How tides affect habitability has become a sub-genre within exoplanetary studies, a theme pushed hard by the gifted trio of Brian Jackson, Rory Barnes and Richard Greenberg (University of Arizona). You may want to browse through earlier Centauri Dreams entries on their work, especially this fascinating take on habitability around M dwarfs, in which the authors consider the possibility that Gliese 581 c was once a relatively benign place, but is now in an orbit that renders life impossible. Orbital evolution is the broad issue, sustained complex life demanding planets with low eccentricities. And orbital evolution can take a lot of time to operate.

Now I see that Brian Jackson has presented new work on tides and habitability at the 40th annual meeting of the Division of Planetary Sciences in Ithaca, NY. Here we push into interesting questions about planets already inside a habitable zone that are nonetheless too hellish to support life, and planets outside that zone that seem too cold to sustain life, but may be able to do so because of tidal effects. Planets in elongated orbits (unlike those in our Solar System, where orbits are relatively circular) undergo tidal stretching when near their star, an effect that diminishes as they move away from it. The result: Friction, generating internal heat that keeps the planet geophysically active.

What we find is that we shouldn’t be too quick to make judgments based upon the presence of liquid water at the surface. Take a planet up to ten times as massive as the Earth — a ‘super Earth’ — and consider the effects of tides upon conditions there. A likely prospect is extreme volcanic activity, which can render a planet already within a star’s habitable zone more like Jupiter’s moon Io than our mild and pacific Earth. This can occur even at relatively low eccentricities. Here’s Jackson on what we may discover when we study the first extrasolar terrestrial planets:

Given the wide range of masses and eccentricities that potentially give rise to extreme volcanism, we might expect that many terrestrial planets will be too volcanically active for life… [T]he most massive terrestrial planets may also be the most heated and thus the most volcanically active. Since the first extra-solar terrestrial planet that is likely to be confirmed will probably be much more massive than the Earth, we might expect it will be volcanically active. Such volcanic activity may be recognizable in the planet’s atmospheric transmission spectrum, similar to Io, whose tenuous atmosphere is largely made of sulfur…

This is drawn from the paper on the team’s work, which also examines the flip side of these effects, that a planet that might otherwise be too cold to sustain life may benefit from tidal effects, which would cause the outgassing of volatiles that could keep its atmosphere viable. For that matter, planets with an ocean under a crust of ice — we can think about Europa as the nearest analog of this process — could maintain warmer water temperatures than would otherwise be possible. Throw in plate tectonics, which can stabilize planetary atmospheres and surface temperatures, and you may have generated what you need to produce a functional biosphere.

And here’s an interesting scenario: A planet whose tidal evolution makes it pass through alternating periods of heating and cooling, such that it may go through an early habitable period, possibly including the development of life. And then, after a long period in which life has been extinguished because of volcanism, the same world may once again become habitable when its orbit circularizes. Thus two separate epochs for the emergence of life may occur on the same planet, although occurring billions of years apart.

The range of outcomes is extensive, as the paper’s summation suggests:

Even with the many simplifying assumptions employed here, these results suggest a wide range of geophysical scenarios. As advancement is made in the understanding of the processes of tidal evolution, in modeling of the geophysics of hypothetical planets, and eventually in the discovery and characterization of actual terrestrial-type planets, these calculations will need to be revisited. In any case, the calculations here show that tidal heating has the potential to be a major factor in governing the internal structures, surfaces and atmospheres of extra-solar terrestrial planets. Accordingly, the effects of tidal heating must be given consideration when evaluating the habitability of such planets.

We shouldn’t be too doctrinaire about the exact specifications for a living planet. As our own system shows, even bizarre outliers like Enceladus may eventually show evidence of a second outbreak of life within reachable distance of our own. Who knows how wide a range of living planets we may find as we zero in on exoplanetary systems? The paper is Jackson et al., “Tidal Heating of Terrestrial Extra-Solar Planets and Implications for their Habitability,” accepted for publication in Monthly Notices of the Royal Astronomical Society and available online.