[BRIEF NOTE] On the likely non-habitability of Gliese 667Cc

[rfmcdonald]

At his blog Drew Ex Machina, physicist Andrew Lepage takes a look at apparently confirmed exoplanet Gliese 667Cc. The planet likely exists, Lepage suggests, but because of tidal heating and its likely size it is not likely to be Earth-like. Think of a hot Neptune instead.

While the spin state and curious cyclically varying orbit would make for interesting diurnal and seasonal cycles on GJ 667Cc, there is a dark side to it that would hamper its habitability. Makarov and Berghea found that tidal heating resulting from the eccentricity of the orbit of GJ 667Cc would amount to 1023.7 joules per year assuming an Earth-like composition. This rather odd notation translates to about 1.6X1016 Watts of heat flow or a factor of about 300 greater than Earth’s current internal heat (i.e. from the decay of radioactive elements as well as left over from its formation). Makarov and Berghea estimate that the temperature of an Earth-like mantle in GJ 667Cc would increase at a rate of 1.6° K per 100,000 years and, barring any limiting mechanism not included in their simple model, the mantle of GJ 667Cc would be rendered completely molten inside of 100 million years. Such a tidal heating rate would seriously affect the potential habitability of GJ 667Cc or even make it impossible.

Another major obstacle for the potential habitability of GJ 667Cc is that it is likely not even a terrestrial planet. As is the case with all planetary discoveries made using precision radial velocity measurements, only a minimum mass or Mpsini can be derived since the inclination of the planet’s orbit with respect to our line of sight, i, is not known from spectral measurements alone. The inclination must be determined by other means. Without a known inclination, only the probability that a planet has a mass in a certain range, such as the mass range consistent with a rocky composition, can be stated.

A recent analysis of Kepler data by Leslie Rogers (California Institute of Technology) has shown that a noticeable change in the composition of planets takes place at around 1.5 times the radius of the Earth (RE). Taking into account the uncertainties in the measured radii and especially the independently measured masses of Kepler objects with radii less than 4 RE selected for her analysis, Rogers found, at a 95% confidence level, that the majority of planets with radii greater than 1.6 RE are likely to possess a substantial volatile envelope rich in water, hydrogen and helium. In other words, such planets are more likely to be mini-Neptunes or gas dwarfs that are extremely unlikely to be habitable and would not be rocky, terrestrial-type planets. Rogers’ conclusions agree well with previous analyses of Kepler data by Marcy et al. as well as Weiss and Marcy published earlier this year. It also supports a theoretical study on the transition from terrestrial to non-rocky planets by Eric Lopez and Jonathan Fortney (University of California – Santa Cruz) that was formally published just a few days ago.