James Webb Telescope Detects Substantial Atmosphere on Ultra-Hot Rocky Exoplanet TOI-561 b

The James Webb Space Telescope (JWST) has delivered compelling evidence confirming the presence of a significant atmosphere surrounding the rocky exoplanet designated TOI-561 b. This groundbreaking observation challenges long-held assumptions suggesting that smaller planets orbiting extremely close to their host stars should be stripped of any gaseous envelopes due to intense stellar radiation.

TOI-561 b is categorized as an ultra-hot super-Earth. It was initially discovered in 2020 and is considered one of the most ancient known planets, having formed approximately 10 billion years ago. The planet boasts a radius about 1.4 times that of Earth. It completes an orbit around its star in under 11 hours, placing it within the rare category of ultra-short-period (USP) exoplanets. Its host star is an orange dwarf, roughly two to three times the age of our Sun. Due to the extremely close proximity, the planet is tidally locked, meaning one hemisphere perpetually faces the star.

A research team led by Johanna Teske of the Carnegie Institution for Science’s Earth and Planets Laboratory utilized the JWST's Near-Infrared Spectrograph (NIRSpec) instrument. Their goal was to measure the temperature of the planet's dayside by analyzing its near-infrared brightness. Theoretical models predicted that if TOI-561 b were merely a bare rock, its dayside temperature should soar to around 2700 °C (4900 °F). However, the observations, conducted in May 2024, revealed a notably cooler temperature of only 1800 °C (3200 °F).

This unexpected thermal deficit strongly suggests the existence of a substantial, volatile-rich gaseous envelope capable of effectively redistributing heat across the planet. Co-author Tim Lichtenberg from the University of Groningen posited that the planet might resemble a “wet lava ball,” implying a composition richer in volatiles than Earth. This characteristic, combined with its anomalously low density, cannot be accounted for solely by its internal structure, such as a small iron core surrounded by a mantle of less dense silicates.

The star TOI-561 is metal-poor, indicating that the planet formed within a distinctly different chemical environment than those found in our own Solar System. Scientists speculate that this composition might be representative of planets that coalesced when the universe was considerably younger. According to Dr. Anjali Piette of the University of Birmingham, the presence of an atmosphere is essential to reconcile all observational data, as powerful winds could be efficiently transporting heat to the planet's nightside.

Researchers hypothesize a dynamic equilibrium exists between a magma ocean and the atmosphere, where gases are continuously outgassed and subsequently reabsorbed. The team plans to dedicate over 37 hours of future JWST observation time to map the heat circulation patterns and refine the atmospheric composition details. The findings of this study have been formally published in The Astrophysical Journal Letters. Co-author Nicole Wallack remarked that this discovery effectively turns established notions about ultra-short-period planets on their head.