A rocky exoplanet so close to its star that its surface is a permanent ocean of molten rock appears to be protected by a layer of highly reflective clouds acting like a planet-wide sunscreen, according to new James Webb Space Telescope observations that challenge assumptions about how such extreme worlds should behave.
The planet, TOI-561 b, orbits roughly 280 light-years from Earth and was already known to be unusually cool on its sun-facing side for a dark, rocky world sitting so close to its star. An international team led by Professor Lisa Dang of the University of Waterloo and Dr. Anjali Piette of the University of Birmingham used JWST to watch the planet across nearly four full orbits over 37 hours, observing its permanently dark nightside for the first time to find out where the missing heat was going.
A measurement that didn't add up — until clouds explained it
The nightside, it turned out, was not warm enough to account for all the heat that should have arrived from the sunlit side. Instead, the team found TOI-561 b reflects approximately 75% of the starlight that reaches it, a figure comparable to Venus. Computer models matched against the telescope data suggest clouds made of silicate minerals — the same class of compounds that make up much of Earth's mantle — form over the dayside and reflect radiation before it can heat the surface, rather than the planet remaining dark and heat-absorbing as expected.
"Our discovery challenges current theories," said Piette, an assistant professor in astronomy at Birmingham. "We would expect a heavily irradiated rocky planet like TOI-561 b to have lost its atmosphere billions of years ago. Instead, the evidence suggests it still has a significant atmosphere today." The findings, published in The Astronomical Journal, raise the question of whether the planet started out with an unusually large reservoir of gas, or whether its molten surface functions as a continuous, planet-wide volcano constantly replenishing what it loses to space.
Dang, who led the project, said the team tracked how the planet's brightness and heat changed across its orbit using a technique called a phase curve. "This technique revealed that the planet reflects a significant amount of light from its star, and that heat is being redistributed around the planet instead of only on the sunlit side," she said. "Together, these point to the presence of an atmosphere."
Researchers still do not know the exact composition of that atmosphere — candidates include a thick carbon dioxide layer resembling a scorching version of Venus, or a sulfur dioxide-dominated mix similar to Jupiter's volcanic moon Io. Dang said further JWST observations at longer wavelengths could help determine which scenario is closer to the truth, and explain how the planet has clung to an atmosphere despite billions of years of intense stellar radiation.
