For 37 hours, the James Webb Space Telescope watched TOI-561 b, a rocky world so close to its star that it races through an orbit in about 10.5 hours. The observations covered more than three full orbits—and revealed a planet that is far less simple than astronomers expected.
Planets with orbits shorter than a day are known as ultra-short-period planets. They face intense radiation from their stars, which should strip away any atmosphere. TOI-561 b, first detected in 2020, appeared to break that rule. Earlier observations hinted at an atmosphere, and a 2025 study using a smaller set of James Webb data suggested it could be rich in gases such as water and carbon dioxide.
The new study, led by Samuel Boucher of the Université de Montréal, used the telescope’s full-orbit view to examine the planet’s atmosphere in greater detail. The paper, “A Highly Reflective Atmosphere on the Lava World TOI-561b Revealed by JWST/NIRSpec Phase Curves,” was published in The Astronomical Journal. The researchers tracked changes in the planet’s light as it moved around its star, including light from its night side—a view that earlier work had not provided.
That view helps address a puzzle. Previous measurements showed that TOI-561 b’s star-facing side was cooler than expected for bare rock exposed so close to a star. An atmosphere could move heat from the dayside to the nightside. Alternatively, a reflective atmosphere could bounce some of the star’s light back into space, leaving the surface cooler. The new observations show that the planet has a highly reflective atmosphere, giving researchers evidence that its atmosphere helps shape the light they see.
The finding matters beyond one unusual planet. Scientists are still trying to learn whether small, rocky worlds can keep atmospheres, and how those atmospheres form. A rocky planet may build a secondary atmosphere through gases released from its interior. On a world with a molten surface, that exchange between the planet and its atmosphere could continue over time. Studying it may reveal processes hidden beneath the surface—processes that are difficult to measure from far away.
TOI-561 b is not a likely stand-in for a calm, habitable world: its star-facing side is thought to be a sea of molten rock. But its unexpected atmosphere offers a rare chance to study how a rocky planet and its interior can affect each other. With more observations, researchers can test how the atmosphere moves heat, reflects starlight and changes alongside the planet beneath it.