The recent results from the Kepler planet hunter have been exciting. Kepler is finding a lot of planets, over a thousand now, including ones close to earth's size and planets orbiting in the "habital zone" around their star where temperatures are expected to be similar to Earth's (allowing liquid water). From these results it has been estimated that there are half a million habital zone planets in the galaxy. It has been argued that the idea of a habital zone is too narrow and that life could exist outside of this range, like perhaps on Jupiter's moon Europa. However, taking this number at face value we can work backwards and estimate the distance to the nearest "hz" planet.
The radius of the milky way is about 50,000 light years (ly) and it is about 1,000 ly thick. Treating the milky way as a simple cylinder gives a volume (pi r^2 h) of 7.85 x 10^12 ly^3. Dividing this space by the number of hz planets gives a volume of 15,700,000 ly^3 per planet. If we imagine this volume as a sphere (4/3 pi r^3) around earth and solve for the radius (r = cuberoot[(3 * 15,700,000 ly^3)/(4 * pi)]) we get a distance of 155 light years. So, simplistically, at face value, we expect the average distance between habital zone planets to be about 155 light years, and the nearest hz planet to Earth to be about 155 ly away.
Earlier this month NASA announced that Kepler confirmed the first earth sized habital zone planet, Kepler-22b. Kepler-22b is 600 ly away, but there should be many other hz planets closer by. The expected number of hz planets goes up quickly after 155 ly, because the volume of a sphere increases dramatically with small increases in the radius and the area of space around Earth quickly contains more possible hz planets. I plotted the relationship below.
What is the probability the planet will have life? Using Earth as an example (our only example), life seemed to appear on Earth as soon as conditions allowed it. Almost immediately as the Earth cooled and liquid water formed. This suggests that if the conditions are right, life is very likely (but this assumption has also been challenged). However, what form will that life be in? For 3/4 of the time since life existed on earth it was only present as single cells. Expecting life that arose independently in space to be similar to life on Earth is likely too narrow a concept. But again taking our only example at face value suggests most (perhaps 3/4) of life on other planets to only be the equivalent of single cell life on Earth. How far away should we expect the first planet with the equivalent of multi-cellular life to be? Multiplying the per planet volume by 4 to account for 1/4 of the hz planets having multi-cell equivalent life and solving for the sphere radius gives (r = cuberoot[(3 * 4*15,700,000 ly^3)/(4 * pi)] = 246 ly). So we expect the nearest multi-cell equivalent containing planet to be 246 ly away.
Is it possible to ever be able to travel those kinds of distances? Project Daedalus and Project Longshot were attempts to conceptually design unmanned interstellar crafts using existing or near existing technology to flyby Bernard's star 6 ly away after a 50 year trip and to orbit Alpha Centauri 4.4 ly away after a 100 year trip. So interstellar travel on the order of ~5 light years to our nearest neighbor stars is being seriously contemplated. Ramping this up to trips of 150 to 250 ly seems impossible but orbiting the earth and traveling to the moon seemed impossible until we did it. Conversely, after 1,000 years of interstellar travel it might be possible for an alien to make it from the nearest planet to earth. However, again by using our only example, the leap from multicellular life to space exploration took a very long time to occur here on earth and is probably very rare elsewhere.
Sputnik, the first human made object to orbit the earth, was only 54 years ago in 1957. When compared to 4 billion years of life on earth this is a tiny fraction of time, 1/80 millionth. Half a million hz planets in the galaxy multiplied by a one in 80 million chance of developing space exploration technology gives 0.00625 or approximately half a percent chance of space capable life per galaxy. To expect an average of one occurrence of space exploring life per galaxy predicts space travel would exist for (1 = 500,000 * x / 4,000,000,000) an average of 8,000 years per planet. 16,000 years to have 2 per galaxy, ... 1 million years to have 125 per galaxy. If on average alien species that are exploring space have been doing so for a million years, and we expect 125 such civilizations in our galaxy, the nearest one is expected to be 4,500 light years away (on this scale consider the galaxy as a flat disc, pi * r^2, r=50,000 ly gives a galaxy area of 7.8 x 10^9 ly^2, dividing by 125 gives an area of 6.2x10^7 area per civilization, converting this back into a radius gives 4,500 ly). Is it possible for a space faring civilization to survive for 1 million years? If so, using approximate Project Daedalus speeds (years of travel ~=10 x ly distance), they would have to travel for approximately 50,000 years to arrive here, or 5% of the age of the alien civilization. How much of space could be explored at those distances within those time frames? Perhaps this is the answer to Fermi's paradox. Life may be common in the universe but we are likely the only ones exploring space in this galaxy. There may be other civilizations exploring space in other galaxies but they are impossibly far away. Even if there are other civilizations exploring space in this galaxy, it would be easy to miss each other because of the immense distances and time scales involved.
Using the time travel technology analogy. Since other civilizations haven't already visited us, it is not possible to develop faster than light space travel (although this may also be a self fulfilling prophecy). This leads to a future vision of humans (or human machines) spread over vast distances in the stars where it is possible to slowly communicate interstellarly and travel to the nearest star in a lifetime, but it is not possible to travel between hz planet star colonies in a single lifetime. Perhaps this diffuse human existence would enhance our probability of survival, if humans were less physically interconnected what happens in one area would take a very long time to affect, or not affect at all, another region of space.
Why should life naturally evolve the capability to explore space? Earth-like life that developed on a planet would be adapted to the planetary conditions and not to space, so what kind of selection would exist for life to evolve to space conditions? Space is tremendously dangerous for earth-like organisms. (Although, a similar argument can be made for sea based life to not evolve to terrestrial life.) As a parallel, it has been suggested that birds species who arrive on isolated pacific islands may evolve flightlessness not only because there are no predators to escape from, but also because of the danger of flying and being blown out to sea. There may be natural selection on habital zone planets not to explore space, and to stay on the planet, the individual organisms that explore space are likely to die. Because humans have cultural inheritance via communication as well as biological inheritance we are able to evolve technology that is not based on individual behavior or genetic capabilities and to get around this biological evolution limitation. Perhaps we would expect other space explores to do the same.
What kind of life might we expect to naturally evolve to space travel? In its simplest sense, life is the ability to reproduce with the chance of new mutations. This can be the biological evolution we are used to, or we can imagine a complex machine that could build copies of itself, or the electronic evolution that can occur with replicating computer programs like a mutating computer virus. Perhaps our idea of life is overly narrow. Might life exist as complex electrical signals transmitting between metallic meteorites? If so what reason is there for this type of life to evolve to try to communicate (or just send out powerful signals) to unknown species in deep space? This could be the asteroid belt equivalent of single cells that we wouldn't notice unless we looked for it. As Dyson suggested, we could possibly build self replicating machines in space, but is there a reason they would evolve to travel interstellarly unless we initially programmed them to do this? These are referred to as Von Neumann Probes and it has been argued by Carl Sagan that any intelligent life would not build them...
If the distance between habital zone planets is too large to cross in a few lifetimes, perhaps in the future we could build machines that could build humans (and other earth life) after traveling to the planet (as is hinted at in A. C. Clark's novel Rendezvous with Rama). Who could then establish a civilization to build new interstellar machines--a long slow spread to the stars. Perhaps we might expect alien civilizations to also do this?
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