Magma Oceans and Rocky Atmospheres: What JWST Is Teaching Us About TOI-561 b

This artist’s illustration depicts the ultra-hot super-Earth exoplanet TOI-561 b as inferred from observations by NASA’s James Webb Space Telescope and other facilities. The data indicate that the planet is enveloped by a dense atmosphere lying above a planet-wide magma ocean.

Astronomers have long assumed that rocky planets orbiting extremely close to their stars are stripped bare of any atmosphere. The intense heat and radiation should quickly erode gases, leaving behind exposed rock. Recent observations from the James Webb Space Telescope (JWST) suggest that this assumption may be incomplete.

TOI-561 b, an ultra-hot “super-Earth,” appears to retain a substantial atmosphere despite surface conditions hot enough to melt rock. If confirmed, this would be the strongest evidence yet that molten rocky planets can hold on to thick atmospheres for billions of years.

TOI-561 b Under Extreme Conditions

TOI-561 b was discovered in 2020 by NASA’s Transiting Exoplanet Survey Satellite (TESS). It lies about 280 light-years from Earth and is roughly 1.4 times Earth’s radius. What makes it remarkable is its orbit: the planet circles its star once every 10.6 hours, placing it extraordinarily close—much closer than Mercury is to the Sun.

Because of this proximity, TOI-561 b is tidally locked. One hemisphere permanently faces the star and experiences extreme heat, while the opposite side remains in constant darkness. Daytime temperatures are high enough to melt silicate rock, forming what scientists believe is a global magma ocean.

The host star itself is unusual. It is very old—around 10 billion years—and formed in a region of the Milky Way with fewer heavy elements than our Sun. Planets formed in such environments are expected to be dense and iron-rich. Yet TOI-561 b is less dense than Earth, suggesting that lighter materials or gases are contributing to its size.

JWST Sees Something Unexpected in TOI-561 b

In 2024, astronomers observed TOI-561 b using JWST’s Near-Infrared Spectrograph. By measuring infrared light emitted from the planet’s dayside as it passed behind its star, they could estimate its surface temperature.

A bare rocky surface should be extremely hot under these conditions. Instead, the observed temperature was hundreds of degrees cooler than expected. The most likely explanation is the presence of an atmosphere that redistributes heat from the dayside to the nightside.

The planet’s infrared spectrum also does not match what scientists would expect from exposed rock alone. While specific molecules have not yet been clearly identified, the data are consistent with a thick atmosphere containing volatile gases, possibly including water vapor, along with high-altitude clouds made of vaporized rock components.

An emission spectrum from JWST’s NIRSpec reveals 3–5 micron infrared light from TOI-561 b, with model comparisons indicating the presence of a volatile-rich atmosphere rather than a bare rocky surface. Source: https://esawebb.org/

A Planet Covered in Molten Rock

The key to TOI-561 b’s atmosphere may lie in its molten surface. At such high temperatures, much of the planet’s outer layer is likely liquid rock. This magma ocean can release gases into the atmosphere, while at the same time absorbing gases back into the melt.

This continuous exchange may allow the planet to maintain its atmosphere even as stellar radiation tries to strip it away. In other words, the magma ocean acts as a long-term reservoir, replenishing atmospheric gases over time.

This idea challenges earlier models, which predicted that planets on such short orbits would quickly lose any volatile material. TOI-561 b suggests that molten rocky planets may be far more resilient than previously thought.

Why This Discovery Matters

Understanding TOI-561 b helps scientists explain why some rocky exoplanets appear larger and less dense than expected. Their extra size may come not from thick interiors, but from substantial atmospheres.

More broadly, this planet offers a glimpse into how rocky worlds may have behaved in the early universe, when stars formed with different chemical compositions than today. It also expands the known diversity of planetary environments, reminding us that “rocky planet” does not always mean Earth-like.

TOI-561 b is not habitable in any familiar sense. Its surface conditions are far too extreme for life as we know it. However, studying such worlds helps astronomers refine models that will eventually be used to interpret cooler, more Earth-like planets.

What Comes Next

Artist’s impression of the ultra-hot super-Earth TOI-561 b, likely featuring a global magma ocean beneath a thick, heat-trapping atmosphere.

Future observations with JWST may be able to identify specific gases in TOI-561 b’s atmosphere and determine how thick it truly is. Astronomers will also compare it with other ultra-short-period planets to see whether magma-supported atmospheres are common or rare.

TOI-561 b shows that even in the most hostile environments, planets can behave in unexpected ways. As JWST continues its mission, it is likely to reveal many more worlds that challenge our assumptions about how planets form, evolve, and survive.

If discoveries like TOI-561 b spark your curiosity, ARC Educator offers a way to dive deeper into space and astrophysics without needing an advanced background. Through clear explanations, hands-on activities, and guided learning, ARC Educator helps students and space enthusiasts explore topics like exoplanets, stars, cosmology, and modern astronomy in an engaging and accessible way. It’s designed for anyone who wants to understand how today’s discoveries are made—and why they matter—while keeping the science exciting and approachable.

-Sayyed Danish

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