Are tidally locked exoplanets more habitable than we once thought?
A team of researchers from theUniversity of Leuven (KU Leuven) in Flanders, Belgium, has discovered alink regarding the level of friction between an exoplanet's loweratmosphere and the surface of tidally-locked exoplanets, and theirpotential for supporting life. The study focuses on exoplanetsorbiting M dwarf stars, aclass of stellar bodies significantly smaller and dimmer, yet muchmore common than Sun-like stars.
The majority of rocky exoplanetsorbiting M dwarf stars only ever show one face to their star. To putthe concept in a familiar context we can look to Earth's satellite,the Moon. One side of the Moon is constantly locked to planet Earththanks to a phenomena known as tidal locking. This iswhy we always see the same face of the Moon no matter when or fromwhere we view it on the surface of our blue marble.
For scientists searching for habitableplanets around M dwarfs, this presents a serious problem, as theinfluence of the star would make the day side of a tidally-lockedexoplanet extremely hot, while the night side would be incrediblycold. As you can imagine, these are not conditions favorable tothe presence of life.
However, last year, a team ofresearchers from KU Leuven discovered that tidally-locked exoplanetscould be rendered habitable by a global air conditioning process inthe atmospheres of the distant worlds. The scientists usedsophisticated computer simulations to model the exoplanetatmospheres, taking into account variations in exoplanet size andspin speed, and were able to identify three potential atmosphericprocesses that could be occurring on satellites orbiting M dwarfstars.
In two of the scenarios, the cold airfrom the night side is transferred to the day side, where it isgradually heated and moved back to the night side of the exoplanet.The constant circulation of air could create a relatively hospitabletemperature. In the third scenario, a powerful air current high in anexoplanet's atmosphere disrupts the heat redistribution process,rendering the planet uninhabitable.
The new study, also carried out byresearchers from KU Leuven, examined the link between an exoplanet'ssurface characteristics and the disruptive current. The team ranhundreds of simulations designed to observe the effect that thefriction between a planet's surface and its lower atmosphere wouldhave on the fragile air conditioning system.
The computer-generated modelscalculated two distinct friction levels. One set of simulations usedan atmosphere to surface friction level similar to that known tooccur on Earth. A more extreme set of simulations saw the frictionvalue set at 10 times that of our planet. The results of the studysuggest that the greater the friction between the lower atmosphereand the surface, the more likely it is that the powerful disruptiveair currents will be suppressed, allowing the cooling process tooccur.
The team stresses that surface friction is not the only factorgoverning the habitability of tidally-locked exoplanets. Forexample, an exoplanet's atmosphere would also require a certaincomposition in order for the cooling process to take place. However,the new study will prove useful in further constraining theparameters for exoplanets capable of hosting life.
Source: KU Leuven