In Simple Terms
A rocky planet 280 light-years away is so close to its star that a year there lasts just 11 hours. New observations suggest it may have an atmosphere and bright clouds that reflect much of the star’s light. Those clouds could help explain how the planet manages to keep an atmosphere despite its extreme heat, but scientists do not yet know what that atmosphere is made of.
A Planet Without a Familiar Cycle of Day and Night
NASA’s TESS mission spotted TOI-561 b in 2020 while searching for planets orbiting stars beyond our solar system. Its name identifies it as a planet associated with the star TOI-561, but its remarkably tight orbit is what makes it stand out. While a year lasts days or months on many planets, this world takes only 11 hours to complete an orbit and return to the same point in its path.
Because it is so close to its star, TOI-561 b is probably tidally locked. That means it takes the same amount of time to spin once on its axis as it does to orbit its star. As a result, one side always faces the star, while the other remains turned away from its direct light. The planet therefore has no familiar daily cycle of sunrise and sunset: one hemisphere is locked in constant daylight, and the other in perpetual darkness.
This arrangement could create an enormous temperature contrast between the planet’s two sides. The star-facing hemisphere receives radiation continuously, while the opposite side gets no direct starlight. If the day side were bare rock with no atmosphere, scientists would expect it to be even hotter than observations indicate. That contrast prompted researchers to ask whether a layer of gas carries some heat to the night side—or whether much of the incoming light is reflected before it can warm the surface.
Webb Watches the Planet Complete Its Orbit
To investigate, the research team used the James Webb Space Telescope to track changes in the planet’s brightness and temperature as it moved along its orbit. The observations lasted 37 hours, long enough to cover nearly four complete orbits. The goal was not simply to take a picture of the planet. Instead, the team measured how the light from the system changed as the planet moved into different positions relative to its star and to the observer.
This kind of measurement helps researchers build a picture of how heat and light are distributed across a planet. As the observed brightness changes over the course of an orbit, scientists can infer differences between the regions that come into view at different times. According to Lisa Dang, a member of the team and a researcher at the University of Waterloo, analysis of those changes showed that the planet reflects a substantial amount of its star’s light and that heat is not confined to the day side. Together, these findings strengthen the case that TOI-561 b has an atmosphere.
Observing nearly the entire orbit was important because it allowed the team to study the planet’s night side for the first time as part of this investigation. The central question was whether the dark hemisphere was warm enough to account for the heat leaving the day side. Webb’s measurements suggested that it was not warm enough to explain by itself all the energy missing from the hot side. Heat carried to the night side, then, was not a sufficient explanation, and reflected light emerged as an important part of the picture.
A Reflective World, Like Venus
The findings suggest that TOI-561 b sends about three-quarters of the starlight that reaches it back into space. That is roughly 75 percent—about as much as Venus reflects, making it the most reflective planet in our solar system. Earth, by comparison, reflects only about 30 percent of the sunlight that reaches it. The comparison is about reflectivity, not about the nature of these worlds: Venus, Earth, and TOI-561 b differ enormously in temperature and atmospheric conditions.
A high level of reflectivity might sound like a detail about appearance, but it helps explain the planet’s heat balance. Light that is reflected does not get converted into heat at the surface. A reflective layer could therefore limit how much energy the star-facing hemisphere absorbs. That may explain why the surface appears less hot than expected for bare rock exposed continuously to its star’s radiation. The possible atmosphere, then, may do more than transport heat: it could also contain clouds that send a large share of the incoming energy back into space.
The team suspects that clouds made of silicate minerals form in the planet’s hot atmosphere and drive the high reflectivity. The proposed materials include silicon dioxide and magnesium silicates, compounds from the same mineral family as familiar substances found on Earth, such as quartz and sand. But these clouds should not be imagined as water clouds like those on Earth. In the planet’s extreme conditions, they would instead consist of mineral particles that have evaporated or condensed in intense heat.
These layers could act as a barrier to radiation, reflecting some of the starlight before the surface absorbs it. They might also help the atmosphere persist, although the available data do not yet reveal all the details of that relationship. The observations support the presence of an atmosphere and point to highly reflective clouds, but they have not yet provided a definitive analysis of the gas’s composition or of how material moves around the planet.
An Atmosphere Between Two Extreme Possibilities
Scientists still do not know what TOI-561 b’s atmosphere is made of. One possibility is a dense atmosphere rich in carbon dioxide, broadly reminiscent of Venus’s atmosphere. Another is an atmosphere rich in sulfur dioxide, which would bring to mind an intensely volcanic environment like that of Jupiter’s moon Io—the most volcanically active body in the solar system.
These comparisons do not mean that TOI-561 b is a copy of Venus or Io. They offer researchers two possible directions for understanding how its atmosphere formed and how it might endure. If volcanic activity releases gases into the atmosphere, it could replenish material the planet loses over time, as has been proposed for Io. If the atmosphere has a different composition, other processes may be responsible for keeping it in place. In either case, identifying its actual ingredients is essential to figuring out what is happening on this world.
That question of persistence is especially important because TOI-561 b receives an enormous amount of energy from its star and has a surface temperature above 1800 degrees Celsius. Scientists had previously thought that a world this hot might not be able to hold on to an atmosphere at all. The new clues raise the possibility that some kind of atmosphere has remained around the planet for billions of years despite the extreme conditions. But the observations do not, by themselves, establish how old the atmosphere is or reveal the details of its history. Instead, they raise a new question: what processes might protect or replenish it?
What Will Future Observations Reveal?
The team believes that observing the planet at longer wavelengths with the James Webb Space Telescope could help distinguish between the proposed explanations. Different materials leave different signatures in light, and analyzing those signatures could show whether the atmosphere is more like a carbon-dioxide-rich haze, has strong traces of sulfur dioxide, or has some other composition that has not yet been identified. Additional measurements could also reveal more about the mineral clouds thought to explain the planet’s high reflectivity.
Knowing the atmosphere’s composition could connect three related mysteries: how so much of the star’s light is reflected, how heat is distributed between the planet’s two hemispheres, and how an atmosphere could persist in such a harsh environment. Each answer would help researchers test a different picture of the planet. Are reflective clouds the dominant factor? Do volcanoes replenish its gases? Or are several processes working together?
Researchers including Lisa Dang of the University of Waterloo took part in the study, and the team’s findings were published in The Astronomical Journal. The results show that studying distant planets does not always require seeing details of their surfaces directly. Changes in light and temperature over the course of an orbit can reveal features that a single snapshot cannot, including a possible atmosphere and the different behavior of a planet’s bright and dark hemispheres.
The Bottom Line
TOI-561 b is an extreme rocky world about 280 light-years away. Discovered in 2020, it completes an orbit in just 11 hours, and its surface temperature exceeds 1800 degrees Celsius, or 3000 degrees Fahrenheit. During 37 hours of observations covering nearly four orbits, the James Webb Space Telescope gathered evidence that the planet reflects about 75 percent of its star’s light and may be surrounded by an atmosphere and clouds made of silicate minerals.
The atmosphere’s composition remains an open question. Some of its ingredients might resemble those in Venus’s atmosphere, or it might bear signs of sulfur dioxide and volcanic activity like that seen on Io. Longer-wavelength observations by Webb could help test these possibilities. For now, the discovery is striking because it suggests that even a blisteringly hot planet may have unexpected ways to reflect light, distribute heat, and perhaps hold on to an atmosphere for a very long time.