Meh... it seems that the "super-earths" around Gliese 581 *can't* be
habitable due to tidal heating. Planet C is likely to be a volcanic
world worse than Io, while Planet D propably is completely molten. (!)

Sent to you by Lucio via Google Reader: Time, Tides and Habitability
via Centauri Dreams by Administrator on 3/14/08
Keep your eye on Gliese 581. Not that the news is necessarily good for
our hopes for habitability around that star -- in fact, a recent paper
suggests quite the opposite. The red dwarf exploded into the public
consciousness with the announcement that one of its planets -- Gl 581 c
-- could conceivably support clement temperatures and water at the
surface, at least in places. But in exploring that possibility, we're
getting a case study of world-class science at work, analyzing data,
offering hypotheses, broadening options. It's an exciting process to
watch.

Gl 581 d is now being analyzed for habitability, while Gl 581 c begins
to appear less and less likely as a home to life. It may take decades
and new space-based observatories for the issue to be resolved, but we
now have a new take on Gl 581 c, embedded in a broader study of tidal
evolution as planetary systems evolve. The study has implications not
just for rocky worlds but for planetary formation in many scenarios.

The work of Brian Jackson, Richard Greenberg and Rory Barnes
(University of Arizona) draws on a key fact: A planet's orbit can be
greatly affected by tides that the planet raises on its star, and on
the tides the star raises on the planet. In fact, tidal distortion and
orbital evolution work together, tidal forces producing internal
heating at the expense of orbital energy. Thus many close-in planets
probably formed further out from their host star than their present
position. In a typical case, say the authors, tidal heating increases
as a planet moves inward and then decreases when the tides circularize
the orbit and shut down the heat mechanism.

But each case will be different, the strength and timing of these
effects determining a planet's properties. The team's intention is to
construct heating histories for planets whose radii have been measured,
sometimes with results that vary from theory. And that gets me back to
Gl 581 c, for in terms of planets with masses less than ten times
Earth's, such heating could have played a role in the planet's
geophysical development. The Arizona team finds that the contribution
of tidal energies on two 'super Earths' -- Gl 581 c and GJ 876 d --
should produce a heat flux with have profound implications:

Among terrestrial-scale planets, we find that tidal heating may have
dominated the geological and geophysical evolution of the planets and
control their current character. The tidal heating rate for GJ 876 d
may be orders of magnitude greater than the magnitude considered by
Valencia et al. to be geophysically significant. For Gl 581 c tidal
heating may yield a surface flux about three times greater than Io's,
suggesting the possibility of major geological activity.

Three times that of Io? Gl 581 c looks less hospitable all the time.
The case of GJ 876 d is even more extreme. This 'super-Earth' of 5.89
Earth masses hasn't been in the habitability picture because its
two-day orbit keeps it far too close to its star for liquid water to
exist. But while the planet has been considered vulnerable to tidal
stresses, I don't think anyone was prepared for what the Arizona team
found:

...radiogenic heating of GJ 876 d might have been adequate to initiate
plate tectonics, but our results indicate that tidal heating may have
been a major contributor to the geological and geophysical character of
the planet. Tidal heat has provided an important component of the heat
budget for this planet, perhaps the dominant component during at least
the past ~108 yr. The tidal heating rate would be so large, in fact,
that GJ 876 d is unlikely to be a solid, rocky body.

I've only focused on two super-Earths here, but the paper also offers
interesting takes on planets like HD 209458 b, whose radius is larger
than predicted, and HAT-P-2 b, whose radius is well below prediction.
Tidal heating histories may help us understand these apparent
anomalies. The paper is Jackson, Greenberg and Barnes, "Tidal Heating
of Extra-Solar Planets," accepted by the Astrophysical Journal
(abstract).

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The more we learn about these issues, the greater the number of complications for planetary habitability multiplies.

If it wasn't for Gerard O'Neill's orbital habitat concepts, I don't think I would hold out much hope for interstellar colonization. One might survey 10,000 worlds and not find a single one suitable for our kind of life.

Regards,

Mike Combs

From: spacesettlers@yahoogroups.com [mailto:spacesettlers@yahoogroups.com] On Behalf Of Lucio
Sent: Friday, March 14, 2008 1:16 PM
To: orions_arm@yahoogroups.com; spacesettlers@yahoogroups.com
Subject: [spacesettlers] Time, Tides and Habitability

Meh... it seems that the "super-earths" around Gliese 581 *can't* be
habitable due to tidal heating. Planet C is likely to be a volcanic
world worse than Io, while Planet D propably is completely molten. (!)

.