[0901.1698] The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?.
A clever use of orbital tidal evolution to constrain the possible structure of the 3 sub-Neptune mass planets around HD 40307. Fast-forwarding the evolution of the system shows it becomes unstable if the planets are too efficient at turning tidal deformations into heat. A terrestrial planet is estimated to turn roughly 1.0%-0.5% of tidal flexing into heat, which doesn’t sound like much, but is enough to liquefy much of Io’s interior, for example. For terrestrial planets very close to their star the situation would be even more dramatic and they’d be largely volcanic. Worse than Io, if you can imagine such a thing.
Ocean planets, for real, which are fluid down to the core, if not beyond, are much more flexible and less able to turn flex into heat. Typically a Jovian or Ice-Giant is estimated to turn 1/20,000th or less of the tidal flex into frictional heating and so the HD 40307 planets, if they were mini-Neptunes, wouldn’t evolve in their orbits as quick.
Sounds like a long chain of inference, I know, but much of the geophysical figuring involved comes from hard-data from the planets we know best, including our own. Nature, however, might know something we don’t…