The fine U. Washington article Some potentially habitable planets began as gaseous, Neptune-like worlds mentions twice that tidal forces can induce inward planet migration.

Two phenomena known to inhibit the potential habitability of planets — tidal forces and vigorous stellar activity — might instead help chances for life on certain planets orbiting low-mass stars, University of Washington astronomers have found.

In a paper published this month in the journal Astrobiology, UW doctoral student Rodrigo Luger and co-author Rory Barnes, research assistant professor, say the two forces could combine to transform uninhabitable “mini-Neptunes” — big planets in outer orbits with solid cores and thick hydrogen atmospheres — into closer-in, gas-free, potentially habitable worlds.

It links to the 2015 open access article published in Astrobiology Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs

The discussion is in the context of gaseous planets between the size of Earth and Neptune. What is the mechanism behind this?

For rocky worlds, tidal forces sap the energy of rotation, resulting in a tidally-locked planet. As this may not be practical for gaseous worlds, where does the energy of tidally-heating the planet (due to stretching caused by the tidal forces) come from? Does it sap energy from the orbital velocity, migrating gaseous worlds closer to their suns? Is there some other mechanism at play?

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    $\begingroup$ Better suited to astronomy.stackexchange.com or physics.stackexchange.com $\endgroup$ – pericynthion Feb 2 '15 at 19:33
  • $\begingroup$ @pericynthion: I suppose that physics would be the place if this SE site didn't have the celestial-mechanics tag, but that tag and the questions tagged with it perfectly fit this question. $\endgroup$ – dotancohen Feb 3 '15 at 10:35
  • $\begingroup$ Hmm, good point! I'm not sure why that category is on this site since it doesn't really fit the general theme. $\endgroup$ – pericynthion Feb 3 '15 at 18:46
  • $\begingroup$ celestial-mechanics is basically on-topic because we (all of us) didn't vote if off the site. And when you have increasing number of questions tagged with it, it also gets increasingly difficult to do so. And possibly unfair, since we can only migrate up to 3 months old questions and others would have to be closed. I can think of reasons why we would want to keep it tho. For one, perturbation effects also apply to spacecraft, either directly affecting trajectory / attitude (radiation pressure, Yarkovsky / YORP, frame drag, tidal force...) or indirectly, affecting navigation, ephemeris... $\endgroup$ – TildalWave Feb 8 '15 at 11:55
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