Earth-sized rocky exoplanets that orbit small, red stars might have intense hurricanes, according to an analysis of exoplanets orbiting M dwarfs – stars much smaller and cooler than our sun.
Thaddeus Komacek at the University of Chicago and his colleagues used hurricane models for Earth to predict the likelihood of their formation on potentially habitable exoplanets.
A key assumption of their modelling is that these exoplanets are covered with liquid water oceans. Although no such exoplanets have yet been identified, previous simulations suggest Earth-sized planets could be abundant around M dwarf stars.
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Because M dwarfs give off less radiation than the sun, potentially habitable exoplanets – those with temperatures relatively similar to Earth – can orbit closer to their star than Earth does.
There are three main factors affecting hurricane formation: the theoretical upper limit on the wind speed a hurricane could generate, called maximum potential intensity; the planetary spin available to generate a cyclonic storm; and vertical wind shear, which is the difference in wind near a planet’s surface and higher up in the atmosphere.
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“The effects of the wind shear and the spin kind of cancel each other out,” says Komacek. “At faster rotation, there’s too much wind shear, and at slower rotation, there’s not enough spin to make the hurricane in the first place,” he says. For that reason, hurricanes are less likely on slowly rotating exoplanets.
The team found that hurricanes are most likely to form on exoplanets that take between eight and 10 Earth days to orbit their star, and which are also tidally locked with their star. That means they rotate at the same rate so one side of the planet is in perpetual daytime.
In future, terrestrial exoplanets that orbit M dwarf stars will be studied using infrared space telescopes such as NASA’s James Webb Space Telescope. If hurricanes occur on these planets, they may affect observations.
“Hurricanes come with lots of clouds, which leads to strong rainfall,” says Komacek. “You would expect the cloud variability to greatly impact the observations of these planets.”
But hurricanes could also enhance the detection of water vapour in the atmosphere of these exoplanets because they effectively transport it to higher altitudes.
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