Recently, an international research team, using the United States National Aeronautics and Space Administration (NASA)’s James Webb Space Telescope (JWST), has uncovered a rocky exoplanet that not only spins rapidly but also has a scorching layer of magma on its surface, potentially covered by a thick and heavy atmosphere. Researchers have indicated that its overall characteristics resemble those of early Earth, challenging previous assumptions of “atmospheric stripping.”
This international team includes institutions such as the University of Birmingham in the United Kingdom, the Kapteyn Astronomical Institute in the Netherlands, and the Carnegie Institution for Science in the United States. The findings of their study on this “super-Earth” with an atmosphere and a molten surface were published on December 11th in the “Astrophysical Journal Letters.”
This “super-Earth” has been named “TOI-561 b” and is located approximately 280 light-years away from the solar system. It has a radius about 1.4 times that of Earth, with a density 3.2 times that of Earth (5.5 grams per cubic centimeter), indicating a rocky composition but with a lower iron content.
It orbits its host star at a distance of less than one million miles (1.6 million kilometers), only around one-fortieth of the distance from Mercury to the Sun, resulting in a revolution period of fewer than 11 hours, classifying it as a rare “ultra-short-period exoplanet.”
Due to being “tidally locked,” one side of the planet is always facing the star, receiving radiation approximately 5,000 times that of Earth. This extreme exposure results in surface temperatures exceeding the melting point of regular rocks, making it a high-temperature rocky exoplanet with a magma ocean. The host star is a low-iron star located in the thick disk region of the Milky Way Galaxy, approximately 10 billion years old, with its surface temperature slightly lower than that of the Sun.
The research team speculates that TOI-561 b might resemble Earth in its early formation, with a relatively smaller iron core and a mantle composed of rocks with lower density than Earth’s interior.
Regarding this, the first author of the paper and scientist at the Carnegie Science Earth and Planets Laboratory, Johanna K. Teske, commented, “What truly sets this planet apart is its remarkably low density. Compared to the metal content of rocky bodies like Earth, its density should not be this low.”
Teske added, “TOI-561 b is different from ultra-short-period planets. It orbits a very old, low-iron star. The star is twice as old as the Sun and located in a region of the Milky Way known as the thick disk, with a chemical environment radically different from our solar system, making the planet’s formation history even more unique.”
Previous theories suggested that small planets extremely close to stars would eventually lose all gases under intense radiation, becoming bare rocky bodies. To verify this, researchers used NASA’s James Webb Space Telescope’s Near Infrared Spectrograph (NIRSpec) to measure the infrared brightness of the dayside of the exoplanet TOI-561 b to estimate its surface temperature.
This observation method utilized the “secondary eclipse” phenomenon, where the system’s overall brightness slightly decreases when the planet moves behind the star. If TOI-561 b were just a bare rocky planet without an atmosphere, the star’s radiant heat would not effectively reach the backlit side, resulting in a predicted temperature exceeding 2,700°C on the dayside.
The results revealed that the dayside temperature of TOI-561 b is only 1,800°C. This significant temperature difference indicates efficient heat distribution. The research team believes that the planet likely harbors a thick, volatile atmosphere, which not only regulates temperature but also gives the planet a much larger apparent radius than its actual rocky core.
Currently, they are analyzing the complete dataset in detail to plot the temperature variations across the entire planet and to more accurately determine the composition of its atmosphere.
Co-author Dr. Anjali A. A. Piette from the University of Birmingham explained, “We need a thick atmosphere rich in volatile substances to explain all the observational results. Strong winds on the planet transport heat to the backlit side, thus cooling the dayside.”
Dr. Piette continued, “Gases like water vapor absorb some of the near-infrared light emitted from the planet’s surface, preventing it from passing through the atmosphere and being detected by telescopes. Therefore, this planet appears colder than expected. Additionally, there may also exist bright silicate clouds that reflect starlight to cool the atmosphere.”
While the measurements from the Webb telescope strongly support the existence of an atmosphere, they also bring up an important mystery: how can a relatively small planet maintain such a thick atmosphere under intense radiation? This question will be one of the team’s main research objectives in the future.
Co-author Tim Lichtenberg from the University of Groningen in the Netherlands concluded, “We believe there is a balance between the magma ocean and the atmosphere of TOI-561 b. Gases escape from the planet’s surface to replenish the atmosphere, which is then absorbed back into the planet’s interior through the magma ocean for circulation. We speculate that this planet is like a ‘sopping wet lava ball,’ possibly with more volatile substances than Earth.”