Webb Telescope Reveals ‘Wet Lava Ball’ Planet with Surprisingly Thick Atmosphere
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A new analysis of data from the NASA/ESA/CSA James Webb Space Telescope suggests the exoplanet TOI-561b, a super-Earth located 280.5 light-years away, is enveloped in a substantial atmosphere above a global magma ocean. This discovery challenges existing models of planetary atmospheres and offers a glimpse into the diverse and often unexpected conditions on worlds beyond our solar system.
Unveiling a Scorching World
TOI-561b orbits a bright star in the constellation Sextans, a star approximately 10 billion years old – twice the age of our Sun – and roughly 80% its size. Officially designated TYC 243-1528-1, the star is categorized as a Galactic thick disk star, a rare population known for its age and composition. The planetary system, which includes at least three confirmed exoplanets (TOI-561b, c, and d), is among the oldest and most metal-poor systems discovered to date in the Milky Way.
The inner planet, TOI-561b, completes an orbit in just 0.44 days. Its mass is 3.2 times that of Earth, with a radius 1.45 times larger, and a density of 5.5 g/cm3, indicating a rocky composition. However, astronomers noted an anomalously low density for the planet. “What really sets this planet apart is its anomalously low density,” one astronomer explained. “It’s not a super-puff, but it is less dense than you would expect if it had an Earth-like composition.”
The Mystery of the Missing Density
Initial theories proposed a small iron core and a less dense mantle to explain the planet’s low density. Researchers also considered the possibility of a thick atmosphere artificially inflating the planet’s apparent size. Despite the intense radiation from its host star, some ultra-short period planets have demonstrated the presence of atmospheres, defying expectations.
To investigate, the team utilized Webb’s NIRSpec (Near-Infrared Spectrograph) instrument to measure the planet’s dayside temperature. This technique, similar to those used in the TRAPPIST-1 system, analyzes the decrease in brightness as the planet passes behind its star. If TOI-561b were a bare rock, its dayside temperature would be expected to reach a scorching 2,700 degrees Celsius (4,900 degrees Fahrenheit). However, observations revealed a temperature closer to 1,800 degrees Celsius (3,200 degrees Fahrenheit) – significantly cooler than predicted.
A Volatile-Rich Atmosphere
An emission spectrum captured by Webb in May 2024 further supported the presence of an atmosphere. The data indicated that a magma ocean alone could not account for the observed temperature difference. “We really need a thick volatile-rich atmosphere to explain all the observations,” stated an astronomer at the University of Birmingham. “Strong winds would cool the dayside by transporting heat over to the nightside.”
The atmosphere likely contains gases like water vapor, which absorb near-infrared light, and potentially silicate clouds that reflect starlight, further contributing to the cooling effect. The question remains, however, how such a small planet, subjected to intense stellar radiation, can retain such a substantial atmosphere. Researchers hypothesize an equilibrium between the magma ocean and the atmosphere, with gases continuously escaping and being replenished. “We think there is an equilibrium between the magma ocean and the atmosphere,” explained an astronomer at the University of Groningen. “At the same time that gases are coming out of the planet to feed the atmosphere, the magma ocean is sucking them back into the interior.”
This suggests TOI-561b is far more volatile-rich than Earth, earning it the descriptive moniker “wet lava ball.” The findings, published today in The Astrophysical Journal Letters, offer a compelling case study for understanding planetary formation and atmospheric retention in extreme environments.
https://doi.org/10.3847/2041-8213/ae0a4c
