![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
rfmcdonaldI think that io9 may have overreached in titling a post. The post "Why we won’t find Earth-2 around a red dwarf star" links to a very interesting paper regarding unconsidered problems facing potentially Earth-like planets around red dwarf stars, "Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating" by Barnes, Mullins, et al., but the paper consider a specific known exoplanet orbiting a red dwarf--Gliese 667C c, covered by me back in February here--and concludes that it could be habitable after all.
What's going on? It all has to do with the habitable zones around stars, the set of orbits in which a planet could plausibly support an Earth-like climate friendly to liquid water. Traditional calculations of a habitable zone have considered the radiant energy produced by a star. For red dwarfs--dim, low mass stars--a planet in the habitable zone would be closely bound by gravitation to its star, quite possibly with one side forever facing its sun in much the same way that one side of the Moon forever faces the Earth and the other forever faces away. This degree of tidal locking wouldn't prevent such a planet from being habitable, as atmospheric models suggest that an atmosphere only slightly denser than Mars would be capable of transporting enough heat to prevent the planet's atmosphere from freezing on the dark side. Other constraints, however, might exist. The authors identify the heat produced by the gravitational tides exerted by a star on such a close planet as a major source of heat.
The authors go on to calculate that it's quite possible for some planets closely orbiting red dwarf stars, especially worlds orbiting low-mass red dwarf stars (less than 20% the mass of our sun, perhaps) and worlds with very eccentric orbits, to be located within the "classical" habitable zone of their star but nonetheless be so heated by the tidal forces exerted by their star as to become "Tidal Venuses", becoming superheated worlds which lose their water to evaporation in space in just hundreds of millions of years. The aforementioned Gliese 667C c is not likely to be such a planet, according to the team's calculations, as its orbit is too distant. Other worlds, as yet undiscovered, may not be so lucky.
What's going on? It all has to do with the habitable zones around stars, the set of orbits in which a planet could plausibly support an Earth-like climate friendly to liquid water. Traditional calculations of a habitable zone have considered the radiant energy produced by a star. For red dwarfs--dim, low mass stars--a planet in the habitable zone would be closely bound by gravitation to its star, quite possibly with one side forever facing its sun in much the same way that one side of the Moon forever faces the Earth and the other forever faces away. This degree of tidal locking wouldn't prevent such a planet from being habitable, as atmospheric models suggest that an atmosphere only slightly denser than Mars would be capable of transporting enough heat to prevent the planet's atmosphere from freezing on the dark side. Other constraints, however, might exist. The authors identify the heat produced by the gravitational tides exerted by a star on such a close planet as a major source of heat.
As a planet moves from periastron, its closest approach to the star, to apoastron, the furthest point, and back again, the gravitational force changes, being inversely proportional to distance squared. This difference creates an oscillating strain on the planet that causes it to undergo periodic deformation. The rigidity of the planet resists the deformation, and friction generates heat. This energy production is called tidal heating.
Tidal heating is responsible for the volcanism on Io (Strom et al. 1979; Laver et al. 2007), which was predicted, using tidal theory, by Peale et al. (1979). Io is a small body orbiting Jupiter with an eccentricity of 0.0041, which is maintained by the gravitational perturbations of its fellow Galilean moons, that shows global volcanism which resurfaces the planet on a timescale of 100 – 105 years (Johnson et al. 4 1979; Blaney et al. 1995; McEwen et al. 2004). The masses of Jupiter and Io are orders of magnitude smaller than a star and terrestrial exoplanet, and thus the latter have a much larger reservoir of orbital and rotational energy available for tidal heating. Moreover, some exoplanets have been found with orbital eccentricities larger than 0.9 (Naef et al. 2001; Jones et al. 2006; Tamuz et al. 2008). Thus, the tidal heating of terrestrial exoplanets may be much more effective than on Io (Jackson et al. 2008c,a; Barnes et al. 2009a, 2010; Heller et al. 2011). This expectation led to the proposition that terrestrial exoplanets with surface heat fluxes as large or larger than Io’s should be classified as “Super-Ios”, rather than “Super-Earths” (Barnes et al. 2009b).
The authors go on to calculate that it's quite possible for some planets closely orbiting red dwarf stars, especially worlds orbiting low-mass red dwarf stars (less than 20% the mass of our sun, perhaps) and worlds with very eccentric orbits, to be located within the "classical" habitable zone of their star but nonetheless be so heated by the tidal forces exerted by their star as to become "Tidal Venuses", becoming superheated worlds which lose their water to evaporation in space in just hundreds of millions of years. The aforementioned Gliese 667C c is not likely to be such a planet, according to the team's calculations, as its orbit is too distant. Other worlds, as yet undiscovered, may not be so lucky.
