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Like Mercury, a nearby ‘lava world’ is probably lacking an atmosphere

WASHINGTON, D.C. — Last year, a nearby rocky world was spotted by NASA’s planet-hunting telescope TESS. Now, astronomers have been able to observe the planet using the Spitzer Space Telescope, taking a closer look at this intriguing world.

The planet, called LHS 3884b, is 49 light-years away in the Indus constellation, orbiting a cool M-dwarf star that’s only about one-fifth the size of our sun. At the time of its initial discovery, astronomers thought it was probably a “lava world” given how close some of the planet’s rocky surface is to its star. The planet has a radius 1.3 times that of Earth.

The Spitzer Space Telescope is in NASA’s Great Observatories family. Spitzer detects infrared light, Hubble captures visible and UV light, Compton was designed for gamma rays, and Chandra sees X-rays.

This artist’s illustration shows LHS 3844b, a rocky nearby exoplanet. It’s 1.3 times the mass of Earth and orbits a cool M-dwarf star.

Spitzer detected light from the planet’s surface as it zipped around its star in a whopping 11-hour orbit. Because the planet has such a short orbit, one side faces the star and is blasted with heat that causes the surface of that side to reach 1,410 degrees Fahrenheit. The heat causes the planet to radiate infrared light, making it perfect for observation by Spitzer.

The side facing away from the star is significantly colder, and little heat is transferred between the two sides of the planet. But if an atmosphere were present, it would allow for the force of wind to carry hot air to the other side of the planet.

Instead, the planet is more like our moon or Mercury, with little or no atmosphere, according to a study published Monday in the journal Nature.

“We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically,” said Laura Kreidberg, lead study author and a researcher at the Harvard and Smithsonian Center for Astrophysics. “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.”

Although the M-dwarf star that the planet is orbiting is cool in temperature compared with other stars, it unleashes flares of ultraviolet light that can destroy a planetary atmosphere. And in this case, the planet is so close to the star that the stellar wind coming off it would probably do away with the atmosphere.

As expected, Spitzer’s data shows that the planet’s dark surface is probably covered in volcanic basalt or magma, or cooled volcanic material, likely from ancient volcano activity. The researchers believe that the planet is “a bare rock, unless a thin atmosphere is continually replenished over time.”

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