Could the closest Sun-like star to Earth hide moon-shrouded oceans under their ice shells?
Scientists using NASA’s James Webb Space Telescope (JWST) have found what may be a Saturn-mass gas planet orbiting Alpha Centauri A, just four light-years from our world. The potential world is about 190 million miles from its star two times Earth from the Sun and therefore securely in the system’s habitable zone. While its -48°C temperature and gaseous composition make it uninhabitable, the discovery has stirred interest about the possibility of life on moons.
“While it may not be possible to live on this target that is Saturn-like and has lots of gas, it may have moons,” stated Dr. Carly Howett of the University of Oxford. “Moons are ubiquitous in our own solar system, and that would make that moon potentially really, really, habitable.” The analogy is good: Saturn’s Enceladus and Jupiter’s Europa, both icy moons with oceans hidden under the surface, are the most promising places to search for life in our own backyard.
They are still hard to find outside the Solar System, but according to theory, they should be everywhere. Fresh N-body simulations of satellite formation indicate that giant planets at the habitable zone can host Mars- to Earth-sized satellites, up to 32% of which can be habitable when tidal heating is incorporated. This ancillary source of energy due to gravitational flexing between a planet and its satellite can preserve subsurface oceans as liquid even at great distances from the heat of a star.
The Alpha Centauri candidate’s binary system location makes the find all the more intriguing. Planet formation in such an environment is notoriously difficult; the gravitational “eggbeater” effect of the companion star can destabilize the protoplanetary disk, increasing collision speeds between planetesimals and generally preventing their accretion. Yet studies by Dr. Roman Rafikov and Dr. Kedron Silsbee suggested that if initial building blocks are a minimum of 10 kilometers in diameter and the disk is nearly circular in shape, planets could still form and endure. “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 Caltech’s Aniket Sanghi, one of the co-lead authors of the Webb detection papers.
The JWST team used MIRI’s coronagraphic mask to block Alpha Centauri A’s glare, revealing an object over 10,000 times fainter than the star. The August 2024 detection was tantalizing, but follow-up observations in early 2025 failed to recover the signal possibly because the planet’s orbit brought it too close to the star to resolve. Simulations suggest an elliptical path between one and two astronomical units, consistent with the initial sighting and with a Saturn-like mass.
If confirmed, this would be the closest directly imaged planet to its star and the closest directly imaged world to Earth. “This would become a touchstone object for exoplanet science, with multiple opportunities for detailed characterization by Webb and other observatories,” wrote NASA Jet Propulsion Laboratory’s Charles Beichman. The next Nancy Grace Roman Space Telescope, launching as early as 2026, will supplement Webb’s infrared observations with visible-light measurements, so size, reflectivity, and possibly atmospheric composition may be better estimated.
For astrobiologists, the real treasure may be the planet’s potential moons. Studies of Enceladus and Europa show that thick crusts of ice can enshroud immense oceans warmed by tidal pressure and maybe by low-temperature hydrothermal vents that could persist for billions of years. Such vents, modeled on the Earth’s Juan de Fuca Ridge system, circulate water through seafloor rocks, warming it and treating it to chemicals conditions on Earth that support flourishing microbial life.
The other possible energy source is radiolysis driven by cosmic rays. On anhydrous or thin atmospheres, energetic particles can penetrate ice and break up water molecules and produce electrons that microbes can utilize. Estimates suggest that on Enceladus, this mechanism would be capable of sustaining 400 millionths of a gram per square centimeter of low-magnitude biomass, but maybe vastly significant to microbial life.
The giant’s moons, if it had any, would be prime targets for such research. In the circumstellar habitable zone, they could potentially have an equal balance of stellar radiation, planetary heat, and tidal energy. Even if their surface were covered by ice and lacked atmosphere, subsurface environments could be energy-dense and stable enough to support life. As Dr. Howett described, it’s something we might be able to see, and potentially visit, in the future.
For the present moment, the planet remains unverified, its image a fleeting glimpse of glinting light in the blinding radiance of a nearby star. But the likelihood that our closest stellar companion might possess not only one giant planet but a group of earth-like satellites ensures that Alpha Centauri A will remain the focus of exoplanetary science for the foreseeable future.
