Some shed tears of joy as the news about the successful landing of the Mars Rover Curiosity came in. Then the most comprehensive 3D map of our observable universe was published. 2012 is literally an astronomical year.

As an layman I would like to know about scientists current understanding about the frequency models that life forms on other planets?

See also:

  • 2
    $\begingroup$ Although we are getting very encouraging signs, there is no indisputable evidence that life has been found other than earthly life. I think Europa is probably more exciting than Mars, where giant reservoirs of water have been found and the temperatures are imaginably hospitable. Too bad its so far away! $\endgroup$
    – shigeta
    Sep 27, 2012 at 20:39
  • 1
    $\begingroup$ Giant reservoirs of water have not been found on Europa, although it is hypothesized that there may be a subsurface water ocean due to the way that the surface ice has deformed. However, I do agree that Europa is an exciting target. Enceladus also, which has been seen to have cryovolcanos spurting out water-ice. I suspect Mars is too dry and cold. We'll see. $\endgroup$
    – Poshpaws
    Sep 28, 2012 at 8:27
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    $\begingroup$ It’s interesting to note that at least among leading astronomers there seems to be no question about this at all: the existence of extraterrestrial life is simply assumed. The only question is about when we find it, not whether (see e.g. some New Scientist issue from earlier this year). $\endgroup$ Sep 30, 2012 at 11:23
  • $\begingroup$ @Konrad Rudolph Astrophysicist Max Tegmark believes that we are alone. Alone as in any other life being beyond the horizon of the visable universe (he's big on multiverses). It is a valid standpoint since we never have observed extraterrestrial life and we don't understand how life originates. He certainly seems to be in the minority, though. $\endgroup$
    – LocalFluff
    Jun 14, 2014 at 9:35
  • $\begingroup$ Wrong forum. Try for astronomy.stackexchange.com. We can't even attempt to answer the bio half yet. $\endgroup$
    – Joshua
    Sep 22, 2015 at 15:21

2 Answers 2


OK, so we know a couple of things about life in the universe. Note, however, that this is not really an answer and is also not very biological in nature.

So, we don't know how life began on the Earth. However, do know that:

  1. The probability of life evolving on a planet in the universe is non-zero (since we exist) and,
  2. So far, we have not found evidence for life elsewhere in the universe.

So this leaves us with two main possibilities:

  • Life is rare in the universe.
  • Life is common in the universe but for some reason we do not detect it.

This latter possibility is basically a restatement of the Fermi Paradox which says

"The universe is very big and there should be lots of alien life out there. Where is everybody?"

In the case that life is common, we can speculate extensively on why we have not detected it so far. For example:

  • We haven't been looking long enough (SETI has been running for only a few decades)
  • We haven't been looking hard enough (the budget of SETI etc. is not large)
  • We haven't been looking in the right way. We are currently looking mainly in the radio regime, maybe aliens broadcast in X (where X is part of the E/M spectrum or something else entirely, e.g. gravity waves, tachyons)
  • Life is common, but intelligent life is rare (this would be my guess)
  • Life is common and intelligent, and doesn't want to answer (why? ask a xenopsychologist)

OK, so these are just some examples of "solutions" to the Fermi Paradox. There are many more which are much more exotic. My personal favourite of these is that we live in a simulated universe and the coders have not included additional alien life in the simulation. It would explain some strange "coincidences" about fundamental physical constants, but what simulation, no matter how good, would ever come up with cauliflower cheese for example?

However, until we actually detect life on another planet we will still be in the dark. The next generation of telescopes will be able to do this for nearby exoplanets if life is present on them and has affected the biosphere in a detectable way.

Edit - You mention "frequency models", but all we can currently say is that the probability of life on other planets is not zero - due to our existence - as I mentioned at the start

Some references:

  • $\begingroup$ "the probability of life on other planets is not zero" We can't even say that. The anthropic principle, you know. We cannot count ourselves as an observation, because our existance is necessary for making any observation. Our existance is perfectly consistent with us being alone. We have zero data. $\endgroup$
    – LocalFluff
    Jun 14, 2014 at 9:39
  • $\begingroup$ Poshpaws, good answer. As I understand we can only "look" into the past, since cosmologically speaking light is really slow. Light also disperses and becomes incoherent with long distances. "Simple" cellular life seems to be an evolutionary prerequisite, and they are not in the habit of sending out powerful radiotransmissions into space (i.e. weak-point of the SETI Proj.). There is also no reason why life should have terraformed a planet to the point of being spectroscopically detectable in the albedo. A lower bound on the number of habitable planets in a galaxy like ours, would be great. $\endgroup$ Jul 23, 2014 at 6:57
  • $\begingroup$ An amusing thing happens. We know that earth is already disappearing as a cosmic radio source. If you substitute 100 years for L in the drake equation, you get expected number of civilizations = 1. That must be us. $\endgroup$
    – Joshua
    Sep 22, 2015 at 15:19

According to this, 1.2% of stars have planets that can support life. According to Google, there are 300,000,000,000 stars in the Milky Way. That means 3,600,000 stars can support life. Each of these stars is estimated to have 1-2 planets that can support life. For the sake of simplicity, I'll use 1.5 to represent this, since it is the average of 1 and 2. 1.5×3,600,000=5,400,000 planets capable of supporting life. Looking around the web, there are wildly different estimates of the odds of life evolving on one of these planets (from 1%-100%), but 100% seems like the most common. This means that, statistically, the odds of extraterrestrial life existing are extremely high.

  • $\begingroup$ One thing is whether a planet is in a habitable zone - something we can begin to estimate with exoplanet observations - another is to know the probability of life evolving on the planet. Until we start to see evidence for extraterrestrial life (e.g. biosignatures) this is still a complete guess. $\endgroup$
    – Poshpaws
    Feb 17, 2013 at 8:08
  • $\begingroup$ Uhh the number of possibly life-supporting planets per star is << 1 because most stars forming planets end up with hot jupiters. $\endgroup$
    – Joshua
    Sep 22, 2015 at 15:20
  • $\begingroup$ Can we extend this to ask (excluding our own solar system), how far away we would have to look before we might reasonably expect to find some form of extra-terrestrial life? In other words, combine the likely number of life-supporting planets with the spatial distribution of stars in the galaxy to derive a mean distance between possibly/likely life-bearing star systems. $\endgroup$
    – Anthony X
    Sep 2, 2019 at 17:10

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