Hi Christine,

Actually, we do not know for certain that out of all of the planets there is only life on Earth. We really haven't been looking for life beyond Earth all that long - only the last 50 years or so. We have sent unmanned spacecraft to all of the major planets in the solar system, and one is currently on its way to Pluto. However, we really have not explored that much of the surfaces of the planets at this point. We also have not thoroughly explored the moons of the Jovian planets either. Saturn's moon Titan is the only one to which we have sent a lander and we really could not explore much of Titan's surface with that mission. Some scientists believe there could be life in oceans under the ice of Jupiter's moon Europa, but we have not been able to explore there yet either. Just because we have not found life on other planets yet does not mean that it does not exist. NASA now has an astrobiology program to search for evidence of life on other worlds both in our solar system and the universe beyond. NASA has a spacecraft called Kepler which is searching for Earth-like or terrestrial planets right now. In February, this mission found candidates for Earth-sized planets orbiting their stars in the region where liquid water could exist. You can read more about NASA's astrobiology program here

http://astrobiology.nasa..../about-astrobiology/

and the Kepler mission here

http://kepler.nasa.gov/

Kris