Recent findings from the NASA/ESA/CSA James Webb Space Telescope indicate that TOI-561b is enveloped by a dense gas blanket above its global magma ocean.

TOI-561 is a luminous star located 280.5 light-years away in the constellation Sextant.

This star is approximately 10 billion years old and has about 80% of the Sun’s mass and size.

It is also known as TYC 243-1528-1 and belongs to a rare category of stars known as the galaxy’s thick disk stars.

TOI-561 hosts at least three exoplanets (TOI-561b, c, and d) and is among the oldest and most metal-poor planetary systems discovered in the Milky Way.

The inner planet, TOI-561b, is classified as a super-Earth with an orbital period of just 0.44 days.

Its mass and radius are 3.2 and 1.45 times that of Earth, with a density of 5.5 g/cm³, consistent with its rocky composition.

“What distinguishes this planet is its notably low density,” remarked Dr. Johanna Teske, an astronomer at the Carnegie Institution for Science.

“It is not significantly bloated, yet it is less dense than would be expected for an Earth-like composition.”

One potential reason for the low density, astronomers suggest, is that it may possess a relatively small iron core and a mantle composed of less dense rock compared to Earth’s.

“TOI-561b is exceptional among ultrashort-period planets as it orbits a substantially older (twice the age of the Sun), iron-poor star within the thick disk region of the Milky Way,” Teske added.

“It likely formed under a vastly different chemical environment than the planets in our solar system.”

Researchers also posit that TOI-561b is encircled by a thick atmosphere, possibly giving it an apparent size larger than its actual one.

Although small planets subjected to intense stellar radiation for billions of years are not anticipated to possess atmospheres, some are exhibiting characteristics beyond mere rocky surfaces or lava.

To investigate the possibility of TOI-561b having an atmosphere, they employed: Webb’s NIRSpec (near infrared spectrometer). This device measures the planet’s daytime temperature through near-infrared brightness.

The technique tracks the decrease in brightness of the star-planet system as the planet transits behind its star, similar to methods used for detecting atmospheres of rocky worlds like the TRAPPIST-1 system.

If TOI-561b were devoid of an atmosphere and comprised entirely of bare rock, daytime temperatures would approach 2,700 degrees Celsius (4,900 degrees Fahrenheit).

However, NIRSpec observations indicate that the planet’s dayside temperature is closer to 1,800 degrees Celsius (3,200 degrees Fahrenheit), indicating it remains extremely hot, but considerably cooler than anticipated.

Emission spectra captured by Webb in May 2024 illustrate the brightness of different wavelengths of near-infrared radiation emitted by the exoplanet TOI-561b. Image credits: NASA / ESA / CSA / Ralf Crawford, STScI / Johanna Teske, Carnegie Institute for Science, Earth and Planets / Anjali Piette, University of Birmingham / Tim Lichtenberg, Groningen / Nicole Wallack, Carnegie Institute for Science, Earth and Planets.

To interpret these findings, the researchers evaluated multiple scenarios.

A magma ocean could redistribute some heat; however, without an atmosphere, the night side is likely solid, limiting heat transfer from the day side.

There may be a thin layer of rock vapor above the magma ocean’s surface, but this alone could cause less significant cooling than observed.

Dr. Anjali Piette, an astronomer at the University of Birmingham, stated, “We truly require a thick atmosphere rich in volatiles to account for all observations.”

“Strong winds could transport heat to the night side while cooling the day side.”

“Gases such as water vapor absorb some wavelengths of near-infrared radiation emitted from the planet’s surface before reaching the atmosphere.”

“Bright silicate clouds might also reflect starlight and cool the atmosphere.”

Although Webb’s findings provide compelling evidence of an atmosphere, the question persists: How can such a small planet exposed to intense radiation maintain an atmosphere, especially one of such significance? Some gas is likely escaping into space, but possibly at a lower rate than expected.

“We believe there is a balance between the magma ocean and the atmosphere,” said Tim Lichtenberg, an astronomer at the University of Groningen.

“As gases escape from the Earth to form the atmosphere, the magma ocean simultaneously reabsorbs them.”

“To account for these observations, this planet would need to be far richer in volatile materials than Earth. It resembles a wet lava ball.”

Findings from this study will be published in today’s Astrophysics Journal Letter.

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Johanna K. Teske et al. 2025. A dense volatile atmosphere over the ultra-hot super-earth TOI-561b. APJL 995, L39; doi: 10.3847/2041-8213/ae0a4c