SETI bioastro: Stars and their habitable zones

From: Larry Klaes (lklaes@bbn.com)
Date: Fri Apr 28 2000 - 10:55:31 PDT


From: "Bruce Moomaw" <moomaw@jps.net>
To: "Europa Icepick Group" <europa@klx.com>
Subject: Re: Star questions
Date: Fri, 28 Apr 2000 00:44:48 -0700
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-----Original Message-----
From: Jayme Blaschke <jblaschke@SWMAIL.SW.ORG>
To: europa@klx.com <europa@klx.com>
Date: Thursday, April 27, 2000 12:13 PM
Subject: Star questions

Two off the wall queries here:

First, can someone give me a star that lies pretty much directly between Sol
and our galaxy core?

And second, any rough guess as to the lower limit of the size of F-type
stars that could viably support life in a solar system? Would planets of
these parent stars differ significantly from our solar system? Would they be
metal poor, smaller, etc? (I know they'd basically be colder). Inquiring
minds want to know.

===========================

        Well, first, F-type stars are generally thought to be a bit too big
to support metazoan (multicelled) life -- the bigger a star is, the faster
it burns its hydrogen and turns into a red giant. And when you get above
the size of G-type stars, this process takes such a short time that there
probably isn't time for metazoan life to evolve on any habitable planet.
(Microbial life is a different matter.)

        When you talk about "lower limits", I think you were thinking of
M-type stars -- the red dwarfs that make up most stars (and are smaller than
G and K-type stars). They're certainly long-lived enough -- but they have
another problem: 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.

        By the way, I'm just finishing up my final report on the
Astrobiology Conference -- dealing with whether planets that can support
multicelled life are rare in the Universe -- in which I mention this, along
with many other points. The damn article has grown to the size of "Lord of
the Rings", but I think I can complete it tomorrow, and with luck you'll see
it in SpaceDaily and SpaceViews next week.

                                                            Bruce Moomaw

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