The James Webb Space Telescope (JWST) continues to revolutionize exoplanet research, providing unprecedented insights into distant worlds. Following its successful commencement of science operations in mid-2022, Webb has already delivered groundbreaking data, including the first clear detection of carbon dioxide on exoplanet WASP-39b, atmospheric water vapor on WASP-96 b, and heavier elements like oxygen and carbon on HD149026b.
Now, astronomers are announcing the most compelling evidence to date for an atmosphere surrounding a rocky exoplanet. This significant discovery centers on TOI-561 b, an ultra-hot super-Earth located approximately 275 light-years away. This planet, about 1.4 times Earth’s radius, orbits its Sun-like star in an astonishingly short period of less than 11 hours. Such planets are classified as ultra-short-period (USP) exoplanets.
Observations utilizing Webb’s Near-Infrared Spectrometer (NIRSpec) indicate that TOI-561 b is enveloped in a global magma ocean, with a substantial atmosphere above it. This finding directly contradicts prevailing theories suggesting that small, rocky planets in such close proximity to their stars would inevitably shed any atmosphere due to intense stellar radiation.
The research, led by Johanna Teske of the Carnegie Institution for Science, reveals that TOI-561 b’s orbit is less than one-fortieth the distance between Mercury and our Sun. Scientists theorize that the planet is tidally locked, meaning one hemisphere perpetually faces its star. This results in dayside temperatures exceeding the melting point of rock, creating a surface of molten lava. Furthermore, measurements of TOI-561 b’s mass and size point to a lower-than-expected density. This could suggest a smaller iron core and a less dense mantle compared to Earth, or it might imply the presence of an atmosphere contributing to its apparent size.
To investigate the potential atmosphere, the team monitored the system for over 37 hours, observing nearly four planetary orbits. They measured subtle changes in brightness as the planet moved behind its star. Without an atmosphere to redistribute heat, the dayside would be an extreme 2,700°C. However, Webb recorded a significantly cooler dayside temperature of around 1,800°C, strongly implying an atmosphere is actively transferring heat from the star-facing side to the cooler nightside.
Scientists hypothesize that TOI-561 b maintains its atmosphere through a dynamic equilibrium between its molten surface and gaseous envelope. Gases released from the magma ocean replenish the atmosphere, while simultaneously, the magma reabsorbs these gases. This unique planetary process might explain how such a volatile-rich world, dubbed a ‘wet lava ball’ by researchers, can retain an atmosphere despite intense stellar bombardment. These findings, from Webb’s Cycle 2 mission, could reshape our understanding of atmospheric retention on rocky exoplanets under extreme conditions.