JWST finds lava world TOI-561 b wrapped in silicate clouds that bounce back 75 percent of its starlight

TOI-561 b is a rocky planet that orbits a star roughly 280 light-years from Earth, and it does so on a brutally short leash. The planet completes one circuit in about 0.44 days, and tidal forces have locked its rotation so that one hemisphere faces the star without end while the other never sees daylight. It is a little larger and heavier than Earth, with a mass of 2.24 Earth masses and a radius of 1.42 Earth radii, and its bulk density of 4.30 grams per cubic centimeter points to a body built largely of rock and metal rather than gas.

Conditions on the star-facing hemisphere are extreme even by the standards of close-in planets. The dayside temperature is 2,048 kelvin, about 1,775 degrees Celsius, hot enough to keep the surface molten. For years the standard picture held that a world in this position should be a stripped, airless cinder. A rocky planet subjected to that much irradiation is expected to lose any primordial atmosphere to escape, and TOI-561 b has had a very long time to lose one. Its host star is old, at least 10 billion years old, and is a metal-poor star belonging to the thick disk of the Milky Way, which makes the planet an ancient survivor rather than a young arrival.

The first hint that something was off came from earlier observations of the dayside. It was substantially cooler than models predicted for a bare, dark rock so close to its star, roughly 2,700 kelvin expected against 2,048 kelvin measured. Two explanations were possible: the missing energy was being carried around to the nightside by an atmosphere, or it was being bounced straight back into space. Distinguishing between those options required watching the planet through a complete orbit, so that the nightside could be measured for the first time.

That is what an international team has now done with the James Webb Space Telescope. The work, led by Professor Lisa Dang at the University of Waterloo in Ontario, Canada, and published in The Astronomical Journal, uses a phase curve to track how the planet brightness and heat change as it circles its star. The paper is titled A Highly Reflective Atmosphere on the Lava World TOI-561 b Revealed by JWST/NIRSpec Phase Curve and carries the DOI 10.3847/1538-3881/ae9763.

Key Facts

The core measurement is a Bond albedo of about 0.74, with an uncertainty range of plus 0.08 and minus 0.10. In plain terms, TOI-561 b bounces back roughly 75 percent of the starlight that falls on it, a reflectivity comparable to Venus in our own solar system. The campaign ran for 37 hours of continuous JWST/NIRSpec observation between 2024 May 1 and 2024 May 3, covering four secondary eclipses and three transits, nearly four complete orbits of the planet.

ScienceDaily reported on October 27, 2026, that the planet appears to keep a substantial atmosphere by reflecting about 75 percent of its starlight, possibly through mineral clouds acting as a sunscreen. The nightside measurement is the decisive piece: it came in at 1,089 kelvin, not warm enough to account for the energy missing from the dayside. Heat transport alone cannot close the budget, so reflection has to make up the difference.

The University of Birmingham reported on October 27, 2026, that the team found evidence of a substantial global atmosphere, explicitly not a habitable one, that reflects a significant amount of incoming starlight. Computer climate simulations run by the collaborators suggest that clouds of silicate minerals, including silicon dioxide (SiO2) and magnesium silicate (MgSiO3), could form on the dayside and act like sunscreen. These are the same family of compounds that make up quartz and sand.

The University of Waterloo reported on October 26, 2026, that Dang described the phase curve as showing both a significant amount of reflected starlight and redistribution of heat around the planet rather than only on the sunlit side, which together point to an atmosphere. Phys.org reported on October 26, 2026, that the results build on earlier evidence that TOI-561 b possesses a thick atmosphere likely replenished by gases released from its molten interior. The composition remains unknown: it could be dense carbon dioxide, or sulfur dioxide of the kind released by volcanic moon Io at Jupiter.

Analysis

What this really means is that the dividing line between planets that keep atmospheres and planets that lose them is not drawn by irradiation alone. TOI-561 b sits deep inside the regime where escape should win, and it has had at least 10 billion years for stripping to run its course. Yet the phase curve says an atmosphere is there, and that it is doing two distinct jobs at once: reflecting energy before it can heat the surface, and moving whatever energy does arrive from the dayside to the nightside.

The reflective cloud layer is the most economical explanation for both signals, but it is not the only conceivable one, and the team is careful to say so. An unusually large initial supply of gases could also explain survival, if the planet started with enough volatiles to outlast billions of years of loss. Anjali Piette, an assistant professor in astronomy at the University of Birmingham, framed the choice directly: whether the planet began with an unusually large reservoir of gas, or whether the ocean of molten rock covering its surface acts as a planet-wide volcano that keeps feeding the atmosphere from below.

The second option is the more provocative, because it turns the magma ocean from a symptom of loss into a mechanism of persistence. A molten surface in contact with a thin envelope can buffer that envelope, releasing gases as pressure drops and reabsorbing them as it rises. If that is what is happening at TOI-561 b, the planet is not a relic that barely survived; it is an active chemical system with a rock cycle standing in for a water cycle.

The bigger picture here is that JWST has moved exoplanet atmospheres from a yes or no question to a question about maintenance. A detection used to be the end of the story. Here the detection is the beginning of an argument about budgets: how much energy arrives, how much leaves, how much is stored, and which reservoir supplies the gas. Those are the kinds of questions astronomers previously could ask only about planets in our own solar system.

Why It Matters

The result matters first because it widens the range of worlds that can hold an atmosphere. Rocky planets around old, metal-poor stars in the thick disk are among the most common environments in the galaxy, and the assumption that they are airless has shaped how astronomers interpret surveys of small planets. A confirmed reflective atmosphere on one of them means that a non-detection elsewhere is now ambiguous: it may mean no atmosphere, or it may mean an atmosphere that is hard to see.

It also matters for how reflective clouds are treated in climate models of hot rocky planets. Silicate clouds have long been invoked in theory as a thermostat, and TOI-561 b provides a Bond albedo, about 0.74, that such models can be tested against rather than merely tuned to. The same number constrains the dayside energy budget, the nightside temperature of 1,089 kelvin, and therefore the strength of any heat redistribution.

Finally, the finding matters because it makes the composition question urgent rather than academic. Carbon dioxide and sulfur dioxide produce different signatures, and telling them apart would say whether TOI-561 b is a hotter Venus or something closer to a scaled-up Io. Either answer would be the first direct look at the chemistry of a magma ocean atmosphere on a planet beyond the solar system.

Next Up

The immediate next step is spectroscopy of the atmosphere itself. The phase curve establishes that an atmosphere exists and how reflective it is, but it does not pin down what the gas is made of. Follow-up observations designed to detect molecular features would test the carbon dioxide and sulfur dioxide scenarios, and would also probe whether the silicate cloud layer leaves its own imprint on the spectrum.

Beyond that, the natural move is to look for the same signature elsewhere. If reflective silicate clouds can keep a heavily irradiated planet alive for 10 billion years, other ultra-hot rocky worlds should show comparably high albedos. Measuring a handful of them would turn a single surprising planet into a population, and would show whether TOI-561 b is an exception or the first clear example of a class.