Over the last two decades, Kepler and other space telescopes have discovered thousands of exoplanets orbiting other stars. Of these, only 52 are considered to be potentially habitable because the planet is the right distance from its host star to support liquid water on its surface. But once an exoplanet is discovered, learning more about its basic characteristics is notoriously difficult, even for the closest potentially habitable exoplanet: Proxima b.
Although astronomers developed a technique for directly observing exoplanet atmospheres last year, this is limited to massive exoplanets that are about twice the size of Jupiter and not at all Earth-like. For now, astronomers must rely on simulations in order to study smaller exoplanets such as Proxima b. But as recent research from the University of Exeter goes to show, there is a great deal that can be learned about the habitability of an exoplanet based on simulations.
"It's amazing we get anything based on exoplanet observation, but the data is very sparse and requires interpretation." Nathan Mayne, a senior lecturer in astrophysics at the University of Exeter, told me over Skype. "Without some sort of model that applies known physics from our atmosphere and the atmospheres in our solar system, you really can't tell a whole lot about what we really care about: the potential conditions in the atmosphere of Proxima b."
To model the climate on Proxima b, Mayne and his colleagues used the Unified Model, one of the most sophisticated numerical models of Earth's climate. But since the Unified Model is made for our planet, and as such takes into account variables like vegetation and the state of our ice caps. For Proxima b, the Exeter crew had to strip the Unified Model down to the basics, taking into account primary climate drivers like convection (the vertical transport of heat and moisture in the atmosphere), rainfall, and heating.
Then, Mayne and co had to plug in parameters for Proxima b, such as its orbital characteristics and the characteristics of its host star, the red dwarf Proxima Centauri. Since the actual atmospheric composition of Proxima b is unknown, the team ran simulations in which the planet was totally covered in water and had an atmosphere similar to Earth, as well as a simulation in which the planet's atmosphere is dominated by nitrogen. Yet just because the team was modeling the climate of Proxima b based on Earth-like atmospheric conditions, that doesn't mean the planet looks anything like ours.
For example, since Proxima b is far closer to its host star than the Earth is to the sun—about 1/20 of the distance—this suggests it might be tidally locked. That means that the same side of the planet is always facing the star, just like one side of the moon is always facing the Earth. After running the simulations and playing with the parameters for atmospheric composition and orbital characteristics, Mayne and his colleagues concurred with previous research which suggested Proxima b likely supported surface liquid water. In this case, the models suggested that the daylight side of the planet would be subject to a perpetual rainstorm if Proxima because the star would be constantly warming up the ocean, resulting in moisture constantly rising to the atmosphere on this side of the planet, where it would form the clouds that would dump the water back into the ocean.
Another theory explored by the Exeter group is if Proxima b has an eccentric 3:2 resonance orbit like Mercury, where the planet rotates three times for every two orbits around the sun. In this case, rather than one side of the planet being warm and receiving constant rainfall, Mayne said that there would be a "smear" of warmer temperatures around the equator, rather than one whole hemisphere.
Moreover, the team found that relatively well-understood processes on Earth such as the ice-albedo feedback—where planetary cooling leads to more ice coverage and thus more sunlight reflected from the planet that in turn leads to more cooling and more ice—wouldn't be likely to apply on Proxima b since its host star's emissions are more in the infrared spectrum than our Sun.
While these results are fascinating and a novel application of Earth climate models to other Earth-like planets (previously the Exeter team had only been using the Unified Model to simulate the climate of giant Hot Jupiters), the simulations are based on a lot of unknowns. Exoplanet climate modeling must be approximate until astronomers get better data about Proxima b, such as the planet's orbital inclination or actual atmospheric composition. For now, though, Mayne and his colleagues are excited to have used simulations to discover that Proxima b is habitable under a far greater array of orbital and atmospheric conditions than previously thought possible.
"If it had an Earth-like atmosphere, Proxima b would certainly be an interesting place to hang out," Mayne said. "If you fancy a planet-wide thunderstorm, maybe it's for you."