Scientists have separated out a trio of key processes to understand how each one keeps heat moving and shapes the atmospheres of tidally locked planets, which have permanent day and night sides.

The researchers simulated how winds transport heat around rocky and gaseous planets, and say that understanding this motion is key for interpreting telescope observations of the atmospheres of tidally locked exoplanets, as well as determining whether these worlds might be habitable. The team reported the findings March 22 in Proceedings of the National Academy of Sciences.

A planet is tidally locked when the same point on the planet always faces its host star. 

"The Earth is spinning quite fast on its axis, and this rate of spin is different from the rate at which the planet goes around the sun, and this means that we have a day-and-night cycle," said Neil Lewis, a Ph.D. student in atmospheric, oceanic and planetary physics at the University of Oxford and last author of the paper. "But [for] these tidally locked planets, the planet spins on its axis at exactly the right rate that it matches its rotation around the sun." 

As a result, one side of the planet is always bathed in sunlight while the other lies in darkness. 

Tidal locking happens when a planet is close enough to its star that the gravitational interaction between the two bodies causes it to bulge. "And then the star's gravity tugs on this bulge and keeps it locked in this day and night setup," Lewis said.

Tidally locked exoplanets tend to be easy for telescopes to spot because they orbit so close to their suns. This means a tidally locked planet passes in front of the star — causing its light to briefly dim — more often than a distant planet, and it also appears relatively larger. 

Being tidally locked has a profound effect on a planet's atmospheric circulation, which is influenced both by the tendency of heated air from the day side to move toward the cooler night side and by the planet's spin.

This motion transports heat, chemicals and clouds and could be key for keeping a planet's atmosphere stable enough to possibly support life. It can also keep a potentially habitable planet's surface temperatures from becoming too extreme. 

"Suppose you weren't able to transport heat very well and you had a very cold night side," Lewis said. "The atmosphere can kind of collapse on the night side because condensation happens when you get cool enough."

Previous research has indicated that atmospheric circulation on tidally locked exoplanets is driven by three main components: a jet of wind going around the planet, a bit like the jet streams found on Earth; vast waves in the atmosphere, described by Lewis as "a bit like waves in the sea"; and direct flow from the day side to the night side, which has some similarities to the tropical Hadley and Walker circulations on Earth.

Lewis and his coauthor Mark Hammond, a postdoctoral fellow in geophysical sciences at the University of Chicago, used a mathematical technique commonly applied to studying Earth's atmosphere to disentangle each feature from the others. 

The team used atmospheric models to explore how the three components contribute to the total circulation on a hypothetical rocky planet and gas giant.

The gas giant was based on a world called HD 189733b that lies 63 light-years from Earth and is a type of exoplanet known as a hot Jupiter. The rocky planet resembled the seven exoplanets orbiting the star TRAPPIST-1.

On the rocky planet, the simulations indicated that, despite having much weaker winds, the day-to-night flow was more important overall for transporting heat than the jet or atmospheric waves.  

"On the hot Jupiter, the heat transport picture is a bit more of a mess, and all the components are contributing," Lewis said. "It shows that even though the net effect is quite similar — you heat on the day side and you take some of it to the night side — on these two types of planets, the way it's getting there is quite different."

In the future, the researchers plan to consider a wide array of tidally locked planets and how much the three atmospheric components depend on characteristics such as a planet's mass, rotation rate and the amount of starlight it receives. 

Jonathan Fortney, a professor of astronomy and astrophysics and director of the University of California, Santa Cruz's Other Worlds Laboratory who was not involved in the research, sees another potential question to investigate on gaseous exoplanets.

"I would be quite interested in seeing how the results are affected by changing the temperature of the deep atmosphere ... as that can vary significantly from planet to planet," he said. "Generally, it is important to examine the details underlying sophisticated 3D atmosphere models, and this is an important work in this direction." 

The study, "The rotational and divergent components of atmospheric circulation on tidally locked planets," published March 22 in Proceedings of the National Academy of Sciences, was authored by Mark Hammond, University of Chicago; and Neil T. Lewis, University of Oxford.