Astronomers using the James Webb Space Telescope (JWST) may have spotted a Saturn-like planet orbiting Alpha Centauri A, the nearest Sun-like star to Earth at just over four light-years away. If verified, this discovery would be groundbreaking, as it would represent the closest known planet orbiting within the habitable zone of such a star.

The finding was discussed in an interview between SETI Institute senior astronomer Dr. Franck Marchis and Dr. Julien Girard of the Space Telescope Science Institute (STScI). The candidate planet was observed with JWST’s Mid-Infrared Instrument (MIRI), which operates at 15 microns – a part of the spectrum blocked from Earth by atmospheric water vapor. Using MIRI’s coronagraph, which blocks the overwhelming starlight, researchers teased out the faint planetary signal.

Alpha Centauri is a three-star system: Alpha Centauri A, Alpha Centauri B, and Proxima Centauri, already known to host planets. The candidate object appears to orbit Alpha Centauri A at around 2 astronomical units (AU) – twice Earth’s distance from the Sun – with a temperature near 225 K (−50 °C). Its brightness suggests it is a gas giant between Saturn and Jupiter in size.

Detecting such planets is notoriously difficult. Signals are weak, and instrumental artifacts can mimic them. To confirm a candidate, astronomers typically re-image the object at different telescope orientations and track its position over time. JWST collected three datasets between 2024 and 2025: the planet appeared once but not in the other two, possibly because it moved too close to the coronagraph’s blind spot. However, earlier observations from the Very Large Telescope also recorded a similar signal, adding weight to the case.

If real, the planet would shed light on planetary dynamics in binary star systems. Since Alpha Centauri A and B orbit each other at 11–35 AU, stable planetary orbits are limited. A gas giant at 2 AU could influence the formation and stability of smaller, potentially habitable worlds.

While unlikely to host life itself, such a planet’s moons could be intriguing habitats. Ongoing JWST monitoring, along with radial velocity techniques, will be crucial for confirmation. Future observatories – such as NASA’s Roman Space Telescope, Project Blue, and the proposed Habitable Worlds Observatory – promise even sharper views.

For more details, read the full article by SETI Institute.