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Requirements for a habitable planet

  1. Mar 20, 2004 #1
    The search for extraterrestrial intelligence has been narrowed down to planets that are similar to earth. If a planet is hazardous to earth life, then it is said to be inhabitable. Is that right?
    If so, WHY? After all, we can only define life according to what we have here on Earth. Something EXTRAterrestrial is not of our world, therefore it doesn't neccesarily have to be able to live in earthly conditions...for that matter, it could live in what seems to us to be the most harshest conditions we can think of.

    so, I don't understand why certain planets have the potential to support life and others don't.
  2. jcsd
  3. Mar 21, 2004 #2
    As I remember from my biology classes, "life" is defined by properties like these:
    1. Reproduction
    2. Metabolism
    3. Sensory perception
    4. Growth
    5. ...? I forgot the rest

    Terrestrial life forms manage to do this using very complex biological and chemical structures, e. g. 1. with DNA (DNA molecules were probably the beginning of life on earth), 2. with encyms (hope that's the correct word). The problem with these molecules is that they are very sensitive; if it's too hot or if there is high-energetic radiation like UV, they brake up. In addition, the chemical reactions need a good medium like water.

    Although it's imaginable that these basic chemical things would differ on another planet, it's hard to believe that they would be much more resistant.
  4. Mar 21, 2004 #3
    Uhmm so a planet like Venus that has acid rain, literally, quite literally, and is in the hundreds of degrees, in temperature, melts landers internal pieces, in short time, you would wonder why this would be seen as in-hospitable to life?...makes sense, untill you know some of the features of the environments of other planets, then it does seem less likely that some of them would support any life at all....Good question though...

    Did that help?
  5. Mar 21, 2004 #4
    because we haven't found anything that we consider to be life on these planets?

    AN idea (yes, it is really scifi but what the heck..):
    Maybe the aliens are moving too fast for us to notice?? like tachyons?
  6. Mar 21, 2004 #5


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    For anything to move that fast it would have to have an extremely high metabolism rate. This would mean that it would have to put out vast amounts of waste heat (easily detectable). This also means a very high body temp; So high that any chemicals complex enough to support life would be ripped apart.
  7. Mar 21, 2004 #6
    Nature has shown (multiple times) that it is very possible to adapt. Does that include ANY situation? Does the ability have limits?

    why is DNA or other chemicals found in earthly life the only option for the sustenance of life?
  8. Mar 21, 2004 #7
    Obviously, you must define what you mean by "life" so that people can explain the conditions needed for it.

    I myself think kuengb's explanation was pretty good.
  9. Mar 21, 2004 #8
    Ditto, Tail.

    I define life as anything with hereditary material. Though isn't it true that viruses (whom I believe have hereditary material) are not considered living?

    But the only problem with this topic is that we can only define life according to earthly life. Our challenge is to think outside the box...surely there must be more than one way for life to exist?[?]
  10. Mar 21, 2004 #9
    Bacteria have been found on Earth in "Extreme conditions" according to what "we" Used to think...we now know better, so it might be possible that some form of life could exist on the planet Venus, just that, given no predation, it would probably cover all of where it could find nutrient, or the face of the planet, if it was available there, to it, so "we" could probably find that easily enough, robotically too, cheaper, which is the next part...where would you look, knowing that you were devoting half a trillion dollars, over the next twenty years? and why??
  11. Mar 21, 2004 #10


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    Yes. Life is based on complex reactions. There has to be enough energy to run those reactions in the environment. But too much energy in the environment will tear the molecules responsible for these reactions apart. Simple rules of chemistry and physics.

    Not necessarially DNA, but there are strong reasons why life tends to be based on the same type of chemicals as Earthly life.

    As stated above, life requires some very complex types of interactions and reactions. In order to do this, the chemicals that form its basis will be complex. Carbon is ideal as a basis for the "backbone" of these types of chemicals because it allows for all kinds of complex molecules. But these chemicals are only stable over certain temperture ranges.

    There has been some suggestion made that silicon could act as the backbone for a different type of life, but the problem is that silicon can't form chains as long as carbon can, which limits its ability to make up the type of complex molecules needed.
  12. Mar 22, 2004 #11
    Ah, very intriguing Janus. What about doping silicon? How does it behave in different tempratures/conditions?

    Brilliant Janus! You gave me a good idea. Is it not true that there is more than one way of doing just about anything?
    say in basic math:
    42/2 = 42 x 1/2
    If we could come up with our own way of making (i guess that's the best word for it?) something similar to DNA and OTHER COMPLEX CHEMICALS, where the result in the end is somewhat equal to DNA and OTHER COMPLEX CHEMICALS (DNA & OCC).....i can't get my ideas together. I'm sure I've been confusing you by now...do you at least get some idea of what I'm talking about?

    Maybe DNA and OTHER COMPLEX CHEMICALS are the more efficient types of chemicals to sustain life. How about the less efficient? Or (if possible) more efficient.
  13. Mar 22, 2004 #12
    I think it would be unwise to limit a search in that way. But I can see the necessity of doing so since, at the moment, we only know how to detect those forms of life.

    However there are forms of life which thrive is exceedingly inhospitible environments in which almost all life on earth can't survive in. I recall a TV program (the Science Channel?) in which they were discussing a certain kind of life form which was discovered deep in the earth's crust. I think it was thousands of kilometers below the earths surface. I believe the temps down there are extreme and I'm not sure whether oxygen is down there. However it has been speculated that these forms of life are responsible for the repopulation of the Earth after an earth killing event, i.e. and event which kills off all forms of life on or near the earths surface, e.g. major asteroid impacts etc. Such massive impacts can destroy life down several miles into the earth's surface and it appears that the earth has had sereral of these happen. After life was wiped off the surface the earth didn't have to start from scratch but merely from the primative life that was on it deep in the earth's crust. I think this life form was discovered while drilling for oil or something. Not quite sure. In any case any search for life should include those forms of life which are capable of living in such extreme environments.

    However I forgot the details of these microbes. If anyone saw that show or remembers the name of this life form that I'm refering to I'd appreciate posting the name of those things. Thanks.
  14. Mar 22, 2004 #13


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    Archaea has been found in the most inhosbitable places on earth. Deep underground, inside hot oil, inside artic ice. Oxygen is not required and I have read about types that eat metals for energy.

    Archaea is probably the microbe you are refering to pmb_phy.
    I am not an expert, but a google search for archaea will tell you everything you want to know.
  15. Mar 22, 2004 #14
    You could also try searching with mooter, cluster responces, selectivity.... www.mooter.com ...
  16. Mar 22, 2004 #15
    try this page(?) Archaea

    seemed good....
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