JWST NIRSpec observations provide strong evidence that the ultra‑hot super‑Earth TOI‑561 b retains a substantial atmosphere. Continuous monitoring in May 2024 captured four secondary eclipses and measured a dayside temperature of about 3,100 °F (1,700 °C), far cooler than the ~4,900 °F (2,700 °C) expected for an airless rock. Models show a volatile‑rich atmosphere with strong heat transport and a cycling exchange with a molten surface best explain the data. The study was published Dec. 11 in The Astrophysical Journal Letters.
JWST Finds Strong Evidence of an Atmosphere on Ultra‑Hot Super‑Earth TOI‑561 b — "Like a Wet Lava Ball"
An artist concept of TOI-561 b and its host star. JWST has found the strongest evidence yet of an atmosphere around the rocky exoplanet, challenging assumptions that ultra-hot super-Earths cannot hold onto air. | Credit: NASA/STScI
A team of astronomers using the James Webb Space Telescope (JWST) reports the strongest evidence yet that a rocky world outside our solar system retains an atmosphere. The planet, an ultra‑hot super‑Earth called TOI‑561 b, challenges the prevailing idea that small, extremely close‑in planets must be completely airless.
TOI‑561 b is the innermost of at least three planets orbiting a roughly 10‑billion‑year‑old star about 280 light‑years from Earth. It circles its star in under 11 hours at roughly one‑fortieth the distance between Mercury and the Sun, placing it among the class of ultra‑short‑period planets whose surface temperatures are high enough to melt rock.
Previous measurements from NASA’s TESS mission indicated TOI‑561 b has a lower bulk density than expected for a purely rocky composition, prompting a closer look. To test for an atmosphere, researchers used JWST’s NIRSpec instrument to measure the planet’s dayside thermal emission.
In May 2024, JWST observed the system continuously for more than 37 hours, covering four full orbits and capturing four secondary eclipses—moments when the planet passed behind its star. By measuring the tiny dip in combined light during each eclipse, the team isolated the planet’s infrared glow and directly determined its dayside temperature.
If TOI‑561 b were a bare, airless rock, models predict a dayside temperature near 4,900 °F (2,700 °C). Instead, JWST measured a significantly cooler dayside around 3,100 °F (1,700 °C). The discrepancy led the team to test a variety of surface and atmospheric scenarios to match the observations.
What Explains the Cooler Dayside?
Only models that include a substantial, volatile‑rich atmosphere with efficient heat redistribution from day to night reproduce the measured temperature. Strong winds moving heat to the nightside would cool the dayside compared with an airless surface. The researchers propose a dynamic exchange between a molten surface and a thin, replenished atmosphere: gases evaporate from a magma ocean to feed the atmosphere while some material is reabsorbed into the interior over time.
"While gases are coming out of the planet to feed the atmosphere, the magma ocean is sucking them back into the interior," said Tim Lichtenberg of the University of Groningen. "It's really like a wet lava ball."
Lead author Johanna Teske of the Carnegie Earth and Planets Lab noted the planet’s chemistry probably differs from that of solar‑system worlds. Co‑author Anjali Piette of the University of Birmingham added that powerful atmospheric winds are needed to transport heat and explain the cooler measured dayside.
Implications
These results open a new window onto the interiors and geological activity of extremely hot rocky exoplanets: by measuring atmospheres, astronomers can infer surface composition, magma‑atmosphere interactions, and volatile inventories. The findings were published Dec. 11 in The Astrophysical Journal Letters.
