Super-Earths can form with sufficient ices to become water worlds, but such planets would have to be constrained to around 10 Earth-masses to avoid forming a thick, hydrogen-helium atmosphere. Assuming an iron-rich planet with an internal structure like Earth, modelling results for the first discovered super-Earth (GJ 876 d) suggest the existence of a minimum threshold in planetary diameter above which a super-Earth "most certainly" has a high water content (where thick layers of water and pressurized ice surround a rocky mantle and core), which would be around 24,000 kilometers (or nearly 15,000 miles) in the particular case of GJ 876 d (Valencia et al, 2007). Given the same mass, water worlds are around 40 to 50 percent larger than rocky planets (Fortney et al, 2007).

© ESO



Larger animation still.



Gliese 581 d orbits within its host
star's habitable zone and so may
have liquid surface water in a deep
global ocean (more).

Water worlds may be most likely type of super-Earth to be potentially habitable for photosynthesis-based Earth-type life (von Bloh et al, 2009 and 2008). In the case of Gl 581 d, modelling simulations indicated that a planet with around eight Earth-masses has a sufficient amount of volatiles that it builds up a much denser atmosphere than an Earth-size planet, which prevents the atmosphere it from freezing out due to tidal locking in an inner orbit around a dim red dwarf. One 2008 study was optimistic that simple microbial life could have developed on Gl 581 d, although "adverse environmental conditions" might prove too much for complex life (von Bloh et al, 2008). As photodissociation of water can produce oxygen molecules (O2) as in Earth-like planets, however, atmospheric accumulation of oxygen without the oxidation of rocks through soil weathering and oxidation of volcanic gases indicate that the detection of O2 would not be a reliable signature of Earth-type life (Alain Léger et al, 2004).

David A. Aguilar, CfA




Large and jumbo
illustrations.




GJ 1214 b has a thick
atmosphere above a
possibly very deep,
hot ocean of water
and ice under crushing
pressure around a
rocky core (more).

By comparison, GJ 1214 b orbits its similarly dim red dwarf star at an even closer distance than Gl 581 d and so the planet is thought to be quite a bit hotter. This planet's discoverers were able to calculate its radius as well as its mass, which they determined are "consistent with a composition of primarily water enshrouded by a hydrogen–helium envelope that is only 0.05 percent of the mass of the planet" that is steamy hot and has been losing mass to space over the lifetime of the planet (Charbonneau et al, 2009). Despite a smaller mass than Gl 581 d, GJ 1214 b is also likely to have a possibly very deep, hot ocean of water and ice under crushing pressure around a rocky core. Model simulations for tidally locked planets (where the same side of the planet is warmed by the host stars, such as dim red dwarfs) suggest that heat transfer by winds efficiently reduces temperature contrasts between day and night sides of the planet (Merlis and Schneider, 2010).