For centuries, human beings have pondered this question. Medieval scholars speculated that other worlds must exist and that some would harbor other forms of life. In our time, advances in science and technology have brought us to the threshold of finding an answer to this timeless question. The recent discovery of numerous planets around stars other than the sun confirms that our solar system is not unique. Indeed, these "exoplanets" appear to be common in our galactic neighborhood. The exoplanets we have discovered so far are giants, like Jupiter and Saturn. They are unlikely to support life as we know it. But some of these planetary systems might also contain smaller, terrestrial planets like Mars and Earth. Over the next 15 years, NASA is embarking on a bold series of missions to find and characterize new worlds. These will be the most sensitive instruments ever built, capable of reaching beyond the bounds of our own solar system.
NASA's Kepler Mission, scheduled to launch in 2009, will survey our region of the Milky Way galaxy to detect and characterize hundreds of Earth-size and smaller planets. It will tell us whether planets like Earth are common or rare in our galaxy. SIM PlanetQuest, to follow Kepler, will measure the distances and positions of stars with unprecedented accuracy. SIM's precision will allow us to locate planets in the habitable zones around nearby stars. Finally, the Terrestrial Planet Finder will build upon the legacy of all that have gone before it. With an imaging power 100 times greater than the Hubble Space Telescope, the Terrestrial Planet Finder observatories will provide the first photographs of nearby planetary systems. We will analyze the atmospheres of these distant worlds, looking for carbon dioxide, water and ozone. The substantial presence of all three gasses would suggest that life is present. Such a discovery would at last provide convincing evidence that we are not alone. We will have found another Earth.
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