Webb Spots Superhot Exoplanet That Could Host Air

NASA’s James Webb Space Telescope has detected tantalizing signs that a tiny, blazing-hot rocky exoplanet orbiting a star some 280 light-years away might be cloaked in as much as 50% water vapor — a discovery that holds implications for understanding the chemistry and conditions on hundreds of well-studied, extreme-temperature exoplanets.

A Scorched World Circling a Sun-like Star Up Close

The planet, TOI-561 b, circles a G-type star some 280 light-years from here and speeds around its orbit in about 11 hours. Ensnared in a tight orbit at less than a million miles from its star, one hemisphere is permanently drenched in the star’s light, and should have an ocean of molten rock.

Despite being relatively small — at 1.5 times the radius of Earth — TOI-561 b is astonishing because of its low density. That odd lightness, say researchers from Carnegie Science and their collaborators, can’t be explained by a mix of rock and metal that’s Earth-like. It belongs to a metal-poor, 10-billion-year-old host star that suggests primordial chemistry, but not even that fully matches the measurements without including volatiles.

A Whisper of Air on a Lava World Hints at Heat Redistribution

Webb’s infrared observations reveal that the planet’s dayside is cooler than what a bare-rock world would be at this distance.

The sunlit face should, if it were lacking air to help move heat around, soar to about 4,900 degrees Fahrenheit. Instead, the telescope measured a temperature of roughly 3,200 degrees — still infernal, hot-in-the-eyes territory but surprisingly temperate.

That mismatch indicates an atmosphere that can redistribute heat from day to night, increase the planet’s effective albedo, or both. A gassy shroud rich in volatile molecules, such as water vapor or carbon-bearing gases, or even rock vapor from the molten surface, might be responsible for the “cooler-than-expected” signal. Reflective clouds of silicate or metal oxides, which could condense in such environments according to laboratory studies and hot-planet models, might further dull the dayside’s heat.

The study, published in The Astrophysical Journal Letters by an international team including Carnegie Science’s Johanna Teske and the University of Birmingham’s Anjali Piette, characterizes TOI-561 b as a “wet lava ball” — a world in which gases could be continuously exchanged between a magma ocean and a thick atmosphere chock-full of gases from evaporating volatile materials.

How Webb Measured the Planet’s Temperature

To study the planet’s heat, the team employed a technique called secondary eclipse: Webb observed small dips in infrared light when the body orbited behind its star, allowing them to capture the planet’s thermal glow by itself.

This creates a brightness temperature and, when combined with the orbit geometry, enables scientists to test how effectively heat is escaping from the dayside of an exoplanet (and the corresponding amount of energy that makes it to the night side).

For a tidally locked, atmosphereless, rocklike planet with low reflectivity, theory predicts a scorching dayside. The cooler temperature indicates major heat transport or a greater-than-expected reflectivity, both signs of an atmosphere. Phase-curve observations — watching the planet’s glow change as it moves in its orbit — will map hot and cool areas, providing a next step to probe winds and cloud patterns.

Why It Matters for Rocky Planets Under Extreme Radiation

In the hunt for exoplanets, it might sound like a good idea to be on the lookout for smaller ones: “It’s easier to find that needle in a haystack,” says Tom Louden from the University of Warwick.

Unfortunately, small planets around their stars get pounded by radiation that can obliterate their atmospheres through hydrodynamic escape. Taken together, measurements of how much light an exoplanet blocks and how much energy the star causes it to absorb can indicate whether these effects come into play in such cases. You’d think, conventional wisdom says, that most should be airless husks. TOI-561 b upends that view, suggesting that on rocky worlds, thick atmospheres can either stick around or be resupplied quickly even under extreme irradiation.

One possible mechanism is recycling from lava to air and back again. Gases dissolved in magma can outgas to replenish the atmosphere as high-flying molecules escape into space. Such a “magma–atmosphere conveyor belt,” long theorized for ultra-hot super-Earths, gets real traction from Webb’s data.

The result provides a key data point for a small but growing class of hellish worlds, cases like 55 Cancri e and K2-141 b in which lavas, rock vapors, and weather may become entwined. For habitability science, these planets are high-pressure (literally) laboratories: Figuring out how atmospheres survive in the harshest conditions will help scientists understand how more moderate, Earth-like planets can maintain air over billions of years.

A Webb-powered leap in mapping ultra-hot rocky exoplanets

JWST, supported by NASA, ESA, and CSA, has already revolutionized exoplanet chemistry — making headlines by sniffing carbon dioxide, sulfur dioxide, and water in the atmospheres of giant planets. Pushing into Earth-size territory is much more difficult: the signals are faint, and thermal contrast is slight. “We have successfully demonstrated that TOI-561 b is within the photometric precision domain for rocky planets around bright stars against scattered light and low-level systematic noise,” the authors say.

Broader context is blossoming: Of the 5,500 confirmed planets on the NASA Exoplanet Archive, only a small fraction are currently small and close enough for these atmospheric tests. Every new measurement of these rocky worlds further constrains models of mass, radius, composition, and the physics by which atmosphere escapes from planets, helping to distinguish between truly airless planets and those wrapped up in gas.

What Scientists Will Try to Learn Next About TOI-561 b

Follow-up with Webb is designed to measure full-orbit phase curves and secondary eclipses in multiple wavebands.

Spectral imprints of water vapor, carbon monoxide, sodium, potassium, or silicon monoxide would expose the atmosphere’s blend and indicate whether clouds made of rock-forming minerals are on hand.

If the team can chart a hotspot that’s out of alignment with the substellar point — the spot directly below the star — that would provide evidence for powerful winds. Pairing this with improved estimates of mass and radius will nail down the planet’s volatile inventory and how that compares to early-formed (around metal-poor stars) worlds.

TOI-561 b is not an abode for life. Yet by demonstrating that even on a lava world, air can exist, Webb has pointed the way to a new frontier for scientists trying to learn how atmospheres come about, change, and survive — key knowledge as astronomers shift their gaze toward cooler, smaller targets in the search for something really Earth-like.