Lava World TOI-561b Reflects Starlight Like Venus, JWST Phase Curve Reveals

A planet so close to its star that a full year lasts barely ten hours — and whose surface is a global ocean of molten rock — has revealed an atmosphere it was never supposed to have, and a reflectivity that matches Venus. The James Webb Space Telescope's first complete orbit-by-orbit observation of the ultra-short-period rocky world TOI-561b returned a Bond albedo of 0.74, meaning the planet reflects three-quarters of all incoming starlight back into space — a figure almost identical to Venus's 0.76. The proposed explanation is high-altitude silicate clouds — fine particles of vaporized rock condensed at altitude. The study, led by Samuel Boucher of the Université de Montréal and Prof. Lisa Dang of the University of Waterloo, was published October 27, 2026 in The Astronomical Journal.

The Phase Curve Technique That Made This Measurement Possible

Most exoplanet atmosphere studies rely on a secondary eclipse — a snapshot of the planet's thermal emission captured as it disappears behind its star, revealing only the hot dayside. A phase curve does something more ambitious: it tracks the planet continuously through multiple full orbits, building a complete map of how brightness and temperature shift as different hemispheres rotate into the telescope's line of sight.

Because TOI-561b completes its orbit in just 0.44 days, a 37-hour continuous JWST observation covers nearly four full passes — enough to characterize not only the dayside but also the nightside, the transition zones, and the efficiency with which the atmosphere transports heat from one hemisphere to the other. The team used JWST's NIRSpec instrument in G395H mode, covering 3 to 5 micrometers of near-infrared light, as part of observing program GO-3860 led by Dr. Johanna K. Teske of the Carnegie Institution for Science.

Silicate Cloud Sunscreen Above a Magma Ocean

JWST's measurement puts TOI-561b's dayside brightness temperature at roughly 2,050 Kelvin (about 1,780°C), according to the Boucher et al. study. If the planet were a bare rock with no atmosphere to transport heat, that value should be approximately 3,200 K. A 1,150-degree deficit points to two processes running at once: an atmosphere reflecting some of the incoming starlight before it ever reaches the surface, and atmospheric circulation moving heat from the permanent dayside to the permanent nightside.

The leading candidate for the reflective layer is silicate mineral clouds. At the temperatures prevailing above TOI-561b's magma ocean, silicon dioxide (SiO₂) and magnesium silicate (MgSiO₃) can vaporize from the molten surface and recondense as fine mineral particles at higher, slightly cooler altitudes — particularly near the day-night terminator, where conditions are marginally less extreme. Those particles scatter near-infrared light efficiently, acting, as the research team describes it, like sunscreen for the molten rock below. Co-author Dr. Anjali Piette of the University of Birmingham has noted that the planet's persistence challenges the prevailing expectation that ultra-short-period rocky worlds should be depleted of any substantial atmosphere by the intense irradiation they receive.

A significant tension remains in the numbers. General circulation models tested by the team — using multiple candidate atmospheric compositions and surface pressures from 0.1 to 10 bar — suggest that silicate condensate clouds can account for a Bond albedo of roughly 0.22. The observed figure is 0.74, more than three times larger. Filling that gap requires physics not yet captured by current models: perhaps a higher cloud fraction than the models produce, additional scattering from yet-unidentified species, or a bulk atmospheric composition that itself contributes to the reflectivity. The paper does not resolve this discrepancy; it establishes it as the central open question.

Why a Lava World Should Have No Atmosphere at All

Ultra-short-period rocky planets like TOI-561b sit at the extreme end of irradiation. At less than one-fortieth of Earth's orbital distance, the planet receives a continuous bombardment of extreme ultraviolet and X-ray radiation powerful enough to ionize atmospheric gases and accelerate them past escape velocity. TOI-561 itself is approximately 10 billion years old — roughly twice the age of our Sun — giving it twice as long as Earth to shed any primordial atmosphere.

The most likely explanation is continuous volatile replenishment from the interior. A global magma ocean would release dissolved volatiles through outgassing at the same time radiation is eroding the upper atmosphere — a dynamic equilibrium between supply and loss that could sustain an envelope for billions of years if the outgassing rate keeps pace. A December 2025 paper by Teske et al., published in The Astrophysical Journal Letters, made the initial case for a thick atmosphere from the dayside emission spectrum alone; Boucher et al. 2026 extends that result with the full orbit phase curve, adding confirmation of global heat redistribution — which requires atmospheric circulation — and measuring the Bond albedo for the first time.

Where TOI-561b Sits Among Confirmed Lava World Atmospheres

Until 2024, no rocky exoplanet outside our solar system had a confirmed atmosphere. JWST changed that with 55 Cancri e, a similarly scorching super-Earth completing one orbit about every 17 hours, whose heat redistribution pattern is consistent with an atmosphere redistributing thermal energy across hemispheres. TOI-561b now joins it as the second rocky exoplanet confirmed to carry an atmosphere — and the first for which a Bond albedo has been derived from a full-orbit phase curve.

K2-141b offers a contrasting case: that lava world appears to host a rock-vapor atmosphere of SiO gas evaporated directly from the surface rather than a bulk volatile envelope, producing a qualitatively different mid-infrared signature — and suggesting that TOI-561b's volatile-rich atmosphere reflects a distinct formation history or interior chemistry.

The Venus comparison deserves precision. Venus achieves its 0.76 Bond albedo through a planet-wide deck of sulfuric acid droplets. TOI-561b reaches nearly the same figure through mineral condensate clouds above a magma ocean surface that would vaporize sulfuric acid instantly. Two very different planets, separated by 280 light-years, arrive at the same observational answer through entirely different physics.

What Future Observations Will Try to Determine

The paper's central unresolved question, as Dang has framed it, is whether TOI-561b's atmosphere "resembles Venus or an extreme Io." A Venus analog would be dominated by CO₂ or SO₂ — stable volatiles that can persist in a hot, dense atmosphere and could explain the high reflectivity through bulk scattering. An Io analog would be sustained primarily by active volcanism, with SO₂ released from eruptions replacing gas continuously lost to space.

Answering the composition question requires observing at longer wavelengths. JWST's MIRI instrument covers 5 to 28 micrometers, where the molecular fingerprints of CO₂ and SO₂ are resolved more clearly than in the NIRSpec near-infrared window, and where mid-infrared data can test whether the factor-of-three Bond albedo gap reflects missing cloud physics, a dominant non-silicate scatterer, or a bulk atmospheric composition the near-infrared alone cannot discriminate.

The Rocky Worlds Director's Discretionary Time Workshop at STScI, running November 16–18, 2026 in Baltimore, will present comparative results from JWST's broader lava world program. Whether TOI-561b's reflective atmosphere proves to be a quirk of its ancient system or a common property of the ultra-short-period class will depend on how many more full-orbit phase curves the community can secure — and whether the models can close the gap between 0.22 and 0.74.