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Say you start with a list of stars. What properties do we believe to be critical in the present existence or future formation of earth-like life in those star systems? How can one reasonably pare down such a list, as to select targets for exploration/colonization?

To be clear, I'm specifically interested in life as we understand it, on a planet, orbiting a star. Intelligence or complexity isn't necessary to the question. If a star could have planets resembling a young earth, with very simple life, that's fine.

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  • $\begingroup$ What do you mean by "life as we understand it"? That's a rather vague descriptor. Also, as Rory Alsop said, there are many ways in which we don't really know what is critical for life to form. What do you think makes this question answerable? $\endgroup$ – called2voyage Sep 5 '13 at 13:11
  • $\begingroup$ Fair criticism. I'm talking about life that resembles Earth life. Not something we'd consider extremeophile, living in intense cold, darkness, heat, vacuum, etc. Not necessarily something that could survive on earth, but something that doesn't require any debate over the definition of life. Something a child might say, "Of course that's alive." And I'm wondering, could such a planet orbit a red dwarf? White dwarf? Red giant? Blue giant? Or are there issues with those stars that would prevent them from having what we might consider habitable planets? $\endgroup$ – Stephen Collings Sep 5 '13 at 13:16
  • $\begingroup$ That description of life still seems too vague to me. Where is the line between "Earth life" and extremophile? Extremophiles are "Earth life". Do viruses count as Earth life? What would a child say is alive? I think that varies. Are plants alive? Some children might not say so. Some children might recognize bacteria as alive. Many extremophiles are bacteria. $\endgroup$ – called2voyage Sep 5 '13 at 13:20
  • $\begingroup$ As to your other questions, it seems like they should be broken out into separate posts. They are far too broad for one question here. $\endgroup$ – called2voyage Sep 5 '13 at 13:21
  • $\begingroup$ My questions about stars are not really "other questions". It's my primary question. $\endgroup$ – Stephen Collings Sep 5 '13 at 13:37
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The simplest answer is Complex Chemistry. You and I can be thought of as bags of very complicated self-sustaining chemical reactions. To that end, we need:

  1. Lots of different chemicals: a ball of hydrogen gas isn't going to do much on it's own.
  2. Density: chemical reactions happen faster when the reactants are densely packed. Liquids work great for this, but so might a gas-giant. A planet with solid, unchanging rock probably wouldn't do much.
  3. Outside source of energy: Energy-input would allow more interesting chemical-reactions to take place. More specifically, sustainable reactions to occur.
  4. Stability, but not too much stability: If a planet goes from 1000 K to 20K each day, it's going to be hard for life to form. Conversely, if a system is totally stable, there isn't going to be enough 'mixing' to get metabolism going.

The search for habitable planets, and for possible extra-terrestrial life-bearing planets is one fraught with HUGE amounts of guesswork. We have no real clue what alien life might look like, so we focus on looking for stuff similar to our own earth. So, things like:

  • Habitable Zone - the planet needs to be in the correct distance to maintain the right temperatures for liquid water to be present
  • Magnetic Field - Something to shield life from intense radiation
  • Presence of water - Again, we think life originated in water, so searches for life tend to focus on this.

Here's some fun food-for-thought: The elements needed for carbon-based life: Carbon, oxygen, nitrogen, hydrogen - are all going to be very common on high-metallicity star-systems. They are readily formed by stellar neuclsynthesis, and -should- be present on most population I and II stars (have a read-through of : http://en.wikipedia.org/wiki/Metallicity)

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Although there is an incredibly wide variety of stars that could have planets orbiting them that could contain life, we don't yet have any examples of life other than what we have on Earth.

And we orbit a yellow dwarf star (a G2 main sequence star) at a mean distance of about 93 million miles.

Which means our only real chance of finding intelligent life that is similar to our own would be to look for similar stars and narrow down the list by only choosing those G2 stars with planets as possible targets for exploration. Then look for ones with planets about the same distance from the sun as Earth. Once we have better instruments, we would then want to look for evidence of water, oxygen etc.

We just have no data about how life in any other environment may evolve. Even the weird and wonderful bacteria we find in extreme environments on Earth are still on Earth.

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    $\begingroup$ I think you misread the question. It's asking for possible criteria for candidate selection, so some of the limits you mention are actually a part of the list that the question is asking about. I.e. if you can't pinpoint a narrow range with one method, you expand this range boundaries, but the method might still help you bring the soup down to stock a bit. ;) $\endgroup$ – TildalWave Sep 6 '13 at 3:43
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Generally, you would want to look at Population I stars. Also, the nature of most variable stars would tend to interfere with evolution on their planets.

Next, the star would have to be old enough for life to have evolved. For main sequence stars, smaller stars have longer lifetimes. As it took approximately 1 billion years for even basic single celled organisms to evolve, nothing larger than an F class star would be a candidate.

It is possible that larger stars, having a larger habitable zone, might have a higher chance of having a suitable planet, but this is pure conjecture.

There is also some debate about the feasibility of life evolving on tidally-locked planets. This may or may not eliminate the smaller M class stars whose habitable zone would be so close to the sun as to cause any planets in that zone to be tidally locked.

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This is a well known exercise, codified as the Drake equation. Excerpt from this wikipedia link:

The Drake equation is:

$$N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L$$

where:

$N$ = the number of civilizations in our galaxy with which radio-communication might be possible (i.e. which are on our current past light cone);

and

$R_{\ast}$ = the average rate of star formation in our galaxy
$f_p$ = the fraction of those stars that have planets
$n_e$ = the average number of planets that can potentially support life per star that has planets
$f_l$ = the fraction of planets that could support life that actually develop life at some point
$f_i$ = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
$f_c$ = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
$L$ = the length of time for which such civilizations release detectable signals into space.

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    $\begingroup$ The Drake Equation is not precisely what the OP is looking for. The OP is more looking for how one might go about estimating $f_p \cdot n_e$, that is how do we know if a star can support life? $\endgroup$ – called2voyage May 13 '14 at 21:25

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