JWST finds reflective, cloudy atmosphere on lava planet TOI-561 b

Researchers used the James Webb Space Telescope to re-observe TOI-561 b and confirmed that the super-Earth lava planet possesses a dense, highly reflective atmosphere containing clouds or haze. The team announced the result on October 27, 2026, and the peer-reviewed findings appeared in The Astronomical Journal on September 29, 2026. TOI-561 b circles a sun-like star roughly 280.5 light-years from Earth in the constellation Sextans.

The planet measures about 1.4 times Earth’s radius and 2.2 times its mass, with a density near 4.3 grams per cubic centimeter. It completes an orbit every 0.44 days, or under 10.5 hours, placing it close to its host star. Surface temperatures exceed 2,000 kelvin, and the measured dayside brightness temperature is about 2,050 K, well below the roughly 3,200 K expected for a dark, airless rock.

Webb’s NIRSpec instrument followed the planet’s infrared glow across four secondary eclipses and three transits, letting Piette’s team assemble a phase curve. The researchers also modeled brightness changes from granulation, the churning of convective cells on the star’s surface, and combined the stellar and planetary signals into a dayside emission spectrum.

Observations indicate TOI-561 b reflects about three-quarters of incoming starlight, a proportion comparable to Venus. General circulation model simulations concluded that heat redistribution alone cannot match the observed dayside fluxes or phase-curve amplitude, and that a high-albedo source with albedo Ab = 0.6 suggests clouds or haze.

By observing the full orbit, the team obtained the first look at TOI-561 b’s nightside, which always faces away from the star, and found it not warm enough to account for the missing heat.

Scientists hypothesize that silicate clouds, potentially composed of silicon dioxide and magnesium silicate or metasilicate, form on the dayside and act like a sunscreen by reflecting incoming radiation. Phys.org describes the cloud layer as vaporized silicate minerals, the same family of compounds that make up quartz and sand.

The atmosphere could be a dense carbon dioxide blanket like a much hotter Venus, or rich in sulfur dioxide like volcanic gas from Jupiter’s moon Io.

Gases escaping from a worldwide magma ocean could continually refill the atmosphere, with the molten-rock surface potentially working like a planet-wide volcano. Other possible explanations include a much thicker primordial atmosphere as a sub-Neptune, formation farther from the star followed by inward migration after the star calmed, or trapping of the original atmosphere in the interior.

TOI-561, also called TYC 243-1528-1, has at least three transiting planets: TOI-561b, TOI-561c and TOI-561d. The system ranks among the oldest and most metal-poor planetary systems found in the Milky Way.

Scientists at the Carnegie Institution for Science in Washington, D.C., announced in December that TOI-561 b is a scorching hot super-Earth lava planet with a thick atmosphere, a surprise because scientists thought it too small and too close to its star to hold one. The new JWST findings strengthen earlier evidence gathered by Lisa Dang, Anjali Piette, and collaborators that the planet has a thick atmosphere that may be continually replenished by gases escaping from its molten interior.

The findings challenge atmospheric-escape theory and current models, which predict that heavily irradiated rocky ultra-short-period planets should have lost any substantial atmosphere billions of years ago.

Lisa Dang leads the project at the University of Waterloo in Ontario, Canada, while Anjali Piette is an assistant professor of astronomy at the University of Birmingham, and Samuel Boucher is the paper’s first author. The international research team included the University of Birmingham, Université de Montréal, University of Waterloo, University of Oxford, Carnegie Institution for Science and its Observatories, Massachusetts Institute of Technology, University of Sydney, University of Exeter, University of Groningen, University of Toronto, University of Hawai‘i, University of Cambridge, and Stanford University. Boucher began the project as an undergraduate research intern at the Trottier Institute for Research on Exoplanets, where Lisa Dang was a Banting Postdoctoral Researcher; she continued supervising him after joining Waterloo, and the internship project grew into the published JWST study.

EarthSky reports that researchers led by the University of Montreal in Canada said on October 27, 2026, that they used the James Webb Space Telescope to re-observe TOI-561 b, which conflicts with other sources that name Lisa Dang of the University of Waterloo as the project lead. Researchers plan to observe TOI-561 b at longer wavelengths with JWST to determine whether its atmosphere resembles Venus or an extreme version of Io and to explain how the planet kept its atmosphere for billions of years.

The paper appeared in The Astronomical Journal, volume 172, issue 4, page 239, with DOI 10.3847/1538-3881/ae9763.

EurekAlert describes the atmosphere as thick but uninhabitable, and notes that reflective silicate clouds may cool the planet and transport heat from dayside to nightside. The findings build on earlier evidence of a thick atmosphere that may be continually replenished by gases escaping from its molten interior.