Life on Other Planets?

Is there life on other planets?  Of course there is.  There are 100 billion stars in this galaxy and it looks like the majority of them have planets.   There are at least 100 billion galaxies in the universe.  that's 10,000,000,000,000,000,000,000 (10 septillion) stars with planets.  most of them will not be inhabited, but even if only one in a trillion is inhabited, that's 10 billion planets with life.  I'm confident will be far higher than that.

The circumstances that allow life as we understand it are fairly narrow: 

• the ambient temperature needs to stay fairly close to the melting point of water: a little above it and not too far below it.  too hot and too much of the water will be vapor.
• the ambient temperature can't vary too much.
• enough gravity that most of the water will stay liquid.
• not so much gravity that complex molecules break down.
• enough solar energy to provide a source of energy
• not so much solar that complex molecules will break down
• enough radiation to generate some mutation.
• not so much that a species can't be stable long enough to reproduce widely.
• has another big planet in the same system that cleans out most of the asteroids
• has a relatively circular orbit (to keep temperature constant)

the only specific chemistry that this requires is water.  Our life is based on carbon and water, but silicon or potentially any other chemistry is conceivable.


So far, there is only one planet that we know meets all these criteria, and it's the one we live on. About 3000 exoplanets have been detected so far and only one of them doesn't have obvious disqualifying factors.  The Kepler spacecraft is looking at 145,000 stars and has only detected planets on a few thousand of them.  The mechanism used wouldn't detect a system with one of the poles pointed at us--around half of the stars examined--and virtually all qualifying planets are below the minimum size that can be detected with Kepler. 


Of course, it may not be necessary for the physics we're familiar with to be relevant at all:

They're Made Out of Meat
A Fire Upon the Deep


The Drake equation: calculates the number of extraterrestrial civilizations that might be able to generate signals that we can detect.   Most of the parameters are unknown.

R_* is the rate of star creation (known)
f_p is the fraction of stars that have planets (still unknown but appears to substantial)
n_e is the number of planets per such star that could contain life (still unknown, but in our system, if we count all moons and planets, it's about 2%)
f_l is the fraction that actually go on to develop life (totally unknown)
f_i is the fraction that develop intelligent life (totally unknown)
f_c is the fraction that release detectable signals into space. (totally unknown)
L is the average length that such civilizations are able to communicate (totally unknown)

he omitted one: fraction that are close enough that any signal might be detected.   It's hard to imagine this coming from anywhere outside of our galaxy and probably only the local quarter.


The first four terms address the same question I'm asking.  The only one that's still completely unknown is f_l, the fraction of planets or moons that actually develop life.  The earliest fossil record found so far of life on earth is about 3.5 Billion years ago.  The first time life as we know it could possibly have formed--the first time that liquid water was available on earth--was about 4.4 Billion years ago.   So we know life started at some time in the first 900m years that it was possible.  There's not much left of the fossil record from that long ago, so it's probable that it was a lot longer than 3.5B year ago.

Life tooled along making stromatolites and other primative things, until about 1.2B years ago, some protozoa figured out how to reproduce sexually.  This created lots of new things: species that would breed true, yet allowed a lot of very small variation, to allow evolution by natural selection to take place.  It took another 650m years for the real excitement to happen, in what's called the Cambrian Explosion, when all of the multicellular phyla of plants and animals evolved. 

So we took at least 3 Billion years to develop above the protazoa stage (and most of that to develop to the protazoa stage), and more than another half billion years to develop intelligence.  if this is typical, chances are pretty good that whatever life we find will be very, very simple.  We know that over 3/4ths of the time that life has been possible on earth, that it has existed.  It is entirely possible that the first life occurred in the first few million years, or even earlier. As it stands, it seems like that 3/4ths is a plausible guess for f_l.

So far, we have only sent missions capable of detecting life to one of the extraterrestrial bodies in our planet where life would be at all possible, Mars, and we know that it's unlikely there because atmospheric pressure is too low to sustain liquid water and temperature too high to allow a permanent frozen shell.   There are a handful of moons that probably do have liquid water, around Jupiter and Saturn, although in all cases, it's under a very thick layer of ice.   Until we find out whether there is life there, we really have no handle on that all important term: f_l.  But we have the beginnings of a handle on some of the others.  Life on about 1 in 200 planets seems possible based on what we know.  And since most stars that have any planets probably have multiple planets, the number of stars with life is probably similar.