Researchers conclude that this discrepancy is likely due to a thick atmosphere above a global magma ocean.
The atmosphere redistributes heat and absorbs near-infrared light, according to findings published in The Astrophysical Journal Letters by Johanna Teske and colleagues at Carnegie Science Earth and Planets Laboratory, the University of Birmingham, and the University of Groningen.
NASA’s James Webb observations reveal cooler dayside and a thick atmosphere on TOI-561 b
Planetary characteristics and orbital properties
According to NASA, TOI-561 b is classified as an ultra-short period super-Earth with a radius approximately 1.4 times that of Earth. Its orbital period is less than 11 hours, placing it less than one million miles from its host star.
The host star is slightly smaller and cooler than the Sun, and the planet is tidally locked, meaning the same side permanently faces the star.
The extreme proximity results in surface temperatures high enough to melt typical rock, creating conditions for a global magma ocean.
The planet’s low density has been attributed to either a small iron core, a mantle composed of less dense rock, or the presence of a surrounding atmosphere.
TOI-561 b orbits an iron-poor star in a region of the Milky Way called the thick disk, and the star’s estimated age is approximately twice that of the Sun.
The planet’s composition may reflect chemical conditions present in the early universe.
Its mass and size place it among a small group of ultra-short-period rocky exoplanets for which direct atmospheric measurements are possible with current observatories.
Atmospheric evidence from Webb observations
To determine the presence of an atmosphere, the research team used Webb’s Near-Infrared Spectrograph (NIRSpec) to measure the emission spectrum of TOI-561 b as it passed behind its star.
The technique identifies the planet’s thermal emission by monitoring the decrease in brightness when the planet is eclipsed by the star.
Results indicate that the dayside temperature is lower than predicted for a bare rock, supporting the hypothesis of a volatile-rich atmosphere.
The atmosphere may consist of gases such as water vapor and other volatiles, which absorb portions of near-infrared light and distribute heat across the planet, as described by Anjali Piette of the University of Birmingham.
The observations also suggest that the atmosphere is thick enough to affect the planet’s apparent radius, making it appear larger than it would as a bare rocky body.
Heat redistribution and atmospheric dynamics
According to NASA, the cooler temperature on the dayside indicates that the heat from the dayside is being carried over to the nightside.
Firstly, intense atmospheric gusts and, secondly, possible silicate clouds might be playing a role in lowering the emitted thermal radiation that is being detected in the near-infrared range by reflecting the stellar light and absorbing the near-infrared wavelengths.
Tim Lichtenberg from the University of Groningen suggests that the magma ocean and the atmosphere might be continually exchanging matter and energy, with the interior releasing gases that replenish the atmosphere while some being taken up again by the molten surface.
The balance between outgassing from the magma ocean and loss to space could maintain the atmosphere over long periods.
Webb’s General Observers Program 3860 captured nearly four full orbits of TOI-561 b over a continuous 37-hour observation period.
This dataset is being analyzed to map temperature variations across the entire planet and to better constrain the atmospheric composition.
The observations are part of NASA’s James Webb Space Telescope program, which is led by NASA in collaboration with ESA (European Space Agency) and CSA (Canadian Space Agency).
The data provide direct measurements of an atmosphere on a rocky exoplanet exposed to intense stellar radiation, marking a significant addition to the study of ultra-short period planets and their thermal and atmospheric properties.
Stay tuned for more updates.