One of the objectives of NASA's Kepler Mission is to establish the number of planetary systems
outside of the Solar System that contain Earth-like planets. At a minimum, an Earth-like planet
would be rocky (vs. gaseous), be about the same radius as Earth, and exist within the habitable
zone of its host star. The habitable zone is the region in which liquid water could exist on a
planet's surface. Liquid water is a key ingredient to life, so planets found within this zone are
more likely to be habitable worlds.
This artist's concept shows what a water-covered Earth-analog planet might look like. Rising out
of a deep blue ocean are several land masses with distinct climate regions, including arid deserts,
tropical rain forests, lush green valleys, frozen tundras, and icy polar caps. Lakes and rivers are
also visible on various continents, as well as steep mountain ranges. Wispy groups of clouds can
be seen forming throughout this planet's mostly clear atmosphere.
Although the Kepler spacecraft is not capable of detecting such detail, it has discovered over
3,200 exoplanet candidates. More than 270 of those are in the habitable zone of their stars. Could
any of those be Earth's twin?
Launched in 2009, the Kepler spacecraft measures the light output of 150,000 stars
simultaneously. The data from each star is then analyzed in order to look for periodic drops
in the light curve being emitted. These drops in brightness could indicate the presence of an
orbiting planet passing in front of its host star, blocking some of its light. This is called a transit.
Three or more transits of equal periods are needed to catalog an object as an exoplanet candidate.