The planet GJ 1214b is the second known super-Earth with a measured mass and radius .
Orbiting a quiet M-star , it receives considerably less mass-loss driving X-ray and UV radiation than CoRoT-7b , so that the interior may be quite dissimilar in composition , including the possibility of a large fraction of water .
We model the interior of GJ \ > 1214b assuming a two-layer ( envelope+rock core ) structure where the envelope material is either H/He , pure water , or a mixture of H/He and H _ { 2 } O .
Within this framework we perform models of the thermal evolution and contraction of the planet .
We discuss possible compositions that are consistent with M _ { p } = 6.55 \ > M _ { \oplus } , R _ { p } = 2.678 \ > R _ { \oplus } , an age \tau = 3 - 10 Gyr , and the irradiation level of the atmosphere .
These conditions require that if water exists in the interior , it must remain in a fluid state , with important consequences for magnetic field generation .
These conditions also require the atmosphere to have a deep isothermal region extending down to 80 - 800 bar , depending on composition .
Our results bolster the suggestion of a metal-enriched H/He atmosphere for the planet , as we find water-world models that lack an H/He atmosphere to require an implausibly large water-to-rock ratio of more than 6:1 .
We instead favor a H/He/H _ { 2 } O envelope with high water mass fraction ( \sim 0.5 - 0.85 ) , similar to recent models of the deep envelope of Uranus and Neptune .
Even with these high water mass fractions in the H/He envelope , generally the bulk composition of the planet can have subsolar water : rock ratios .
Dry , water-enriched , and pure water envelope models differ to an observationally significant level in their tidal Love numbers k _ { 2 } of respectively \sim 0.018 , \sim 0.15 , and \sim 0.7 .