The rocky exoplanet TOI-561 b is so intensely irradiated that astronomers expected it to have lost its atmosphere long ago. But observations with the James Webb Space Telescope suggest the scorching world may still be wrapped in a substantial atmosphere—and that bright clouds could be reflecting much of the star’s light back into space.
A report from the University of Birmingham describes the work of an international research team that monitored the planet for 37 hours, observing nearly four orbits. TOI-561 b lies about 280 light-years from Earth. The study, led by Professor Lisa Dang of the University of Waterloo, was published in The Astronomical Journal. Dr. Anjali Piette, an astrophysicist at the University of Birmingham, was among the researchers.
The team’s measurements suggest the planet reflects about 75% of the light reaching it, a proportion the report compares with Venus. That result helps explain a puzzle from earlier measurements: TOI-561 b’s dayside was cooler than expected for a dark, bare rock orbiting so close to its star.
To investigate the discrepancy, the researchers tracked how the planet’s brightness and heat changed as it moved around its orbit. This technique, called a phase curve, let them examine the nightside, which receives no direct starlight because the same side of the planet always faces its star. The report says the nightside was not warm enough to account for all the heat missing from the dayside. The observations instead point to both strong reflection and heat moving from the permanently lit hemisphere toward the dark one.
Those patterns support the possibility of an atmosphere, but they do not reveal exactly what it contains. The report describes two possibilities: a carbon-dioxide-rich atmosphere, broadly reminiscent of Venus but under much hotter conditions, or one dominated by sulfur dioxide, a gas associated with volcanic activity on Jupiter’s moon Io. The atmosphere’s composition remains unknown.
Computer climate models offer one possible explanation for the high reflectivity. They suggest that clouds containing silicate minerals, including silicon dioxide and magnesium silicate, could form above the dayside and reflect incoming light before it heats the surface. The cloud layer remains a proposed explanation from simulations, rather than a confirmed view of clouds on the planet.
The finding matters because an atmosphere around a heavily irradiated rocky world raises a question about how such an atmosphere could last. The report says TOI-561 b’s conditions would be far too hostile for life as we know it. Its interest here is as a test of how rocky planets behave under extreme heat and radiation, and how atmospheres might survive or be renewed.
One possibility is that gases escaping from the planet’s global ocean of molten rock continually replenish the atmosphere. The molten interior could act like a planet-wide source of volcanic gases, countering some atmospheric loss to space. That explanation is still a hypothesis: the report says researchers do not yet know whether the planet began with an unusually large supply of gases or is replenishing them from within.
The team hopes that further Webb observations at longer wavelengths will help establish the atmosphere’s chemical composition. Those measurements could also help distinguish whether TOI-561 b is more like an extremely hot Venus or a more intense version of Io, and clarify how it has retained an atmosphere despite billions of years of radiation.
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