For a planet to be amenable to life, a number of factors need to be "just right." A good candidate should be a terrestrial (rocky) planet. Ideally, it should measure between half and twice the size of Earth, but the important thing is that it's massive enough to hold an atmosphere but not so big that it bloats into a gas giant like Jupiter or an ice giant like Neptune.
It should also be located in the habitable zone, a distance from the parent star where the surface temperature won't freeze liquid water or boil it off. The location of this zone varies according to the star's characteristics.
Kepler's Search for Exoplanets
Kepler is the first NASA mission capable of finding Earth-sized planets around other stars. Its main goal is to generate a base estimate, or census, of the number of such planets orbiting within habitable zones, where conditions are right for liquid water to exist.
The instrument package doesn't orbit the Earth in a satellite: It's housed within a spacecraft 9 feet (2.7 meters) in diameter and 15.3 feet (4.7 meters) high that orbits the sun, trailing our home planet.
Kepler uses a very wide field telescope and a photometer (light meter) to measure brightness variations in more than 156,000 stars simultaneously [source: Ames Research Center, NASA Finds Earth-size Planet Candidates]. It takes these readings every 30 minutes because transits can require from an hour to half a day, depending on the planet's orbit and the type of star involved.
Mission scientists also employ spectroscopic data from ground-based observatories to help confirm planet candidates and use stellar observations to remove other confounding factors, such as binary stars (a pair of stars that revolve around a common center of mass).
The Cygnus-Lyra neighborhood was chosen as the study area because it's well-populated with stars and lies high enough above the Earth's orbital plane that the sun, Earth and moon won't get in the way of Kepler's observations. The stars are between 600 and 3,000 light-years away. From our perspective, they cover an area equivalent to 1/400 of the sky [source: Harwood].
Kepler detects planets via the photometric or transit method, which means that it detects the small drop-off in a star's brightness that occurs when an orbiting planet passes between its star and us. Once the data analysis identifies a dimming event, scientists look for further dips of the same magnitude, duration and period to confirm the planet's existence.
This is no mean feat: An Earth-sized planet crossing in front of a sun-sized star dims its light by a mere 0.01 percent. NASA folks like to say that detecting such a tiny dip is like spotting a flea crawling across a headlight from several miles away. Jupiter-sized planets cast a bigger shadow. Even so, viewed from outside our solar system, Jupiter's transit only diminishes our sun's brightness by 1 to 2 percent [source: Ames Research Center, FAQ].
There's more. For the transit method to work, a planet must pass almost perfectly along our line of sight, the chances of which are around 0.5 percent for an Earth-sized planet (in an Earth-sized orbit) and 10 percent for a Jupiter-sized planet (if it orbits near its star) [source: Ames Research Center, FAQ].
To put it another way: Even if we checked out 100,000 stars that actually had Earthlike planets, we would only be able to "see" 500 of them via the transit method. Using probabilities like these, scientists can estimate the planet population of our galaxy from Kepler's observations.