Are we alone in the universe or is there life out there waiting to be discovered?
NASA is getting ready to launch the Kepler telescope on an ambitious, first-of-its-kind mission: to search for Earth-size planets in our galaxy, orbiting stars at the right distances to have water on their surface.
These planets are too small and too difficult to be seen with past telescopes. But they are precisely the kinds of planets on which life could exist.
Kepler is NASA's $600 million telescope, scheduled to launch this week from the Kennedy Space Center. Its potential is enormous, according to astronomer Debra Fischer of San Francisco State University.
"What excites me is for the first time, we are going to have a mission that can take a full census of the kind of planets that exist around other stars," she said.
Jon Morse, NASA's top astrophysicist, believes Kepler will revolutionize what we know about the universe.
"Kepler will push back the boundaries of the unknown in our patch of the Milky Way Galaxy," Morse told ABC News. "And its findings may fundamentally alter humanity's view of itself."
Kepler's telescope will work by detecting starlight, and analyzing minute changes in brightness in the galaxy while scanning space for planets like Earth.
Kepler will trail Earth as it orbits the sun, on a mission that will last from three-and-a-half years, possibly going as long as six years.
During the mission, Kepler will measure changes in the brightness of more than 100,000 stars, every 30 minutes, searching for "winks" in light that happen when a planet passes in front of its star.
That is how Kepler will know when it has found another Earth.
The Hubble Space Telescope, which has a different mission, can't do this.
"Hubble is really amazing for reviewing the structure of the universe [and] formation of galaxies," Fischer said.
"Hubble can't afford to do what Kepler is doing. Kepler has a smaller telescope and will stare at only one field, a large area of the sky in the constellations Cygnus and Lyra."
Fischer said that will take years.
"The hardest thing is going to be finding Earths around a star," she added. "If Kepler is flying for three years, an Earth in the habitable zone would take [a] year's orbit to spot, so it will take three transits in three years to detect another Earth.
Kepler may not find ET but it may find ET's home.