Researchers find planets by examining the brightness of stars as a function of time; brightness drops when a potential planet crosses the star.
Three transits are required for a planet confirmation. The period of the transit of the newly discovered second earth 'Kepler 22-b' was 7.4 hours. It did not appear to give off its own light, indicating it is a planet and not a star.
Scientists do not yet have a measurement of the mass of Kepler 22-b, which would tell them more about the composition of the planet. This summer, when the planet's star will be high in the sky, ground-based telescopes can attempt to get its mass.
The planet is even more mysterious because its radius is between that of Earth and Uranus and Neptune, both of which have radii about four times the size of Earth's. So we don't know what a planet in this size range typically looks like.
Is life restricted to Earth, or could it exist somewhere like Kepler 22-b? It may be that the characteristics of Earth, with its particular position in the solar system, particular magnetic field strength and presence of larger planets like Jupiter make Earth very rare in having life. But it's also possible that life in the universe is common, and we're only one example.
"As soon as we find an independent example of life somewhere else, we'll know it's ubiquitous throughout the universe. We're all looking for No. 2," said Jill Tarter of the SETI Institute.