Webb data indicates atmospheric gases above a molten rocky exoplanet

Observations from the James Webb Space Telescope have provided the strongest evidence yet for an atmosphere surrounding the ultra-hot rocky exoplanet TOI-561 b.

The planet, classified as a super-Earth, has a radius approximately 1.4 times that of Earth and orbits a Sun-like star around 275 light-years from Earth.

TOI-561 b completes an orbit in less than 11 hours, placing it in the category of ultra-short period exoplanets.

Measurements from Webb’s Near-Infrared Spectrometer (NIRSpec) indicate that the planet is covered by a global magma ocean, with a dense layer of gases above it.

Research led by Johanna Teske from the Carnegie Institution for Science, involving multiple institutions including the Waterloo Centre for Astrophysics, Oxford University, and the Kapteyn Astronomical Institute, was reported in The Astrophysical Journal Letters on December 11, 2025.

Orbital and physical characteristics

TOI-561​‍​‌‍​‍‌​‍​‌‍​‍‌ b is a planet that shows tidal locking features; in other words, its dayside is always turned to its star, while the nightside is kept dark.

The close orbit, which is less than one-fortieth the distance between Mercury and the Sun, causes the dayside to have a temperature exceeding the melting point of rock, thus producing a molten surface.

An examination of the planet's mass and radius reveals that it has a low bulk density. Therefore, scientists propose that this could be due to a small iron core and a mantle composed of lighter rock than Earth.

The star around which the planet orbits is approximately 10.5 billion years old and has a low iron content; thus, TOI-561 b must have formed in a different chemical environment than the Solar System.

Teske referred to the planet as being formed in the Milky Way region known as the thick disk. Thus, it was the Universe when it was relatively young that provided the conditions for the planet's formation.

Observational techniques

To study the planet’s atmosphere, Webb observed TOI-561 b over 37 hours, covering nearly four complete orbits.

The team monitored the reduction in system brightness as the planet passed behind its star, a method that complements the transit technique traditionally used to detect exoplanets.

Without an atmosphere, the planet’s dayside would reach temperatures around 2,700 degrees Celsius, as heat transfer to the nightside would be minimal.

NIRSpec data showed a lower dayside temperature of approximately 1,800 degrees Celsius, suggesting heat distribution across the planet’s surface through atmospheric processes.

These measurements provide quantitative evidence that a layer of gases exists above the molten surface.

Atmospheric composition and heat transfer

The​‍​‌‍​‍‌​‍​‌‍​‍‌ NIRSpec emission spectrum captured TOI-561 b's near-infrared light and showed absorption features that matched the atmospheric gases.

According to the statement by co-author Anjali Piette of the University of Birmingham, a thick, volatile-rich atmosphere is the only explanation for the observed temperature variations.

Heat that would normally flow from the hot side of the planet to the cold side is redirected by strong winds, and gases like water vapor absorb certain wavelengths of near-infrared radiation emitted from the molten surface.

Besides that, the existence of silicate clouds, for example, can help cool the planet by reflecting the starlight coming ​‍​‌‍​‍‌​‍​‌‍​‍‌in.

Interaction between magma ocean and atmosphere

Research indicates an equilibrium exists between TOI-561 b’s magma ocean and its atmosphere.

Co-author Tim Lichtenberg of the University of Groningen explained that gases released from the magma feed the atmosphere, while some are reabsorbed into the planet’s interior.

This dynamic may explain how a small, tidally-locked planet exposed to extreme stellar radiation can retain an atmosphere.

The high volatile content of the atmosphere, necessary to produce the measured heat distribution and spectral absorption, suggests a composition distinct from terrestrial planets in the Solar System.

These findings were obtained as part of Webb’s General Observers Program 3860, with ongoing analysis aimed at determining atmospheric composition and temperature variations between the day and night sides.

Stay tuned for more updates.