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The most prevalent star in our galaxy is a red dwarf, smaller and darker than the Sun. Because these small stars emit much less radiation, the region where the planets might have liquid water on their surface is much closer to the star. In these exosolar systems, the habitable zone is usually closer to the star than Mercury of our sun.
This fits well with our current technology, which is best for identifying planets close to their host stars. But he asked whether these nearby planets could actually be habitable, since red dwarf stars are prone to violent outbursts. Researchers have closely examined a planet in orbit close to a red dwarf and have discovered that it resembles a bare rock, suggesting that its star may have removed any atmosphere that existed before.
Redistribution
The study of the atmosphere of an exoplanet usually involves observation of it creating a partial eclipse of its host star. In these cases, part of the starlight passes through the atmosphere of the planet, which allows us to have an idea of its composition. If there is no sign of this kind of change, we generally infer that the planet does not have an atmosphere.
But this is not the only way to determine if an atmosphere is present. The new work focuses on one of the effects of an atmosphere: it redistributes heat. It is an essential element on the planets close to their host star, which are often locked by tidal points, whose rotation period is identical to the length of their orbit, so that ## EQU1 ## 39, one side remains permanently oriented towards the star. Without atmosphere, all this heat is simply returned to the same side and the opposite side remains freezing. With an atmosphere, some of this heat will be transported to the outside of the planet, warming it up.
This approach seems ideal for a recently discovered exoplanet, LHS 3844b, a super-Earth whose radius is 1.3 times larger than that of our planet. A team of researchers arranged for the Spitzer Space Telescope to fix the star and its companion nearby for 100 hours. Since the planet only takes 11 hours to complete an orbit, several complete orbit recordings have been recorded, the planet having various orientations relative to its star, including partially eclipsing it and moving behind it. the star from the point of view of the Earth.
This allowed researchers to track radiation from the system when the planet had slightly different orientations, with different fractions of its sides hot and cold facing the Earth.
On the sunny side
The amount of infrared radiation from the planet has allowed researchers to estimate the temperature on both sides of the planet. On the warm side, temperatures seem to be around 1000 Kelvin (725 ° C). On the other side of the planet, the temperature was, within the limits of measurement error, impossible to distinguish from zero.
As you can imagine, this is not compatible with a redistribution of heat towards the colder side of the planet. If its atmosphere is rich in carbon dioxide, the data indicate that it must be less dense than the sparse atmosphere of Mars. Even in the absence of greenhouse gases, the atmosphere can not be much denser than that of the Earth, even though the planet is much larger.
But the news is not better for a thin atmosphere. The researchers modeled the rate of atmospheric evacuation given the amount of radiation that it receives from its host star. They find that any atmosphere consistent with current limits based on heat redistribution is unstable and will be completely eroded by starlight. "Since the data excludes thick atmospheres," the researchers write, "and that thin atmospheres are unstable throughout the life of the planet, the LHS 3844b is probably a bare rock, unless thin atmosphere is continually restored over time ".
Not satisfied with this conclusion, the researcher then analyzed the light reflected by the planet to determine the type of rock we are examining. The most compatible material with infrared data is basalt, a rock produced by volcanic activity. In this respect, the planet looks a lot like Mercury or the dark regions of the Moon. This could result from ongoing volcanic activity, or it could be the product of the planet that spent some time at the beginning of its history with an ocean of magma on its surface.
All of this does not exclude an atmosphere for each planet around a red dwarf. LHS 3844b is extremely close to its host planet. It is therefore possible that planets a little further away have a better chance of atmospheric survival. But this is only the second time that there is a publication describing the successful use of atmospheric research in this way. It is therefore an important validation of our ability to study new planets.
Nature, 2019. DOI: 10.1038 / s41586-019-1497-4 (About DOIs).
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