An artist's illustration of an Earth-like planet. The search for planets that are similar to Earth is one of NASA's main goals. Many planets have already been discovered orbiting other stars, but so far only larger planets (the size of Jupiter or larger) have been found. New missions are being planned by NASA which will be able to detect smaller Earth-sized planets. Some of these missions will also try to detect signs of life on these planets by studying emissions in their atmospheres.
BILLIONS OF EARTHS COULD BE OUT THERE!!
The four basic types of terrestrial planets (Most earth-like planets)
There are four basic types of terrestrial planets imaginable around other stars. Upper left: A dry, barren desert world in a system devoid of water. Upper right: A warm and moist jungle world covered with plant life and a rich carbon dioxide atmosphere, and located at the inner edge of the habitable zone. Lower left: A frozen, ice-covered planet just outside the habitable zone. Lower right: A water world with large oceans and little land mass in the middle of the habitable zone. (Click image to enlarge) Image Credit: NASA.