Tidal Heating Could Kill 'Habitable' Alien Worlds
Although red dwarfs are often pegged as "ideal" places to hunt for habitable exoplanets, it turns out that this type of star — which is the most numerous class of star in our galaxy — may have a sinister side.
Far from promoting the evolution of biology within their habitable zones, powerful tides could literally sterilize orbiting exoplanets, cooking them from the inside-out, Nature News reports.
In fact, according to astrophysicist Rory Barnes, of the University of Washington in Seattle, the size of habitable zones around some red dwarfs could shrink by up to a half.
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The habitable zone surrounding a star is the distance at which a world can orbit with liquid water existing on its surface. Too close to the star, the water evaporates; too far from the star, the water freezes.
According to Earth Brand™ life, biology requires liquid water to evolve. So a planet that orbits inside this "Goldilocks Zone" (not too hot, not too cold) has a pretty decent stab at hosting life (as we know it, at least).
So, as red dwarfs are small and dim, our exoplanet-hunting missions have a better chance at spotting exoplanets orbiting within a red dwarf's habitable zone. For liquid water to exist on a red dwarf exoplanet, it would need to have a very compact orbit, giving us a better chance of detecting it.
But when it comes to red dwarfs, argues Barnes, you can't take anything for granted.
Just because a world — even a bona fide "Earth-like" exoplanet containing all the good stuff that should allow life to thrive — could be detected in a red dwarf's habitable zone, the extreme tidal forces exerted on this world could render it uninhabitable. The gravitational field in the close vicinity of a red dwarf would generate heat inside habitable zone exoplanets, making them too hot, thereby rending them useless as potential habitats.
Tidal heating isn't a problem for Earth, as our sun's habitable zone isn't as compact as a red dwarf's. But tidal heating is a factor for the Jovian moon Io, for example.
As Io's orbit is slightly eccentric (or elongated), there are points in its orbit that take it closer to Jupiter than others. During its orbit, Io is squashed and stretched like putty by tidal variance, causing frictional heating in its core. This internal heat source makes Io the most volcanic place in the solar system.
"I'm just scaling that Io–Jupiter system up by a factor of 1,000 in mass," Barnes said at this week's American Astronomical Society Meeting of the Division on Dynamical Astronomy at Timberline Lodge, Ore. "It's the same process, on steroids."
Barnes also noted that this tidal interaction can, over time, make eccentric orbits more circular, thereby removing the tidal variance and cooling the exoplanet's core. But this doesn't save the exoplanet's habitability — the damage has already been done. All the water would have likely been lost to space during its hot tidal heating phase, leaving a sterile planet in its wake. Should exoplanet hunters then discover an exoplanet with a circular orbit within a red dwarf's habitable zone, they'd have no idea whether it had been sterilized or not.
However, there may be situations where this tidal heating may be beneficial to the evolution of life. If, say, an ice-encrusted world orbits a red dwarf just outside the habitable zone boundary (i.e., it's "too cold"), this tidal interaction could gently heat the core, melting the base of the ice. A situation similar to the Jovian moon Europa — where subsurface oceans are covered in a thick protective layer of ice — could present itself.
Whether or not this factor is a major player in the hunt for truly habitable worlds around red dwarf stars remains to be seen, but it's certainly something that needs to be considered during the hunt for Earth 2.0.
Source: Nature News
Image credit: NASA/JPL-Caltech