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?

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.

  • $\begingroup$ - Star light is not a prerequisite for having life. With regards to properties for harboring life shouldn't we be asking this about the planets. I mean i don't see any relation between formation of planets around a star and the star itself. To put it in a different way,not every planet has a star. $\endgroup$ – Hubble07 Aug 26 '13 at 15:53
  • $\begingroup$ Without a star, where does the planet get the energy necessary to sustain life? On the Earth, almost all the energy used by higher forms of life originates in photosynthesis, which depends on the sun light. $\endgroup$ – becko Aug 26 '13 at 17:19
  • $\begingroup$ Life could have evolved at hydrothermal vents. The heat that sustains the vents comes from radioactive decay in the planet's core. Complex life probably couldn't exist without photosynthesis, but simple life could. $\endgroup$ – John Rennie Aug 26 '13 at 17:49
  • $\begingroup$ "quantify the odds" I think you misspelled "guess at" or perhaps "guesstimate". There are good reasons to guess that some systems are more likely than others but we have very nearly zero data (Earth doesn't count for anthropic reasons). $\endgroup$ – dmckee Aug 26 '13 at 18:43

Historically, this was discussed in terms of the Drake equation. The question corresponds to finding the product $f_p n_e f_l$, where $f_p$ is the fraction of stars having planets, $n_e$ the average number of planets that could potentially support life, and $f_l$ is the fraction that actually develop life. Current research seems to show that $f_p$ is of order unity. It's hard to estimate $n_e$ because current technology is best suited for detecting hot jupiters, and we also don't know whether earthlike conditions are necessary in order to support life -- although they are certainly necessary in order to support earthlike life as in the question. I don't think we have any hard evidence about $f_l$; if microbial life is proved to exist or to have existed on Mars, that would suggest that $f_l$ is rather large. Ward 2009 gives a pessimistic estimate for $n_e$, based on factors that they think make our planet special, such as a large moon and the right concentration of heavy elements in the primordial cloud from which our solar system formed. One possible interpretation of the Fermi paradox is that the product given by the full Drake equation is small, i.e., we're the only technological civilization in our galaxy.

Ward and Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe

  • $\begingroup$ For those not in the know, the Fermi paradox states that if there are all of these earth-like planets harboring life, why aren't we able to listen to Polaris-ian radio right now? $\endgroup$ – Jerry Schirmer Aug 26 '13 at 17:14
  • $\begingroup$ Probability is meaningless here, same with school level Drake equation. According to it Mr. Drake also should not exist :D Obviously other life forms exist "everywhere" in the galaxy and cosmos. Why should they contact us? You think we are charming? You think we are charismatic in a cosmic way? Maybe. In 5000 years. There is no way to have any meaningful conclusion based on logic about something unknown until you meet this unknown. $\endgroup$ – Asphir Dom Aug 26 '13 at 20:56
  • $\begingroup$ Probability is meaningless here, same with school level Drake equation. According to it Mr. Drake also should not exist I don't understand what you mean by this. It sounds like you might be misunderstanding something. $\endgroup$ – Ben Crowell Aug 26 '13 at 22:04
  • $\begingroup$ Probability works only for the given space of events, having statistics at hand so to speak. For single event it does not work. Drake equation is irrelevant. Real answer is we don't know the odds. If you put numbers in Drake equation our civilization should also be non existent. $\endgroup$ – Asphir Dom Aug 26 '13 at 22:39
  • $\begingroup$ @AsphirDom: I don't understand sentences #1 and 2. Sentences #3 and 4 would need to be expanded on at considerable length in order to convince anyone. Sentence #5 is false and contradicts sentence #4. $\endgroup$ – Ben Crowell Aug 26 '13 at 23:30

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