“We are faced with the case of a disappearing planet!” explained Aniket Sanghi of the California Institute of Technology, whose words translated both the excitement and disappointment of one of the most technically challenging exoplanet searches ever undertaken.
The James Webb Space Telescope’s Mid-Infrared Instrument (MIRI) has produced what may be a watershed moment in the direct imaging of exoplanets: a dim, Saturn-mass planet provisionally called Alpha Centauri Ab, orbiting Alpha Centauri A the nearest Sun-like star to the Earth in its habitable zone. At only 4.25 light-years distant, the Alpha Centauri system has been the most beguiling quarry for astronomers, but its brightness and intricate triple-star dynamics have prevented its planets from being seen.
The August 2024 detection required a custom observing sequence, designed specifically to overcome the glare of Alpha Centauri A and the contaminating light from its binary companion, Alpha Centauri B. MIRI’s coronagraphic mask blocked the primary star’s light, while advanced subtraction techniques removed the residual halo from both stars. What remained was an object more than 10,000 times fainter than Alpha Centauri A, sitting roughly two astronomical units away about twice the Earth-Sun distance.
February and April 2025 follow-up observations did not recover the signal. Instead of ruling out the discovery, the team simulated millions of orbits, including both the Webb data and a 2019 candidate sighting by the Very Large Telescope. The simulations revealed that in approximately half of the stable orbital arrangements, the planet would have passed too close to its star within those later observation windows, making it undetectable by Webb’s telescopes.
If proven, Alpha Centauri Ab would break several records: the nearest directly imaged planet to a Sun-like star, most temperature- and age-compatible with our own gas giants, and the closest such world to our own. Its membership in a binary system would also challenge current models of planet formation. Under such conditions, tidal perturbations from the companion star can truncate protoplanetary disks, rendering the existence of massive planets in the habitable zone a mystery.
The candidate’s calculated mass the equivalent of Saturn’s 95 Earth masses and its elliptical orbit of one to two astronomical units point toward a gas giant, not a rocky Earth twin. While that rules out a habitable surface, gas giants in our Solar System host dozens of moons, some with subsurface oceans. As exoplanet researcher Mary Anne Limbach explained, “Moon formation around giant planets generally should be quite common.” A large enough moon might even preserve an atmosphere and liquid water, although as Columbia University’s David Kipping warned, “you need this planet to have an unexpectedly big moon” in order for it to approach being Earth-like.
The Webb observations also searched for exozodiacal dust tiny comet and asteroid debris that may masquerade as planetary signals and hide small worlds. No dust was found, but the detection limit fell to levels ten times lower than any previous measurement for this star, opening the way to future surveys for rocky planets in the system.
Direct imaging is still the most difficult technique in exoplanet hunting, especially for bright stars near Earth. Most previously known exoplanets were discovered indirectly, through transit dimming of starlight or gravitational wobble caused by orbiting planets. Webb’s capability to directly image a planet so close to a host star assuming the detection is confirmed would be a turning point. As NASA’s Jet Propulsion Laboratory’s Charles Beichman said, “This would become a touchstone object for exoplanet science, with multiple opportunities for detailed characterization by Webb and other observatories.”
Those observatories will soon be joined by NASA’s Nancy Grace Roman Space Telescope, launching in 2027, which will add visible-light pictures to Webb’s infrared measurements. Collectively, they might characterize Alpha Centauri Ab’s size, reflectivity, and orbit, and potentially even spot moons.
For now, the faint point of light seen in August 2024 remains an enigma a possible world in the “Goldilocks zone” of our nearest stellar neighbor, awaiting its reappearance in the coming years. As Sanghi reflected, “Of all the directly imaged planets, this would be the closest to its star seen so far… and nearest to our home, Earth.”
