For decades, astronomers assumed that small, rocky planets in precarious proximity to their stars were destined to be barren, airless worlds. The reasoning was simple: intense stellar radiation would strip away an atmosphere in quick order and leave behind exposed rock or a molten surface. But now, new observations from the James Webb Space Telescope have overturned that expectation, hinting that the ultra-hot super-Earth TOI-561 b might be blanketed by a thick, volatile-rich atmosphere above a global magma ocean.
TOI‑561 b is an extreme specimen in the exoplanet catalog. At a radius about 1.4 times Earth’s and a mass roughly twice as large, it completes an orbit in less than 11 hours. Still, its proximity to said star less than one‑fortieth that between Mercury and the Sun means it is almost certainly tidally locked, with one hemisphere in perpetual daylight. On that dayside, temperatures soar far beyond the melting point of rock, creating a continuous ocean of molten silicates. The planet’s host star is ancient, about 10 billion years old, and iron‑poor, placing it in the Milky Way’s thick disk. As lead author Johanna Teske noted, “It must have formed in a very different chemical environment from planets in our own solar system.”
In May 2024, the JWST team used its NIRSpec on TOI‑561 b, watching the system for more than 37 hours and catching four full orbits. The key moments were “secondary eclipses” where the planet passed behind its star and its own infrared light was very briefly lost. Measuring the minuscule drop in overall brightness allowed scientists to extract the planet’s thermal emission and find its dayside temperature: if it had no atmosphere, models predicted a scorching 4,900 °F (2,700 °C). But JWST found it is much cooler, ~3,200 °F (1,800 °C).
That discrepancy begged an explanation. A naked magma ocean can redistribute some heat, but without an atmosphere, the nightside would rapidly solidify, choking off substantial thermal transport. A thin rock‑vapor layer could also exist, yet its cooling effect would be too tiny. Co‑author Anjali Piette elaborated, “We really need a thick volatile‑rich atmosphere to explain all the observations. Strong winds would cool the dayside by transporting heat over to the nightside. Gases like water vapor would absorb some wavelengths of near‑infrared light emitted by the surface before they make it all the way up through the atmosphere. The planet would look colder because the telescope detects less light, but it’s also possible that there are bright silicate clouds that cool the atmosphere by reflecting starlight.”
It is surprising to see such an atmosphere on such a small, ultra‑short‑period planet. Atmospheric escape models indicate that the intense irradiation should strip volatiles on timescales well below the age of the system. A possible means of retention from such stripping is through continuous replenishment from the interior. Coauthor Tim Lichtenberg noted a dynamic equilibrium: “While gases are coming out of the planet to feed the atmosphere, the magma ocean is sucking them back into the interior. This planet must be much, much more volatile‑rich than Earth to explain the observations. It’s really like a wet lava ball.”
This finding also cuts across broader studies of magma ocean worlds and atmospheric retention. In those environments, molten surfaces and thick atmospheres can exchange material, potentially stabilizing volatile inventories against loss. Thermal redistribution models incorporating wind-driven heat transport between hemispheres predict that higher surface pressures enable more uniform temperatures. The measured cooling on the dayside of TOI-561 b fits into those scenarios with a pressure exceeding the ~10-bar threshold where complete day-night heat circulation is expected.
JWST data also puts in sharp focus the power of secondary eclipse spectroscopy for rocky exoplanets: directly sampling infrared light emitted by these worlds lets astronomers constrain both temperature and, in a series of forthcoming analyses, atmospheric composition. Mining the full dataset, the team is trying to map temperature variation across the planet and to search out spectral fingerprints of specific molecules. Such measurements would show if water vapor, carbon-bearing gases, or exotic silicate clouds dominate TOI‑561 b’s atmosphere.
TOI‑561 b is thus a vivid demonstration for science and space‑exploration enthusiasts of how advanced instruments can overthrow ingrained assumptions: instead of sterile rock, Webb has revealed a dynamic, extreme world where seas of molten lava and thick gases interact in an intricate balance-an otherworldly “wet lava ball” defying the cosmic shoreline separating airless bodies from those with atmospheres.
