A new analysis of observations from the NASA/ESA/CSA James Webb Space Telescope suggests that TOI-561b, a rocky exoplanet orbiting extremely close to its star, may still possess a substantial, highly reflective atmosphere. The planet completes an orbit in just over 10 hours, yet its measured dayside temperature is much lower than expected for bare rock with no atmospheric heat transport.
TOI-561, also known as TYC 243-1528-1, is a bright star about 280.5 light-years away in the constellation Sextans. The star is approximately 10 billion years old and has about 80% of the Sun’s mass and size.
At least three transiting planets—TOI-561b, TOI-561c and TOI-561d—orbit the star. The system is among the oldest and most metal-poor planetary systems known in the Milky Way.
TOI-561b is the innermost planet and is classified as a super-Earth. It has an orbital period of only 0.44 days, or just over 10 hours. The planet is about 1.4 times Earth’s radius and 2.2 times its mass, giving it an unusually low density of approximately 4.3 grams per cubic centimeter.
A surprisingly persistent atmosphere
According to current theory, a rocky world exposed to such intense radiation should have lost a substantial atmosphere billions of years ago.
“Our discovery challenges current theory,” said Dr Anjali Piett, an astronomer at the University of Birmingham. “Severely irradiated rocky planets like TOI-561b likely lost their atmospheres billions of years ago.”
“On the contrary, there is evidence that the atmosphere remains significant today,” Piett said. “This raises big questions about whether the planet started out with an unusually large supply of gas, or whether the oceans of lava covering the surface act as planet-wide volcanoes, constantly feeding the atmosphere.”
Using Webb’s NIRSpec instrument, Piett and her colleagues tracked the planet’s brightness during four eclipses and three transits. They used these observations to construct a phase curve, which shows how the planet’s brightness changes throughout its orbit.
The researchers also modeled fluctuations in the star’s light caused by granulation—the motion of convective cells at the stellar surface. Combining the stellar and planetary signals allowed them to obtain a daytime emission spectrum for TOI-561b.
The planet’s dayside temperature is about 2,050 K, substantially below the approximately 3,200 K expected for dark, bare rock without heat transport. The difference suggests that the planet may have an atmosphere capable of reflecting some of the incoming starlight and redistributing heat.
Mineral clouds may reflect starlight
To investigate the planet’s brightness, the astronomers compared their observations with computer simulations of its climate. The simulations suggest that clouds made of silicate minerals, including silicon dioxide and magnesium silicate, could form on the dayside and act like a planetary sunscreen.
“We measured how the planet’s brightness and heat changed throughout its orbit,” said Professor Lisa Dunn, an astronomer at the University of Waterloo in Ontario. “This technique, known as phase curving, revealed that the planet reflects a large amount of light from the star, and that heat is redistributed around the planet rather than just on the sunlit side. Together, these point to the presence of an atmosphere.”
“The next big question we will try to answer is whether TOI-561b’s atmosphere is similar to Venus or an extreme version of Io,” Dunn said.
“Using Webb to observe the planet at longer wavelengths could provide insights and help explain how such a hot planet has maintained an atmosphere for billions of years.”
The team’s findings are described in a paper published in The Astronomical Journal.
Samuel Boucher et al., 2026, “Highly reflective atmosphere of lava world TOI-561b revealed by JWST/NIRSpec phase curve,” The Astronomical Journal 172, 239. doi: 10.3847/1538-3881/ae9763.
Source: www.sci.news
