JWST phase curves point to a reflective atmosphere on lava world TOI-561 b

Full-orbit observations with the James Webb Space Telescope (JWST) indicate that the ultra-short-period planet TOI-561 b has a reflective atmosphere, despite the intense radiation it receives from its star. The study, led by Samuel Boucher at the Université de Montréal and published in The Astronomical Journal, reports that clouds or haze probably reflect much of the incoming starlight. The result confirms earlier hints of an atmosphere and sharpens a puzzle for models of atmospheric loss.

An unlikely place for an atmosphere

Ultra-short-period (USP) planets, found in large numbers by Kepler and TESS, complete an orbit in under a day. Their proximity to their stars means they are bathed in extreme radiation, and theory predicts that rocky examples should have lost any substantial gas envelope. TOI-561 b orbits in roughly 10.5 hours. A 2025 study based on limited JWST data had already suggested that it hosts an atmosphere containing volatiles such as water and carbon dioxide, based partly on its bulk density and dayside emission spectrum.

What the phase curve shows

The team observed the planet for 37 hours, covering more than three full orbits. Such a phase curve tracks how brightness and heat change as the planet circles its star, and it gave the first view of the planet's nightside.

Earlier work had found the dayside to be much cooler than expected for bare rock this close to a star. Either heat was being reflected away, or an atmosphere was carrying it to the nightside. The new data showed that the nightside was not warm enough to account for the missing energy, so roughly three-quarters of the incident light must be reflected. Comparisons with a general circulation model supported this: heat redistribution alone could not match the dayside fluxes or the amplitude of the phase curve, and the model needed a high albedo of about 0.6.

The authors suggest that clouds of MgSiO₃ and SiO₂ could form on the dayside, with SiO₂ in particular helping to explain the high reflectivity. Lisa Dang of the University of Waterloo, project lead, said the measurements show both substantial reflected light and heat being shared around the planet, which together point to an atmosphere.

Challenging escape theory

The finding sits uneasily with expectations. Atmospheric-escape theory predicts that strongly irradiated super-Earths should lack volatile envelopes. JWST surveys of rocky planets have so far been read as suggesting a population of bare rocks, although most of those targets orbit M dwarfs, which deliver less irradiation than TOI-561 b receives. TOI-561 b orbits a late G or early K dwarf, a more sun-like star.

The authors outline four possible explanations, noting that more than one may apply:

• The planet began as a volatile-rich sub-Neptune and has been gradually stripped.
• The atmosphere is secondary, and such atmospheres persist while primary hydrogen-helium envelopes are lost more readily.
• The planet migrated inwards as its young, more active star calmed, reducing its total radiation exposure.
• Volatiles accreted early were sequestered in the interior during a magma ocean phase and later outgassed.

Why it matters and what comes next

Secondary atmospheres on lava worlds may carry information about processes inside the planet that remote observations cannot otherwise reach. The authors note that the composition of such an atmosphere could constrain geochemical evolution, including the redox state of the magma ocean. That connects directly to wider questions about how rocky planets evolve and how habitable conditions arise.

The next step is to establish what kind of atmosphere this is. Dang said the team wants to learn whether it resembles Venus or an extreme version of Io, and that observing at longer wavelengths with JWST could help explain how such a hot planet has retained gas for billions of years. The cloud composition and the albedo value remain model-dependent inferences, so further spectra will be needed to test them.

Source: This oddball reflective lava world is challenging to understand (Phys.org, 1 November 2026).

Image: AI-generated illustration.