To date, the Kepler space telescope has found more than 1,200 likely planets orbiting stars beyond the sun — quite a haul for a satellite that's been flying for just over two years. The true prize Kepler is hunting for, of course, is not just any planet, but one that's a twin of Earth — about the size of our world, orbiting in a zone where the temperature range is like ours.
All that would make it a prime place to look for life. Finding such a not-too-hot, not-too-cold world is probably just a matter of time, but even then, there will be one more factor to consider: how old the planet is.
In the biology game, planetary age can be everything. If alien astronomers had discovered Earth when it was just a billion years old, the only life they'd be able to find would be the most primitive of microbes. If they waited another billion years, they'd see the effects of cyanobacteria pumping oxygen into the atmosphere. At about 4 billion, multicellular organisms would arise. And if the aliens wanted someone to talk to, they'd have to wait until Earth had been around for 4.6 billion years, when humans began communicating with radio signals.
The same limitations are true for contemporary earthlings studying other worlds. Trying to determine the age of a planet directly is well-nigh impossible, particularly at the vast cosmic distances at which Kepler is forced to do its work. But there's a way around that problem, and it begins with the fact that all planets are about the same age as the star they orbit; if you know how old the parent sun is, therefore, you know the age of its litter of worlds.
http://www.time.com/time/health/article/0,8599,2074984,00.html?xid=newsletter-weekly
