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Stars That Have Life-Bearing Planets

Just about everyone has looked at the sky and wondered about life among the stars. After all, it's a pretty large universe out there, so even if the chances are very small, the odds are good that life evolved elsewhere. Calculating the odds is one thing, but actually finding evidence of life on planets around other stars is quite a challenge. The question is so important, though, that efforts are being made to address the challenge.

Conditions for Life
- It is difficult for humans, who evolved in a water-rich environment within a certain temperature range, to predict with certainty how life could evolve in other environments, built on other principles. So the search for life around other stars is really the search for life similar to our own. Some of the criteria are things like: a stable star with a reasonable percentage of heavy elements, a planet with an orbit and mass that allows the existence of water, and the presence of oxygen in the planet's atmosphere.
Searching for Planets
- In 1995, scientists discovered the first evidence for an extrasolar planet --- a planet orbiting a star other than the sun. As of July 2011, more than 560 extrasolar planets have been detected. All of these discoveries have been made through indirect measurements: measuring changes in the image or position of the star. Those methods are accurate, but they are limited to detecting relatively large planets. The effect of Earth-like planets on their stars is too small to measure with these methods. Nevertheless, the first step in finding signs of life on other planets is to find other planets.
Goldilocks
- In 2010, scientists announced the first "Goldilocks" planet --- one with just the right characteristics to support the existence of liquid water. There is some controversy around the discovery, but even if this one --- Gliese 581g --- turns out to be inaccurate, sooner or later a planet with the right characteristics will be found. But that's just the discovery of a planet with the possibility of having a long history of water. More will be needed to find evidence to support the presence of life.
How to Finish the Search
- Scientists have tried to identify the signature of life. That is, how will the presence of life influence the atmosphere of another planet? The most obvious signature is the presence of a lot of oxygen in the atmosphere. Oxygen can get into an atmosphere through other means, but without the presence of life to replenish it, it rapidly gets absorbed into minerals. Measuring the atmosphere around a planet millions and billions of kilometers away is a technological challenge. It is on the edge of current technological capabilities, and NASA is working on making it possible.
The Answer So Far
- Lots of chances for life --- but it hasn't been discoverd yet.
  
  So far, out of all the billions of stars in the universe, people have evidence of life on one planet around one star. The one planet is Earth and the star is our sun. Although the chances are good that signs of life will eventually be found on planets around other stars, the technology is not quite ready to seriously begin that search. With dedication to the program, the search can begin in earnest in just a few years.

Astronomy