Roughly 280 light-years away, a rocky planet hot enough to be called a lava world sends back about three-quarters of the energy it receives from its star. That is the reading from James Webb Space Telescope observations of TOI-561 b, a planet that circles its star in under 11 hours and that astronomers would expect to be a bare, scorched rock. Instead the data point to a bright, reflective atmosphere, and to clouds of mineral droplets as the favoured explanation.
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The figure is the paper’s own, and it carries error bars: a Bond albedo of 0.74, with a plausible range running from about 0.64 to 0.82.
A Bond albedo of 0.74, read off a 37-hour light curve
Webb’s NIRSpec instrument watched TOI-561 b continuously for 37 hours, covering nearly four orbits, and mapped its emission between 3 and 5 microns as the planet swung around its star. A record of brightness through a full orbit is called a phase curve, and this one gave a first look at the planet’s nightside. Samuel Boucher is first author of the paper, titled “A Highly Reflective Atmosphere on the Lava World TOI-561b Revealed by JWST/NIRSpec Phase Curves,” and Lisa Dang of the University of Waterloo led the project. The preprint was submitted to arXiv on 21 August 2026, revised on 28 August, and the published version is in The Astronomical Journal, volume 172, issue 4, article 239 (DOI 10.3847/1538-3881/ae9763).
The preprint’s abstract says only that the observations favour a “high Bond albedo.” The number sits in the full text: in the paper’s results and its Table 1, a fit to both NIRSpec bands together gives a Bond albedo of 0.74, with an uncertainty of +0.08 and −0.10. Bond albedo is the fraction of all incoming stellar energy a planet reflects, which is why it can be read as “about 75%” of the starlight reaching the planet, the phrasing in the University of Birmingham’s account of the work.
How the number was reached matters for how far it can be trusted. The 37-hour stretch let the team model the star’s granulation, the phase curve and the dayside emission together, and the resulting dayside spectrum is described as more robust to stellar variability and in agreement with earlier fits that used eclipses alone. The albedo itself comes from an energy-balance argument and comparison with general circulation models, which favour a high Bond albedo together with a moderate redistribution of heat around the planet, and the two quantities are fitted together.
Individual bands tell a looser story. The shorter-wavelength band alone gives 0.79 and the longer one 0.52, and slices within them range from 0.23 to 0.87.
Silicate clouds as the favoured explanation
The mineral clouds are an interpretation layered on that measurement. The paper says silicate clouds of silicon dioxide and magnesium silicate could form on the dayside near the terminator, the line between day and night, and that the high Bond albedo is “suggestive of reflective clouds.” Boucher and colleagues point to LTT 9779 b, another hot planet whose high geometric albedo of about 0.8 has been attributed to silicate clouds. In general circulation model runs, the phase curves that best match TOI-561 b are those with a Bond albedo of 0.6.
Researchers describe such clouds as a natural sunscreen, and the University of Birmingham release, carried by ScienceDaily on 27 October 2026, compares the planet’s reflectivity to that of Venus. The paper is more cautious about the mechanism than the release’s shorthand. The models need a Bond albedo above 0.5 to match the dayside flux in one band and around 0.4 in the other, and raising the albedo further still cannot reproduce the low nightside emission.
An atmosphere that should not be there
Anjali Piette, an assistant professor at the University of Birmingham and co-author, said the discovery “challenges current theories,” because a heavily irradiated rocky planet like this would be expected to have lost its atmosphere billions of years ago. The paper concludes that the planet appears to carry a global reflective atmosphere, which would require volatile exchange with its interior to be maintained. Birmingham’s account adds that gases could be continually resupplied from a global magma ocean and that heat appears to move from the dayside toward the nightside.
A separate paper is easy to confuse with this one. Birmingham’s December 2025 release on TOI-561 b reported evidence of a thick atmosphere in an earlier, separate paper; the 75% result belongs to the August 2026 phase-curve paper.
What stays unresolved
The composition of the gas is unknown, with carbon dioxide and sulfur dioxide among the candidates. Dang said observing the planet at longer wavelengths with Webb could give insight into how a planet this hot has held on to an atmosphere for billions of years, and she described the team’s measurement as how the planet’s brightness and heat changed throughout its orbit.
So the claim has three layers of different strength: a Bond albedo of 0.74, with a range from 0.64 to 0.82; clouds as the leading explanation in the models; and an atmosphere that the authors say still needs a source of replenishment. Only the first is a measurement.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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