecting their orbital dynamics.

While a cold gas giant is unlikely to host life, its moons, if large and geologically active, could be potential habitats.

Next Steps in Observation

Further JWST observations, combined with complementary methods like radial velocity measurements (detecting stellar wobbles caused by orbiting planets), will be required to confirm the candidate. Future missions could also contribute:

• Nancy Grace Roman Space Telescope: Launching in 2027, Roman’s coronagraph instrument will pioneer space-based wavefront control, enabling direct imaging of Jupiter-sized planets in reflected light.
• SETI Institute’s Project Blue: A proposed dedicated telescope designed to directly image planets around Alpha Centauri A and B in visible light at contrasts of 10⁻¹⁰ after prolonged observation.
• Habitable Worlds Observatory: A future flagship mission aiming to directly image and characterize Earth-like planets, building on JWST and Roman technologies.

Why Alpha Centauri Matters

Alpha Centauri’s proximity offers unmatched opportunities for follow-up study. Planets here could be prime targets for atmospheric characterization via spectroscopy, and their closeness raises the possibility of future interstellar probes. As Dr. Girard noted, “The Alpha Centauri system will always be observed. It’s our nearest laboratory for studying exoplanets in detail.”

Watch the full conversation with Dr. Julien Girard on the SETI Institute’s YouTube channel to learn more about this discovery and its implications.