A new method might tell scientists whether or not a rocky exoplanet—the kind of exoplanet most similar to Earth—has an atmosphere.
In the past decade, astronomers have spotted lots of seemingly Earth-like rocky exoplanets orbiting in the “habitable zone,” the region around a star with balmy temperatures that might allow water to exist as a liquid on a planet’s surface. But unlike Earth, many of these planets sit close to small, cool, and active M-dwarf stars, whose radiation and stellar winds might obliterate any chance of these planets forming atmospheres. Whether or not these rocky exoplanets can hold onto an atmosphere is now a major question in the field.
“We’ve never characterised the atmospheres of a rocky planet other than those in our solar system,” study author Eliza Kempton, University of Maryland assistant professor, told Gizmodo. “This is the first major stepping stone towards characterising rocky planets in the habitable zone.”
The researchers propose watching known rocky exoplanets right before they pass behind their parent star, comparing that amount of light to the light seen from the star alone. Calculations of the distance between the star and the planet tell them how much radiation the planet receives, and the difference between the light astronomers see before and after the eclipse tells them how much light the planet has re-radiated during its daytime. If the planet re-radiates all of the light, then it probably doesn’t have an atmosphere. But if some of the light has gone missing, it’s evidence that an atmosphere has redistributed some of the energy to the rear of the planet or scattered it. The researchers published their methods and the theory behind them in four papers in the Astrophysical Journal (1, 2, 3, 4).
Others have attempted to find atmospheres on exoplanets before; this past year, a team led by scientist Laura Kreidberg determined that the exoplanet LHS 3844b had no atmosphere by looking at the light it emitted while completing an orbit around its star. But the team behind the new method wanted to speed up the search. The James Webb Space Telescope, capable of telling astronomers more about exoplanets than ever before, will hopefully launch in 2021, and astronomers will surely compete aggressively for its precious observing time. So this team of researchers focused on devising a survey method that could provide a simple yes-or-no response for tens of exoplanets in a single observing run.
The method won’t tell us what’s in the atmosphere, like whether or not it’s been altered by the presence of life or whether it looks like our own atmosphere, Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University who was not involved in the study, told Gizmodo in an email. But it’s still useful, she said. “This method can be used to prioritise which planets to observe in more detail.”
There are other limits to the method, Kempton said. Rocky planets in the habitable zone might contain water-altered substances like clay whose light-reflecting behaviour fools researchers into thinking there’s an atmosphere. Astronomers won’t be using this strategy to survey the habitable zone planets, the ones with water that we hope have signs of life. Instead, they will focus on planets closer to their stars.
But that’s ok—researchers are taking things one step at a time. “What we want to do with [James Webb Space Telescope] is start with an easier target,” Kempton said. “Planets that are somewhat warmer than habitable will be easier to characterise, take less telescope time, and get higher-fidelity results.” Astronomers want to know whether these rocky exoplanets orbiting these small, cool M-dwarfs have atmospheres at all. If they do have atmospheres, then it’s a reason to keep looking at the habitable zone planets.
The James Webb Space Telescope might have the unique opportunity to quickly identify which exoplanets have atmospheres. Let’s just hope it launches on time.