NASA's Spitzer Space Telescope sheds light on Moon-like exoplanet
NASA's Spitzer Space Telescope has provided new insights into a rocky exoplanet that may be very similar to the planet Mercury or Earth's Moon. Orbiting an M-type dwarf star 48.6 light years from Earth in the constellation of Indus, LHS 3844b is 1.3 times the mass of our planet, probably airless, and covered with lava plains resembling the lunar maria.
With the number of exoplanets found now well into the thousands and many more expected to be discovered, tallying up one more is becoming a bit old hat, but newer space and ground telescopes allow astronomers to not only confirm the existence of exoplanets, but also tell us something about their nature.
LHS 3844b was found in 2018 by the NASA's Transiting Exoplanet Satellite Survey (TESS) mission by measuring the dip in its parent star's light curve as the planet passed in front of it. That's good enough for the catalogs, but when the infrared Spitzer telescope was turned on the exoplanet, things got a bit more interesting.
For one thing, its parent star is of a type that's not only the most common and long-lived in the Milky Way galaxy, but also home to a very large percentage of exoplanets. LHS 3844b revolves around its star in a "year" that lasts only 11 hours at so close a distance that NASA says its tidally locked and only shows one face to its star – just as the Moon does to the Earth.
It's also so close the day side has a surface temperature of 1,410° F (770° C). That's very handy because M-type stars tend to be not only small, but also on the cool side, so the infrared light being radiated from LHS 3844b can be seen directly by Spitzer. Not only that, but the telescope can see with enough resolution to differentiate between the day and night sides.
According to NASA, the research team found that the night side is very cold. This means that there is probably little or no atmosphere. Otherwise, storms would arise and the winds would blow onto the cold dark side and warm it. In the case of LHS 3844b, it matches computer models of the sort of temperature profile you'd expect on an airless world. As to what happened to the atmosphere, that is another question.
"We've got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven't been able to study them empirically," says Laura Kreidberg, 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."
The basic mechanism is that M dwarfs emit high levels of ultraviolet radiation as well as frequent solar flares – both of which can strip away the atmosphere of worlds that aren't very massive or protected by powerful magnetic fields. The Spitzer data indicates that although an atmosphere might exist under very peculiar circumstances, the most likely conclusion is that the planet has very little, if any air.
In addition to this, the light coming from LHS 3844b shows that it's quite dark, suggesting that its surface is similar to the Moon, with great lava fields that have cooled into basalt. As to whether the planet is typical of those orbiting M-type stars, that remains to be seen.
"I'm still hopeful that other planets around M dwarfs could keep their atmospheres," says Kreidberg. "The terrestrial planets in our solar system are enormously diverse, and I expect the same will be true for exoplanet systems."
The research was published in Nature.