Planets, in particular habitable planets, are so common in works of science fiction that there's a tendency to assume that they'd be common in the real Universe. There is little hard data to support that notion--not yet anyway. Just 15 years ago, the only planets astronomers knew where the nine that orbited one star: Sol. (I'm not attempting to promote Pluto-back-to-full-fledged-planethood, but it was considered a planet back then, hence the inclusion.) We have now identified over 490 planets (and counting) orbiting other stars. So although stars with planets seem to be fairly ubiquitous, perhaps even the rule rather than the exception, that still raises the question of the abundance of habitable planets.

Until recently the detection methods astronomers used for finding extrasolar planets has had a distinct bias--the planets we've found tend to be large, Jupiter-like, and close to their parent stars. Now the Kepler spacecraft

has just begun its search for extrasolar Earths and, in a very short time, has already found over 700 candidate stars that could have Earth-sized planets

. As followup studies examine these candidate stars further, is it only a matter of time until another "Earth" is detected? Certainly, but we may have to sift through a lot of near-misses first. New Scientist has a interesting article on whether or not there is a Moore's Law for Science

, using the extrasolar planet hunt as backdrop for examining whether or not previous rates of scientific discovery can be used as predictors of future performance. The article says:

Their calculations suggest there is a 50 per cent chance that the first habitable exo-Earth will be found by May 2011, a 75 per cent chance it will be found by 2020, and a 95 per cent chance it will be found by 2264.

Is there a 75% chance we'll find an Earth-sized planet by 2020? Almost certainly, given the performance of the Kepler spacecraft and the fact that astronomers have found a planet only 1.5 times larger already

. A habitable exo-Earth? Not so fast. Enter CO2. Earth-sized planets that are situated in the habitable zones

, or Goldilocks zones

, of their parent stars are too small and too warm to hold onto the two most common gases: hydrogen and helium. Terrestrial planet atmospheres, at least the ones with which we are familiar, are formed initially from the volatile compounds

commonly found in, and delivered by, ices from comets: in particular water (H2O), ammonia (NH3), methane (CH4), and carbon dioxide (CO2). At a molecular level, CO2 is, by far, the most massive of those four compounds. So, like dry ice (also CO2) fog at a Halloween party,

CO2 sinks to the bottom of a planet's atmosphere, displacing other gases that can eventually escape into space. When we look at our neighbors, both Venus and Mars have atmospheres composed mostly of CO2. Venus is so hot that the molecules of most gases easily reach escape velocity. (Carbon dioxide is a greenhouse gas that traps the radiant heat from the sun efficiently, driving the temperature of Venus to approximately 860 degrees Fahrenheit planetwide.) Although Mars is far colder, it has only 37% of Earth's gravity; it's so small that the molecules of most gases escape, just as on bigger, hotter Venus. Earth, too, likely had an atmosphere composed chiefly of CO2 in its youth, and if it still had that atmosphere, it would be too hot for life. Earth was "lucky", though: in its infancy Earth was struck by a Mars-sized object

and stripped of its Venus-like atmosphere. What are the odds that events like this are common throughout the galaxy? So we're not necessarily on the brink of finding Caprica, Minbar

, Risa

or, luckily, Skaro

. Even though our detection methods are likely to turn up numerous Earth-sized planets in the very near future, they're unlikley to be Earth-like. Yes, it's only a matter of time until we find the first exo-Earth, but given the relative abundances and properties of the most common gases that form terrestrial planet atmospheres, we may run across a lot of extrasolar Venuses first.