Astronomers at the Carnegie Endowment have found convincing evidence for an atmosphere on a rocky planet outside our solar system. The results are published at Astrophysical Journal Letters.
TOI-561 b is an ancient super-Earth twice the mass of Earth, but it is very different from our planet due to its close proximity to the star. This planet orbits just 1/40 the distance from Mercury to the Sun and completes a complete rotation in 10.56 hours. One side is always illuminated, while the other side is in eternal darkness.
“Astronomers thought that such a small and hot planet would not be able to maintain an atmosphere for long. Our observations show that TOI-561 b is surrounded by a dense layer of gas, which rules out ideas of extremely short-period planets,” said Nicole Wallack, a researcher at the Carnegie Endowment.
Harsh conditions and unusual structures
The planet is less dense than Earth, despite having a similar mass.
“This is not a hyperinflationary world, but its density is lower than expected for an Earth-like planet,” explains lead author Joanna Teske.
Scientists think this may be due to the small iron core and lighter silicate coating.
The star TOI-561 is twice as old as the Sun and poor in iron, making the planet a unique candidate for studying the formation of the ancient world.
“TOI-561 b formed in a chemical environment different from our solar system,” Teske noted. “This is a window into the early stages of planetary evolution in the Universe.”
Atmospheric and surface cooling
To study the atmosphere, researchers used the NIRSpec near-infrared spectrometer on the JWST telescope. They measured the temperature of the planet's dayside during the secondary eclipse. If the surface were bare rock, the temperature would reach nearly 2700°C. Actual temperature is about 3200 – 1800°C.
“Strong winds transport heat toward the night, and water vapor and silicate clouds absorb and reflect light. This reduces temperatures and creates the cooling effect we see,” explains co-author Anjali Piett.
The magma ocean acts as a thermostat
TOI-561 b is likely covered by a magma ocean interacting with the atmosphere.
“The gas leaves the planet but the magma brings some back. The planet is like a ball of wet lava,” said Tim Lichtenberg from the University of Groningen.
This helps the atmosphere remain stable despite the star's intense radiation.
The fire surface can evaporate volatile substances, then condense in the atmosphere, circulate the gas and stabilize the temperature. Taken together, this makes TOI-561 b one of the few known extreme worlds with a dense atmosphere near the star.
Long-term sustainability and scientific significance
The team observed the planet for 37 hours continuously, recording nearly four orbits. The data is currently being analyzed to map temperatures across the entire surface and clarify the chemical composition of the atmosphere.
“What's interesting is that these observations raise more questions than they answer,” Teske said. We see a world that has existed for billions of years and has retained its atmosphere despite extreme conditions.”
This finding shows that small rocky planets can maintain dense atmospheres for billions of years, with magma oceans playing an important role in this process. TOI-561 b becomes a unique object for studying the early stages of planet formation, demonstrating how chemistry, atmosphere and magma interact to create stable extreme worlds.
