Re: searching for alien life

From: Anders Sandberg (asa@nada.kth.se)
Date: Wed Aug 06 2003 - 01:42:09 MDT

  • Next message: Jeff Davis: "Re: searching for alien life"

    On Tue, Aug 05, 2003 at 06:32:32PM -0500, Kevin Freels wrote:
    >
    > Now I am not a physicist, but doesn't the fluidity of water depend on
    > more than jus tthe distance from it's star and the heat from it?

    Water is a fluid for a certain range of pressures and temperatures; it
    is the green region in the phase diagram
    http://www.martin.chaplin.btinternet.co.uk/phase.html

    So you could get liquid water in a larger temperature range by having a
    greater pressure; above 21.671 MPa you pass the critical point and
    liquid and vapor pass seamlessly into each other (see Hal Clement's
    novel _Close to Critical_).

    You can also extend the range of the planets by having the temperature
    kept in a milder range as you say. The problem is that warm planets will
    have water vapor in their atmospheres and it is a greenhouse gas, so you
    get a positive feedback. Cold planets will tend to get much ice, and it
    is a good reflector and will make the planet colder, producing a
    snowball (where liquid water can still persist under the surface, as it
    likely did on Earth in the precambrian and right now on Europa).

    > It seems to me that the people conducting these searches are limiting
    > themselves by putting a "habital" zone around a star.

    Most searches are system-wise, so the exact extent of the lifezone is
    not important. But plenty of arguments about the probability of life
    depends on its average size, and a larger lifezone makes life more
    likely.

    However, people are indeed expanding the range of places where life
    would not be unexpected. Some have suggested moons of gas giants a la
    Europa, and brown dwarves might have lifezones of their own.

    -- 
    -----------------------------------------------------------------------
    Anders Sandberg                                      Towards Ascension!
    asa@nada.kth.se                            http://www.nada.kth.se/~asa/
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