Given that terrestrial planets are produced through random collisions, we might reasonably ask, ''How massive can they become?'' First, this will depend upon the amount of material that resides in the disk, and second, it will depend upon where the Jovian planets form and how rapidly their orbits evolve inward toward the central proto-star.
Planetary migration is certainly an important effect in planetary formation, since many of the newly found exoplanets contain ''hot Jupiters,'' an expression invented to encapsulate the observation that they are found very close in toward the parent star, often with orbital radii of just a few tenths of an astronomical unit. The problem with hot Jupiters is that they could not possibly have formed where they are found, and hence it is now clear that they formed deeper within the disk, in the region where water ice is stable, and then migrated inward as a result of gravitational interactions with the disk itself.
This inward movement of the massive Jupiter planets will have tended to scatter any terrestrial planets that might have formed closer in. At first it was thought that this would mean that Earthlike planets must be very rare, but more recent detailed calculations by, for example, Martyn Fogg and Richard Nelson (Queen Mary, University of London, UK) have shown6 that terrestrial planets can survive the gravitational stirring effects produced by a migrating Jupiter, and there are, in fact, no longer any specific reasons to believe that terrestrial planets are uncommon objects. Fogg and Nelson also find from their numerical models that the inward migration of a Jovian planet might also result in the formation of hot super-Earths.
The key factor that shapes the final appearance of a terrestrial planet is whether it grows too rapidly and thus acquires sufficient mass that it begins to accrete a hydrogen and helium gas envelope. If a terrestrial planet begins to acquire a massive hydrogen envelope a runaway accretion effect comes into play, and the planet becomes heavier and heavier, eventually becoming a Jovian-type planet. The limit at which this runaway accretion effect begins seems to be at about 10 times the mass of the Earth. Hence, the largest Earth-like exoplanets that might be found in orbit around other stars will have masses no greater than about 6 x 1025 kg.
Super-Earths will be quite different in some important ways to the one-Earth mass planet that we evolved upon. First, a 10 Me super-Earth planet will be about 85% larger than Earth,7 and it may well have a global ocean. A planet-covering ocean is likely to arise because a massive planet will have a higher surface gravity and a less ridged crust than a low-mass planet, resulting in it having a more stunted topography. It will also experience a much higher atmospheric pressure at its surface than that found on a low-mass planet. All these effects, although the exact details are still far from clear, are likely to combine to produce a deep, planet-covering ocean. We comeback to look at the potential habitability of super-Earth and ocean-world exoplanets in Chapter 8, since in principle they would make ideal sites for interstellar terraforming projects (should that day ever arrive).
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