Scientists detect atmosphere on molten rocky exoplanet

Researchers using NASA’s James Webb Space Telescope have detected the strongest evidence yet for an atmosphere on a rocky planet outside our solar system.

Observations of the ultra-hot super-Earth TOI-561 b suggest the exoplanet is surrounded by a thick blanket of gases above a global magma ocean.

Publishing their findings in The Astrophysical Journal Letters, researchers say that the results help to explain the planet’s unusually low density and challenge the prevailing wisdom that relatively small planets so close to their stars cannot sustain atmospheres.

With a radius 1.4 times Earth’s, and an orbital period less than 11 hours, TOI-561 b falls into a rare class of objects known as ultra-short period exoplanets.

Although its host star is only slightly smaller and cooler than the Sun, TOI-561 b orbits so close to the star — less than one million miles or one-fortieth the distance between Mercury and the Sun — that it must be tidally locked, with the temperature of its permanent dayside far exceeding the melting temperature of typical rock.

Co-author Dr Anjali Piette, from the University of Birmingham, said: “We really need a thick volatile-rich atmosphere to explain all the observations. Strong winds would cool the dayside by transporting heat over to the nightside.

“Gases like water vapour would absorb some wavelengths of near-infrared light emitted by the surface before they make it all the way up through the atmosphere. The planet would look colder because the telescope detects less light, but it’s also possible that there are bright silicate clouds that cool the atmosphere by reflecting starlight.”

One explanation the team considered for the planet’s low density was that it could have a relatively small iron core and a mantle made of rock that is not as dense as rock within Earth.

Lead author Johanna Teske, staff scientist at Carnegie Science Earth and Planets Laboratory, said: “What really sets this planet apart is its anomalously low density. It is less dense than you would expect if it had an Earth-like composition.

“TOI-561 b is distinct among ultra-short period planets in that it orbits a very old, iron-poor star – twice as old as our sun - in a region of the Milky Way known as the thick disk. It must have formed in a very different chemical environment from planets in our own solar system.”

The planet's composition could be representative of planets that formed when the universe was relatively young.

The team also suspected that TOI-561 b might be surrounded by a thick atmosphere that makes it look larger than it is. Although small planets that have spent billions of years baking in blazing stellar radiation are not expected to have atmospheres, some show signs that they are not just bare rock or lava.

Using Webb’s NIRSpec (Near-Infrared Spectrograph) to measure the planet’s dayside temperature based on its near-infrared brightness, researchers tested the hypothesis that TOI-561 b has an atmosphere. The technique involves measuring the decrease in brightness of the star-planet system as the planet moves behind the star - similar to that used to search for atmospheres in the TRAPPIST-1 system and on other rocky worlds.

If TOI-561 b is a bare rock with no atmosphere to carry heat around to the nightside, its dayside temperature should be approaching 4,900 degrees Fahrenheit (2,700 degrees Celsius). But the NIRSpec observations show that the planet’s dayside appears to be closer to 3,200 degrees Fahrenheit (1,800 degrees Celsius) — still extremely hot, but far cooler than expected.

To explain the results, the team considered a few different scenarios. The magma ocean could circulate some heat, but without an atmosphere, the nightside would be solid, limiting flow away from the dayside. A thin layer of rock vapour on the surface of the magma ocean is also possible, but on its own would have a much smaller cooling effect than observed.

Compelling evidence

While the Webb observations provide compelling evidence for such an atmosphere, the question remains: how can a small planet exposed to such intense radiation hold on to any atmosphere at all, let alone one so substantial?

Co-author Tim Lichtenberg, from the University of Groningen, Netherlands, said: “We think there is an equilibrium between the magma ocean and the atmosphere. While gases are coming out of the planet to feed the atmosphere, the magma ocean is sucking them back into the interior. This planet must be much, much more volatile-rich than Earth to explain the observations. It's really like a wet lava ball.”

These are the first results from Webb’s General Observers Program 3860, which involved observing the system continuously for more than 37 hours while TOI-561 b completed nearly four full orbits of the star. The team is analysing the full data set to map the temperature all the way around the planet and narrow down the composition of the atmosphere.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).