The rocky exoplanet the size of the Earth lacks atmosphere

Astronomers from MIT, Harvard University and elsewhere have been looking for signs of an atmosphere on a rocky exoplanet, the size of the Earth, without finding any.

Atmospheres have already been detected on planets much larger than ours, including several hot Jupiter and sub-Neptunes, all mainly ice and gas. But this is the first time scientists have been able to determine whether an earth-sized terrestrial planet, external to our solar system, had an atmosphere.

The planet in question, LHS 3844b, was discovered in 2018 by NASA's Transiting Exoplanet Survey Survey (TESS) satellite. It is about 1.3 times larger than the Earth. The planet surrounds its star in just 11 hours, making it one of the fastest exoplanets in orbit in the world. The star itself is a cool little dwarf M who resides just 49 light-years from Earth.

In an article published today in NatureThe team reports that LHS 3844b probably does not have a thick, Venus-like atmosphere, or a thin, Earth-like atmosphere. Instead, the planet probably looks more like Mercury – a thick, burning rock. If an atmosphere existed, the researchers explain that the radiation of the star probably took it early in the formation of the planet.

"We basically found a hot, gas-free planet around," says co-author Daniel Koll, a postdoctoral fellow at the Department of Earth, Atmospheric and Planetary Sciences at MIT. "This is the first time we know in detail the atmosphere of a planet around these M-dwarfs, which are the most common type of stars, making LHS 3844b the most common type of rock planet in the Galaxy."

Could any form of life succeed in settling in such a barren desert? Koll and his colleagues believe that this is extremely unlikely, because the absence of atmosphere would instantly remove all organisms on the surface of the planet. But that does not mean that other terrestrial exoplanets are also without coverage.

"We never thought this planet would be welcoming for life," says lead author Laura Kreidberg, a researcher at the Harvard Center for Astrophysics. "It's more a question of whether this whole category of planets around smaller stars has an atmosphere or not. And our technique is a robust way of assessing whether an atmosphere is present or not. "

Kreidberg and Koll are MIT co-authors: Jason Crossfield, David Berardo, Xueying "Sherry" Guo, George Ricker, Sara Seager and Roland Vanderspek, as well as Harvard colleagues from the University of Texas at Austin Jet Propulsion. Laboratory, Caltech, Stanford University, the University of Maryland, and Vanderbilt University.

Exposed faces

In 2018, LHS 3844b was part of the first series of extrasolar worlds confirmed by TESS, a satellite developed by MIT that monitors the transits of thousands of brightest and closest stars, periodic dives in the light of stars that can signal the presence of a planet orbiting the star. , momentarily blocking its light.

Kreidberg and his team have identified LHS 3844b as an ideal laboratory because its star is bright and close, and therefore a source against which scientists could study the planet in detail. As LHS 3844b is extremely close to its star, and therefore incredibly hot, Kreidberg and Koll felt that it should emit enough heat to reveal clues indicating where it is harboring an atmosphere.

Caricature illustrating the luminosity measured in time (white line) corresponding to a perfectly locked planet orbiting its parent star. The brightness increases as the hot day of the planet comes to view. Image: Laura Kreidberg

LHS 3844b is a perfectly locked planet, which means it has a permanent day and night side, just as the moon always has the same face as the Earth. If an atmosphere existed, it would circulate the heat around the planet and the heat emitted by the sides day and night would be about the same. In the absence of atmosphere, the day would be much warmer than the evening.

As the planet revolves around its star, an observer can see different faces of this planet. When she comes out from behind her star, the day of the planet is exposed. Then, as she turns around the star, the planet pivots to show her night side, before returning to expose her day side before going back behind the star.

The researchers estimated that if they could measure the heat from the different faces of the planet during its orbit, they could determine the difference in temperature between the day and night side and possibly if the planet has a atmosphere.

To test this idea, the team used NASA's Spitzer Space Telescope, an instrument for measuring infrared radiation, or heat, and directed the telescope on LHS 3844b for 100 hours, capturing about 10 orbits of the planet. They measured the heat released by the different faces of the planet through each orbit.

"This has been done a lot for planets closer to Jupiter's size, but it's the very first time this measurement is made for a terrestrial planet around a dwarf star M," says Koll.

"Somewhere over there"

Researchers have calculated from their measurements that the walk of the day is 1000 Kelvin, about 1340 degrees Fahrenheit, while the night falls to 0 K or -460 F. The drastic temperature difference indicates that the planet does not It does not have a thick atmosphere, similar to that of Venus, which would distribute heat evenly across the planet.

The team simulated various scenarios involving thinner atmospheres of different compositions, but found that none of them produced a heat distribution corresponding to their observations, indicating that the planet also does not contain a atmosphere as fine as the Earth.

There was a scenario in which an extremely thin atmosphere, similar to that of Mars, could produce the drastic heat difference of the planet. However, the team discovered that it was unlikely that such a thin atmosphere could remain stable because the star's radiation would quickly dissipate any trace of gas around the planet.

The researchers concluded that LHS 3844b is essentially an extremely hot bare rock. But what kind of rock could it be? In a final step, the team sought to identify its composition by measuring the reflectivity of the planet's surface. As we know on Earth, different minerals reflect light to varying degrees: basalt, a solid black lava, reflects very little light, while lighter rocks, such as granite, containing minerals such as quartz, have a higher reflectivity. The team measured the relationship between the brightness of the star and that of the planet to calculate the reflectivity of the planet.

"We can see that a surface resembling granite is excluded by our data, while a dark material like a lava field in Hawaii, the lunar mare or the surface of Mercury fits our observations," says Kreidberg.

Although the group concluded that there is no atmosphere – and therefore no life – on LHS 3844b, they argue that this may not be the case for similar terrestrial exoplanets in orbit around cold M-dwarfs. They hope to apply their technique to other rocky exoplanets, including those that are farther from their stars and have a better chance of retaining an atmosphere.

"Atmospheres help protect life and protect it from ultraviolet light," says Kreidberg. "I'd be delighted to detect an atmosphere on a planet, even if it's a bit too hot or too cold, because that would indicate, yes, that some terrestrial exoplanets can have an atmosphere, and probably somewhere, there will have one this is the right temperature, which could keep liquid water. "