SETI bioastro: Re: Star questions

From: Larry Klaes (
Date: Mon May 01 2000 - 05:53:37 PDT

From: "Andrew LePage" <>
To: Larry Klaes <>
Date: Fri, 28 Apr 2000 18:49:24 -0400
Subject: Re: Star questions
Priority: normal
X-mailer: Pegasus Mail for Win32 (v3.12c)


This little tid bit from Bruce Moomaw caught my eye:

> they're so dim that any planet in their
> >habitable zone is so close to the star that the tides will stop its
> >rotation in just a few hundred million years and it will have a permanent
> >dayside and nightside (as Mercury was supposed to have). This means that
> >the nightside should get permanently below freezing -- and that means
> >that all the planet's dayside water will soon evaporate, drift to the
> >nightside and freeze permanently, leaving the dayside a waterless desert.
> > There may be one way out of this. James Kasting and Darren
> > Williams
> >have concluded that a planet in the outer parts of its star's Habitable
> >Zone will build up a thicker CO2 atmosphere because cold slows down the
> >rate at which the CO2 belched from its volcanoes reacts with rock and
> >liquid water to turn into carbonates -- and they also think that such a
> >thick CO2 atmosphere does a very good job of equalizing temperatures, so
> >that such planets, even if they have highly tilted spin axes, lack the
> >grotesque pole-equator temperature extremes that such a tilted planet
> >would have at Earth's distance from the Sun. So a planet in the outer
> >fringes of an M-class star's Habitable Zone might conceivably have CO2
> >air thick enough to keep its nightside above freezing -- but I haven't
> >seen any studies of this specific question.

Actually, there have already been studies on the effects of
synchronous rotation on the habitability of planets orbiting M-type
stars. Monoj Joshi and Robert Haberle of Ames Research Center
and their collegues did some fairly sophisticated atmospheric
simulations (on par with those done more recently by Kasting and
Williams) and found that it was possible for synchronous rotators
to be habitable with elevated levels of CO2 in their atmospheres
(especially planets that are more Mars-sized than Earth-sized).
The results from this paper also imply that slow rotators (e.g. the
large moons of EGPs or planets locked in spin-orbit coupling like
Mercury) would even be more likely to be habitable.

The paper, "Simulations of the Atmospheres of Synchronously
Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for
Atmospheric Collapse and the Implications for Habitability", can be
found in Vol. 129, No. 2, pp. 450 - 465 of the October 1997 issue of

Feel free to pass this along.


Andrew LePage
Gentleman Astronomer
Newburyport, MA

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