- NASA's James Webb Telescope found strong evidence of an atmosphere on rocky exoplanet TOI-561 b
- TOI-561 b is 1.4 times Earth's size and orbits its star in less than 11 hours
- The planet's low density suggests a small iron core and a thick atmosphere
In a major discovery, scientists using NASA's James Webb Space Telescope have found the strongest evidence so far that a rocky planet outside our solar system has an atmosphere. According to NASA, the finding is important because such small, extremely hot planets were earlier believed to be unable to hold on to thick layers of gas.
This planet, TOI-561 b, belongs to an extremely rare category called ultra-short-period exoplanets. It is approximately 1.4 times larger than Earth and orbits its star in less than 11 hours.
The planet is located very close to its star, less than a million miles away. Its star is slightly smaller and less hot than the Sun.
Due to this proximity, TOI-561 b is tidally locked, meaning only one side of it is always facing the star. This permanently dayside of the planet is extremely hot, where rocks are likely to melt and an ocean of magma may be spread across its entire surface.
Scientists were surprised by TOI-561 b's unusually low density. According to lead scientist Johanna Teske, this planet isn't quite as light as "super-puff" planets, but it still has a much lower density than planets with Earth-like composition.
One possible reason for this is that the planet's iron core is relatively small and its rocky crust less dense than Earth's. This argument is even stronger because TOI-561 b orbits an extremely old, iron-poor star. This star is located in the "thick disk" region of the Milky Way galaxy. It is believed that the planet formed when the universe was young, and its chemical environment was different from today.
However, this reason alone doesn't fully explain the planet's low density.
Researchers also suspect that a thick atmosphere may exist around TOI-561 b, making it appear larger than its actual size. This is surprising, because planets so small and so close to their stars are typically thought to be merely solid rock after billions of years of intense radiation.
To test this hypothesis, scientists used the James Webb Space Telescope's near-infrared spectrograph. They measured the temperature of the planet's dayside at a time when the planet moved behind its star and the overall system's brightness dropped.
If the planet had no atmosphere, its dayside should have been about 4,900 degrees Fahrenheit. But data from Webb recorded a temperature of about 3,200 degrees Fahrenheit. This temperature is still extremely high, but much lower than expected, suggesting that some process is helping to cool the planet.
The team considered several possibilities to explain this temperature difference. Heat flow through a magma ocean alone was not considered sufficient. A thin layer of rocky steam was also possible, but its effect would have been limited.
The most convincing explanation is a dense atmosphere rich in volatile elements. According to co-author Anjali Piette, strong winds could transport heat from the dayside to the nightside. Gases like water vapor absorb some of the heat, making the planet appear cooler from a distance. Furthermore, bright clouds made of silicate particles could reflect starlight and help cool the atmosphere.
The question remains as to how such a small planet can maintain a thick atmosphere despite such intense radiation. Scientists believe that a balance exists between the magma ocean and the atmosphere. When gases escape into the atmosphere, the molten surface draws some of them back into the planet.
According to co-author Tim Lichtenberg, TOI-561 b contains far more volatile elements than Earth. They described the planet as a "ball of wet lava," where gases constantly flow between the surface and the atmosphere.
These findings emerge from initial observations made under Webb's General Observers Program 3860. During this period, the James Webb Telescope observed the system for more than 37 hours, with TOI-561 b completing approximately four orbits.
Scientists are now analysing the entire data in depth to create a complete map of the temperature around the planet and better understand the composition of its atmosphere.
