Ocean Planets

The models by Sotin et al., (2007) allow the simulation of the internal structure of ocean planets, whose existence was first suggested by Leger et al. (2004). These planets would have formed in the outer regions of the stellar system, where condensation of water took place, and subsequently migrated inwards. The discovery of an exoplanet of 5.5 Earth masses (Beaulieu et al., 2006), reinforces this assumption:

Fig. 7.9 Density, pressure, and temperature profiles calculated for an Earth-like exoplanet and for an ocean planet. In both cases, the mass of the exoplanet is 1 Earth mass. The models are compared with the PREM model used for the Earth, and incorporate seismic data (After Sotin et al., 2006)

The planet lies 5 AU from its star, in a region where the expected temperature is around 50 K; so water should be trapped in the form of ice in the planetesimals, and where it forms a significant fraction of them.

The calculations considered a mass fraction of H2O of 50 per cent. They showed that an ocean planet of one Earth mass would have a radius of 8000 km rather than 6400 km; if its mass is 10 Earth masses, its radius is 15 000 km instead of 12 000 km for a Super-Earth. The depth of the liquid ocean varies from 3000 km to 6000 km for ocean planets whose mass varies between 1 and 10 Earth masses.

Figure 7.9 shows density, pressure and temperature profiles calculated for terrestrial-type planet, and for an ocean planet.

Figure 7.10 shows the mass-radius relationship for terrestrial-type exoplanets and for ocean planets. It will be seen that regardless of the mass of the object, the radius of an ocean planet of equal mass, is 26 per cent greater than that of a terrestrial-type exoplanet. It should, therefore, be possible, in principle, to distinguish between exoplanets of the terrestrial and ocean types, from simultaneous velocimetric and transit observations.

It will also be seen that the evolution of the two curves in Fig. 7.9 is very similar. The figure shows that Ganymede, Callisto, and Titan should have about 50 per cent water by mass. So there are 'ocean satellites' in the Solar System, but their method of formation is different from that envisaged for ocean-type exoplanets: they formed directly when the local nebula that accompanied the formation of their giant planet collapsed, and have not undergone any - or at least very little - migration. Because

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