The latest data from NASA’s Spitzer Space Telescope shows the exoplanet’s surface may resemble those of Earth’s Moon or Mercury. The planet likely has little to no atmosphere and could be covered in the same cooled volcanic material found in the dark areas of the Moon’s surface. Discovered in 2018 by NASA’s Transiting Exoplanet Satellite Survey (TESS) mission, planet LHS 3844b is found 48.6 light-years from Earth and is 1.3 times the size of Earth.
The alien exoplanet orbits a small, cool type of star called an M dwarf.
I’m still hopeful that other planets around M dwarfs could keep their atmospheres
Dr Laura Kreidberg
As the Milky Way’s most common and long-lived type of star, M dwarfs may host a high percentage of the total number of planets in the galaxy.
TESS found the planet via the transit method, using the observed light of a parent star which dims because of a planet orbiting between the star and Earth.
Another method – detecting light directly from a planet’s surface – is difficult because the star is so much brighter and drowns out the planet’s light.
But during follow-up observations, Spitzer was able to detect light from the surface of LHS 3844b.
The planet makes one full revolution around its parent star in just 11 hours.
With such a tight orbit, LHS 3844b is most likely “tidally locked,” where one side of a planet permanently faces the star.
The star-facing side is about 770C (1,410F), meaning the planet radiates a lot of infrared light, capable of being detected by the Spitzer infrared telescope.
The planet’s parent star is relatively cool, making direct observation of LHS 3844b’s dayside possible.
This observation marks the first time Spitzer data have been able to provide information about the atmosphere of a terrestrial world around an M dwarf.
By measuring the temperature difference between the planet’s hot and cold sides, NASA discovered there is a negligible amount of heat being transferred between the two.
If an atmosphere were present, hot air on the dayside would naturally expand, generating winds that would transfer heat around the planet.
On a rocky world with little to no atmosphere, like the Moon, there is no air present to transfer heat.
Dr Laura Kreidberg, a researcher at the Harvard and Smithsonian Centre for Astrophysics, and lead author of the new study, said: “The temperature contrast on this planet is about as big as it can possibly be.
“That matches beautifully with our model of a bare rock with no atmosphere.”
Understanding the factors that could preserve or destroy planetary atmospheres is part of how scientists plan to search for habitable environments beyond our solar system.
Earth’s atmosphere is the reason liquid water can exist on the surface, enabling life to thrive.
On the other hand, the atmospheric pressure of Mars is now less than one percent of Earth’s, and the oceans and rivers that once dotted the Red Planet’s surface have disappeared.
Dr Kreidberg added: ”We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically.
“Now, with LHS 3844b, we have a terrestrial planet outside our solar system where for the first time we can determine observationally that an atmosphere is not present.”
Compared to Sun-like stars, M dwarfs emit high levels of ultraviolet light, which is harmful to life and can erode a planet’s atmosphere.
M dwarfs are particularly violent in their youth, belching bursts of radiation and particles capable of stripping away budding planetary atmospheres.
The Spitzer observations rule out an atmosphere with more than 10 times the pressure of Earth’s.
An atmosphere between one and 10 bars on LHS 3844b has been almost entirely ruled out as well.
However, the authors note there’s a slim chance it could exist if the stellar and planetary properties were to meet some very specific and unlikely criteria.
The NASA researchers believe with the planet so close to a star, a thin atmosphere would be destroyed by the star’s intense radiation and outflow of stellar winds.
Dr Kreidberg added: “I’m still hopeful that other planets around M dwarfs could keep their atmospheres.
“The terrestrial planets in our solar system are enormously diverse, and I expect the same will be true for exoplanet systems.”
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