Webb Telescope Detects Atmosphere on Lava Planet TOI-561 b, Challenging Exoplanet Formation Theories
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The James Webb Space Telescope has provided the strongest evidence yet of an atmosphere surrounding a rocky exoplanet, a scorching world dubbed TOI-561 b, potentially rewriting our understanding of atmospheric retention on smaller planets. Observations suggest this “super-Earth” is covered in a global magma ocean beneath a thick gaseous blanket, defying previous assumptions about the ability of planets close to their stars to hold onto atmospheres.
A ‘Wet Lava Ball’ 275 Light-Years Away
TOI-561 b, located 275 light-years from Earth, is approximately 1.4 times the radius of our planet and orbits a Sun-like star. Classified as an ultra-short period (USP) exoplanet, it completes an orbit in less than 11 hours – a remarkably swift journey. This proximity to its star, less than one-fortieth the distance between Mercury and the Sun, initially led scientists to believe the planet would be unable to sustain an atmosphere. However, data collected by Webb’s Near-Infrared Spectrometer (NIRSpec) paints a different picture.
The research, published December 11th in The Astrophysical Journal Letters, was spearheaded by Johanna Teske and a collaborative team from institutions including the Carnegie Institution for Science, the University of Waterloo, and Oxford University. Their findings indicate a dayside temperature of around 1,800 °C (3,200 °F), significantly cooler than the anticipated 2,700 °C (4,900 °F) if no atmosphere were present.
Tidal Locking and a Surprisingly Low Density
Because of its close orbit, TOI-561 b is likely tidally locked, meaning one side perpetually faces its star. This results in extreme temperature differences, with the dayside hot enough to melt rock, creating a vast magma ocean. Further analysis of the planet’s size and mass revealed a surprisingly low density, suggesting a relatively small iron core and a mantle composed of less dense rock than Earth’s.
“TOI-561 b is distinct among ultra-short period planets in that it orbits a very old (twice as old as the Sun), iron-poor star in a region of the Milky Way known as the thick disk,” explained a senior researcher involved in the study. “It must have formed in a very different chemical environment from the planets in our own solar system.” This unique composition suggests the planet could offer insights into planetary formation during the early universe.
An Atmosphere in Equilibrium?
The existence of a substantial atmosphere on TOI-561 b presents a puzzle. How can a small, intensely irradiated planet retain such a dense gaseous envelope? Researchers propose a dynamic equilibrium between the magma ocean and the atmosphere. “We think there is an equilibrium between the magma ocean and the atmosphere,” stated a co-author from the University of Groningen. “While gases are coming out of the planet to feed the atmosphere, the magma ocean is sucking them back into the interior.”
This process, coupled with a high concentration of volatile compounds, leads researchers to describe the planet as a “wet lava ball.” Another possibility considered is that the atmosphere itself is contributing to the planet’s apparent size, a phenomenon observed in “super-puff” gas giants.
Measuring Heat Transfer and Future Research
To investigate this further, the team observed TOI-561 b for over 37 hours, tracking nearly four complete orbits. They measured the decrease in brightness as the planet passed behind its star – a technique mirroring the Transit Method used to initially detect exoplanets. This allowed them to assess heat transfer between the dayside and nightside.
According to a researcher from the University of Birmingham, “We really need a thick, volatile-rich atmosphere to explain all the observations. Strong winds would cool the dayside by transporting heat over to the nightside. Gases like water vapour would absorb some wavelengths of near-infrared light emitted by the surface before they make it all the way up through the atmosphere.” The possibility of silicate clouds reflecting starlight and further cooling the atmosphere is also being investigated.
These initial findings stem from Webb’s General Observers (GO) Program 3860, part of its Cycle 2 programs. The team is currently analyzing the complete dataset to map temperatures across both sides of the planet and further characterize the atmospheric composition. This ongoing research promises to refine our understanding of exoplanet atmospheres and the diverse conditions under which planets can form and evolve.
