The most common star in our galaxy is a red dwarf, smaller and dimmer than Earth. Because these small stars put out much less radiation, the region where planets could have liquid water on their surfaces is much closer to the star. In these exosolar systems, the habitable zone is typically closer to the star than Mercury is to our Sun.
That's a good match to our current technology, which is best at identifying planets close to their host stars. But it has raised questions about whether these close-in planets could actually be habitable, given that red dwarf stars are prone to violent outbursts. Now, researchers have taken a close look at a planet orbiting close to a red dwarf and have found that it looks like a bare rock, suggesting that its star may have stripped off any atmosphere that once existed.
Studying the atmosphere of an exoplanet typically involves observations of it creating a partial eclipse of its host star. In these cases, some of the starlight passes through the planet's atmosphere, allowing us to get a sense of its composition. If there's no sign of this sort of change, then we typically infer that the planet doesn't have an atmosphere.
But that's not the only way to determine if an atmosphere is present. The new work focuses on one of the effects an atmosphere has: it redistributes heat. This is a critical feature on planets near their host star, which are often tidally locked, their rotation period identical to their orbit length, so that one side remains permanently oriented toward the star. With no atmosphere, all that heat simply radiates back off the same side, and the opposite side remains frigid. With an atmosphere, some of that heat will be carried around to the far side of the planet, warming it.
This approach seems to be ideal for a recently discovered exoplanet, LHS 3844b, a super-Earth with a radius 1.3 times larger than that of our home planet. So a team of researchers arranged to have the Spitzer Space Telescope stare at the star and its nearby companion for 100 hours. Given that the planet only takes 11 hours to complete an orbit, this provided several complete records of orbits, with the planet in various orientations relative to its star, including partly eclipsing it and moving behind the star from Earth's perspective.
This let the researchers track the radiation coming from the system when the planet had slightly different orientations, with different fractions of its hot and cool sides oriented toward Earth.
On the sunny side
The amount of infrared radiation coming from the planet allowed the researchers to estimate the temperature of the two different sides of the planet. On the hot side, temperatures seem to be in the neighborhood of 1,000 Kelvin (725°C). On the other side of the planet, the temperature was, within the limits of measurement error, indistinguishable from zero.
As you might imagine, that's not consistent with heat being redistributed to the cooler side of the planet. If its atmosphere is rich in the greenhouse gas carbon dioxide, then the data indicates that it has to be less dense than Mars' sparse atmosphere. Even without any greenhouse gases, the atmosphere can't possibly be much more dense than Earth's, even though the planet is substantially larger.
But the news isn't much better for a thin atmosphere. The researchers modeled the rate of atmospheric escape given the amount of radiation it receives from its host star. They find that any atmosphere that's consistent with the present limits based on heat redistribution is unstable and will be completely eroded by the star's light. "Given that thick atmospheres are ruled out by the data," the researchers write, "and thin atmospheres are unstable over the planet’s lifetime, LHS 3844b is probably a bare rock, unless a thin atmosphere is continually replenished over time."
Not content with that conclusion, the researcher then analyzed the light reflected from the planet to figure out what kind of rock we're looking at. The material most consistent with the infrared data is basalt, a rock produced by volcanic activity. In this regard, the planet looks a lot like Mercury, or the dark regions on the Moon. This could result from ongoing volcanic activity, or it could be the product of the planet having spent some time early in its history with a magma ocean on its surface.
All this doesn't rule out an atmosphere for every planet around a red dwarf. LHS 3844b is extremely close to its host planet, so it's possible that planets a bit farther out will have a better chance of atmospheric survival. But this is only the second time that there has been a publication describing the successful use of searching for atmospheres this way, so it provides an important validation of our ability to study additional planets.