The Hubble Space Telescope has caught a lava world slipping behind its star — the first time Hubble has detected the secondary eclipse of a rocky exoplanet, a measurement that lets astronomers take the temperature of a world they will never visit.
The planet is TOI-2431b, a lava world with 6.2 Earth masses and 1.54 Earth radii, orbiting a K-type star every 5.4 hours — the shortest-period planet with a measured mass and radius. Its dayside brightness temperature is 2,524 Kelvin, about 2,251°C, consistent with a world that absorbs all the incoming energy from its star and redistributes none of it to the night side. The data show zero nightside emission.
Watching a planet disappear
A secondary eclipse is the moment a planet passes behind its star, and for a brief window the star’s light is all there is — the planet’s own glow vanishes from the total. Hubble measured it with its WFC3 G141 near-infrared phase curve, combined with visible light from TESS, which first detected the planet in 2019. The work was led by Sebastian Zieba of the Center for Astrophysics | Harvard & Smithsonian, and published in the Astrophysical Journal Letters on 30 October 2026.
What the data do not rule out is a rock-vapour atmosphere or a reflective surface — both remain possible, and follow-up with the James Webb Space Telescope’s MIRI instrument is planned. The measurement is a first, not a final answer.
A world of molten rock
TOI-2431b is a world where the dayside is hot enough to melt rock, and the nightside, as far as Hubble can tell, gives off nothing at all. It is the kind of planet that makes the familiar categories — rocky, gaseous, habitable — look thin. Hubble has spent decades looking outward; this time it looked at a planet vanishing behind its star, and measured the heat of a world that never cools down.
The measurement matters because it turns a distant point of light into a place with a temperature, a dayside and a nightside. TOI-2431b will never be visited, and no telescope will ever photograph its surface. But Hubble watched it disappear behind its star and measured the heat it left behind — 2,524 Kelvin on the dayside, nothing on the night side — and that is enough to know what kind of world it is.
