Researchers were surprised to discover that an “Earth twin” exoplanet in another star system has a thick and dense atmosphere, even though its close proximity to its star should have caused it to evaporate into space. Its surface is extremely hot and likely composed of molten rock.
So far, astronomers have discovered more than 5,000 exoplanets, but only now has a planet been found with strong evidence of a significant atmosphere.
What makes this even more surprising is that the planet is extremely close to its star, in a region where conventional understanding predicts temperatures too high for a stable atmosphere to exist.
TOI-561b, the closest planet to its star, is slightly larger than Earth and rocky. It orbits the star at about one-hundredth the distance of Earth from the Sun and completes a full orbit in just 11 Earth hours. This implies that its surface temperature on the day side should reach roughly 2,700 degrees Celsius
An international team of astrophysicists from the United States and the United Kingdom used the James Webb Space Telescope to investigate whether the planet has an atmosphere.
For a planet this close to its star, it is nearly impossible to directly observe or analyze its atmospheric composition. Instead, researchers looked for indirect evidence, and their findings suggest a thick and dense atmosphere.
The team used the telescope’s spectrometer to estimate the temperature on the planet’s star-facing side. If the planet were heated directly with no atmosphere, the temperature would be around 2,700 degrees Celsius. If a thick atmosphere spreads and transfers some of the heat to the dark side, the temperature would be lower.
Measurements published last weekend show that the star-facing side reaches about 1,800 degrees Celsius. While still hot enough to melt rock, this is significantly cooler than expected without an atmosphere.
To test these observations, researchers considered three scenarios: no atmosphere, a thin atmosphere primarily composed of vaporized rock, and a thick atmosphere. The first scenario predicts very limited heat transfer to the night side. The thin atmosphere scenario would also move far less heat than observed.
The thick atmosphere scenario explains the data best. “We really need a dense atmosphere rich in volatile elements to explain all the observations,” said Anjali Piette from the University of Birmingham, one of the study leaders.
“Strong winds can cool the day side by moving heat to the dark side.
The atmosphere may also contain bright clouds of silicon oxides that reflect sunlight and provide additional cooling.”
Despite strong evidence for a thick atmosphere on TOI-561b, it is still unclear how a planet can maintain such an atmosphere so close to its star at temperatures that would normally cause massive evaporation.
It is possible the planet is losing gas, but at a slower rate than expected.
“We think there is a balance between a magma ocean and the planet’s atmosphere.
Molten rocks release gas, but the magma ocean recaptures many of the vapors and returns them to the planet,” explained team member Tim Lichtenberg from the University of Groningen.
“To match the observations, the planet must be far richer in volatile elements than Earth it’s like a ball of wet lava.”
The study is based on observations made with the space telescope in May 2024, lasting nearly 37 continuous hours almost four TOI-561b days, meaning the planet completed almost four full orbits during the observation.
Researchers continue to analyze the data, hoping that detailed temperature measurements at different points in the orbit will strengthen their findings and provide insight into the planet’s atmospheric composition.