Planets that orbit close to their stars might lose their atmospheres along with any chance of life, but new models show a way in which these planets may retain their atmospheres and habitability.

When scientists search the cosmos for potentially habitable planets or alien life, they look to terrestrial planets. A number of rocky, possibly Earth-like, planets orbit around M dwarf stars, a smaller type of star than our Sun. These exoplanets may seem like good possibilities for finding alien life, but because of an M dwarf’s small size, planets orbit close to the star and can become tidally locked. This could potentially lead to the planets’ atmospheres collapsing entirely, rendering them uninhabitable.

At this point, said Daniel Koll, a graduate student at the University of Chicago, nobody knows whether tidally locked exoplanets are habitable. To try and answer this question, Koll researches how atmospheres behave on tidally locked planets and what conditions could prevent atmospheric collapse.

Two options exist for how the atmospheres of tidally locked planets can behave: atmospheric collapse or global circulation of the atmosphere. “In general, we don’t really understand the atmospheric physics,” said Koll at his poster presentation during the American Geophysical Union’s 2015 Fall Meeting in San Francisco, Calif. In fact, we don’t fully understand the atmospheric physics of our own planet, he said. So not only does his work help scientists understand where they can look for life, it could also help them understand our own planet’s atmospheric dynamics.

Other researchers have already run general circulation models, which people use to model our own changing climate, for these types of tidally locked exoplanets. What Koll did was devise models based on various basic physical conditions that could occur and see which line up best with those circulation models. Koll says two options exist for how the atmospheres of tidally locked planets can behave: atmospheric collapse or global circulation of the atmosphere.

Entire Atmospheric Collapse

The first option is that the atmosphere collapses entirely. Tidally locked planets always present the same side to their star, much like one side of the Moon always faces Earth, because strong gravitational gradients cause the planet’s orbital and rotational periods to synchronize. The nightside of a locked planet can get very cold, Koll said—temperatures can reach as low as 30 kelvins.

Temperatures that low are below the condensation point of the atmosphere, Koll said. When this happens, the atmosphere freezes and forms ice or nitrogen glaciers on the planet’s surface, leaving no gaseous atmosphere. But the planet doesn’t just settle with one half of it lacking an atmosphere. The atmosphere tries to attain a pressure equilibrium as gas moves from the dayside to the nightside, only to freeze once again. That process would repeat over and over until no atmosphere remained on either side of the planet.

Global Circulation Can Maintain Atmospheres

The other option for a tidally locked atmosphere, the option that Koll studies, is that the planet can effectively redistribute its atmosphere to its nightside without it condensing down. To understand what physics governs these atmospheres, Koll built different properties into his models.

The first was pretty simple: the nightside isn’t one uniform temperature. This nonuniformity brought his model’s results closer to the general circulation model’s line, but not close enough. He next approached the system as a heat engine that works in a way similar to how hurricanes operate on Earth. The atmosphere on the dayside heats up and moves over to the nightside, where it cools down as the planet and atmosphere radiate more heat out into space than heat comes in.

When the cooling atmosphere sinks closer to the planet’s surface, the waves of its movement slow and lose energy. That lost energy emerges as heat, which then warms the atmosphere again so it can make its way back to the dayside. This type of global circulation would retain a planet’s atmosphere because it never gets cold enough to condense out. When Koll plotted his modeling of this behavior, it lined up exactly to the global circulation model baseline.

“More greenhouse gases in the atmosphere make the atmosphere better at moving heat over to the nightside,” said Koll. Planets with thick atmospheres can circulate and retain their atmospheres better as a result. Getting this close to the global circulation models is quite a feat.

A Near-Perfect Fit

Not only did Koll’s models line up almost perfectly with the circulation models, but he also used fairly simple models that he writes out by hand in a composition book. By using such relatively simple models and having such accurate results, Koll feels that he and his colleague are gaining a solid understanding of how the atmosphere behaves on tidally locked planets. This knowledge could help determine which planets could possibly contain life or be habitable.

“Getting this close to the global circulation models is quite a feat,” said Cedric Gillmann, a planetary scientist from the Royal Observatory of Belgium. Even though Koll’s line looks almost too good to be true, Gillmann says that it’s possible to get such accurate models if the basic parameters are correct, which in Koll’s case they appear to be. “It’s a small miracle,” said Gillmann, who, along with Koll, is looking forward to the day when technology allows people to actually observe these exoplanets and their atmospheres.

– Cody Sullivan, Writer Intern

Citation: Sullivan, C. (2015), Atmospheres can collapse on the dark sides of planets, Eos, 96, doi:10.1029/2015EO042551. Published on 29 December 2015.