Over the last few years, researchers have discovered a treasure trove of rocky planets orbiting red dwarfs, small stars with a radius 60 percent smaller than our Sun. Despite their size, these stars are very active – so much so that many believe the planets around them may struggle to keep an atmosphere. Now, new research published in Nature strengthens this idea.
A Harvard-led team of astronomers looked at 100 hours of observations of exoplanet LHS 3844b to identify signs of an atmosphere. They confidently ruled out a dense atmosphere (10 times what we have on Earth) and a less dense atmosphere. The model that best fits the data is a barren rocky planet, similar to Mercury but with a much hotter dayside pushing 770 °C ( over 1400 °F).
"This is the very first time we've been able to say conclusively whether a terrestrial exoplanet has an atmosphere or not!" lead author Dr Laura Kreidberg, a Clay Fellow at the Center for Astrophysics, Harvard and Smithsonian, told IFLScience.
The planet has a radius about 1.3 times that of Earth's own and orbits a red dwarf named LHS 3844, located 48 light-years away in the constellation Indus in the southern sky. It was among the first exoplanets discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) after it launched last year.
The planet orbits the star in 11 hours, being just 10 million kilometers (6.3 million miles) away from it. That’s about 7 percent of the distance between the Earth and Sun. Even though its star is much dimmer than our Sun, it gets a lot more radiation.
The question now is if this new research is applicable to similar Earth-sized planets such as Proxima b or the TRAPPIST-1 planets that have been discovered over the last few years. These planets are further away from their stars than LHS 3844b. By being less irradiated, they might have better luck at maintaining their atmosphere against erosion from the stellar wind.
"It's hard to generalize from a sample of one. I would say that our result confirms theoretical predictions that hot terrestrial planets around M-dwarfs have a tough time holding onto their atmospheres," Dr Kreidberg explained. "We need to make this measurement for more planets, then we can take advantage of the big sample size of exoplanets to evaluate how often and under what circumstances they keep their atmospheres."
The study of this planet's lack of atmosphere was conducted with NASA’s infrared observatory Spitzer, whose mission is about to end. The much-advanced replacement for it, the James Webb Space Telescope (JWST), will hopefully be in space in 2021. Recent estimations suggest the excellent capabilities of JWST will lead to rapid observations of Earth-sized planet atmospheres. A study also suggests it might be able to characterize all seven planets' atmospheres in the TRAPPIST-1 system in just one year.