AAS Nova

First Detection of a Rocky Exoplanet’s Secondary Eclipse by Hubble

By on Features
Share:
Illustration of a lava world exoplanet
Illustration of a lava world exoplanet orbiting close to its host star. [ESO/L. Calçada; CC BY 4.0]

The Hubble Space Telescope has detected the secondary eclipse of a rocky exoplanet for the first time. These observations place constraints on the possible atmosphere of the lava world TOI-2431b.

Valuable Eclipses

Thousands of exoplanets have been discovered via the transit method, which captures the temporary dimming of a star’s light caused by a planet passing in front of a star from our perspective. Far fewer planets have been spotted passing behind their stars — what’s known as a secondary eclipse — creating a much subtler dip in the combined light from the star and the planet.

A planet’s secondary eclipse encodes valuable information about its dayside temperature, the reflectivity of its surface, and the properties of its atmosphere. Recently, Sebastian Zieba (Center for Astrophysics | Harvard & Smithsonian) and collaborators reported on Hubble Space Telescope observations of the extreme exoplanet TOI-2431b, marking the first time that Hubble has detected the secondary eclipse of a rocky exoplanet.

A First for Hubble

phase curve of TOI-2431b and its host star

The combined light from TOI-2431b and its host star over all orbital phases. The secondary eclipse is located at 0.00. Click to enlarge. [Adapted from Zieba et al. 2026]

TOI-2431b orbits its K-type host star every 5.4 hours, making it the shortest-period planet with a measured mass (6.2 Earth masses) and radius (1.54 Earth radii). It’s expected to be a lava world: a rocky planet with such a high temperature that all or part of its surface is molten.

These planets orbit their host stars so closely that searing radiation and stellar winds have long since stripped away their primordial atmospheres, but observations have demonstrated that these planets can have atmospheres — albeit deeply un-Earth-like ones composed of superheated rock vapor.

plot of dayside brightness temperature as a function of maximum dayside temperature for rocky exoplanets

TOI-2431b (purple star) stands out from other lava worlds in terms of its dayside brightness temperature. The light purple star shows the estimated dayside temperature for an arbitrary nonzero albedo. [Zieba et al. 2026]

Zieba and coauthors used Hubble to observe TOI-2431b and combined these near-infrared observations with visible-light data from the Transiting Exoplanet Survey Satellite (TESS), which first detected TOI-2431b in 2019. The planet’s secondary eclipse suggests a dayside brightness temperature of 2524K — hot enough to melt many types of rocks — and this temperature is roughly equal to expectations for a world that absorbs all incident energy and redistributes none of it. This interpretation would make TOI-2431b an outlier among the lava worlds observed so far, which generally appear to have a mechanism — a reflective surface, a cloudy atmosphere, or a way to transport heat efficiently from day to night — acting to lower their dayside temperatures.

Atmosphere or Not?

simulated emission spectra for TOI-2431b

Modeled optical-to-infrared emission spectrum of TOI-2431b. The no-atmosphere and rock-vapor atmosphere models are indistinguishable with current Hubble and TESS data (red and blue circles) but may be distinguishable with JWST (simulated observation plotted with a green diamond). Click to enlarge. [Zieba et al. 2026]

These observations don’t definitively rule out an atmosphere or a somewhat reflective surface, since at the near-infrared and visible wavelengths to which Hubble and TESS are sensitive, it’s not possible to disentangle the thermal emission from the planet and the starlight reflected by the planet. Zieba and collaborators modeled a possible rock-vapor atmosphere for TOI-2431b, which showed that the planet’s emission with and without an atmosphere is similar at the wavelengths probed by Hubble and TESS.

While the current data are consistent with TOI-2431b having no atmosphere, JWST will push our observations into the wavelength realm in which a possible rock-vapor atmosphere could be detected. This work represents an exciting first for Hubble, demonstrating its ability to place constraints on hot rocky planets and setting the stage for further insights from JWST.

Citation

“The First Hubble Detection of a Secondary Eclipse from a Rocky Exoplanet: The 5.4 hr Planet TOI-2431 b,” Sebastian Zieba et al 2026 ApJL 1011 L32. doi:10.3847/2041-8213/aeb4ab