Are tidally locked exoplanets more habitable than we once thought?

Graphic displaying the atmospheric processes at work on a simulated exoplanet with a surface friction similar to Earth (on the left) and with 10 times Earth's friction value (right)(Credit: KU Leuven - Ludmila Carone and Leen Decin)

A team of researchers from the University of Leuven (KU Leuven) in Flanders, Belgium, has discovered a link regarding the level of friction between an exoplanet's lower atmosphere and the surface of tidally-locked exoplanets, and their potential for supporting life. The study focuses on exoplanets orbiting M dwarf stars, a class of stellar bodies significantly smaller and dimmer, yet much more common than Sun-like stars.

The majority of rocky exoplanets orbiting M dwarf stars only ever show one face to their star. To put the concept in a familiar context we can look to Earth's satellite, the Moon. One side of the Moon is constantly locked to planet Earth thanks to a phenomena known as tidal locking. This is why we always see the same face of the Moon no matter when or from where we view it on the surface of our blue marble.

For scientists searching for habitable planets around M dwarfs, this presents a serious problem, as the influence of the star would make the day side of a tidally-locked exoplanet extremely hot, while the night side would be incredibly cold. As you can imagine, these are not conditions favorable to the presence of life.

However, last year, a team of researchers from KU Leuven discovered that tidally-locked exoplanets could be rendered habitable by a global air conditioning process in the atmospheres of the distant worlds. The scientists used sophisticated computer simulations to model the exoplanet atmospheres, taking into account variations in exoplanet size and spin speed, and were able to identify three potential atmospheric processes that could be occurring on satellites orbiting M dwarf stars.

In two of the scenarios, the cold air from the night side is transferred to the day side, where it is gradually heated and moved back to the night side of the exoplanet. The constant circulation of air could create a relatively hospitable temperature. In the third scenario, a powerful air current high in an exoplanet's atmosphere disrupts the heat redistribution process, rendering the planet uninhabitable.

The new study, also carried out by researchers from KU Leuven, examined the link between an exoplanet's surface characteristics and the disruptive current. The team ran hundreds of simulations designed to observe the effect that the friction between a planet's surface and its lower atmosphere would have on the fragile air conditioning system.

The computer-generated models calculated two distinct friction levels. One set of simulations used an atmosphere to surface friction level similar to that known to occur on Earth. A more extreme set of simulations saw the friction value set at 10 times that of our planet. The results of the study suggest that the greater the friction between the lower atmosphere and the surface, the more likely it is that the powerful disruptive air currents will be suppressed, allowing the cooling process to occur.

The team stresses that surface friction is not the only factor governing the habitability of tidally-locked exoplanets. For example, an exoplanet's atmosphere would also require a certain composition in order for the cooling process to take place. However, the new study will prove useful in further constraining the parameters for exoplanets capable of hosting life.

Source: KU Leuven

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