March 6, 2009. A balmy Florida evening, and my family and I stood on Cocoa Beach, looking northward toward the Cape Canaveral Air Force Station. We were part of a seaside crowd gathered to witness the launch of NASA’s Kepler Space Telescope. As the fireball appeared and slowly began to rise in the distance, we cheered with our fellow observers. About 30 seconds later, we felt the ground rumble and heard the deep roar, watching the Delta II rocket climb into the night sky and accelerate as it headed out over the ocean.
Kepler went on to spend nine years in deep space searching for galactic neighbors like us: Earth-sized planets orbiting Sun-like stars. Kepler watched a patch of the Milky Way galaxy that included millions of stars. It beamed back data on nearly 200,000 of them and found more than 2,300 exoplanets—planets outside our solar system.
“With data from Kepler, we have more precise and detailed information than we’d ever had before,” says astrophysicist Eric Ford, who was part of the Kepler science team. Ford and his colleagues at Penn State’s Center for Exoplanets and Habitable Worlds are building on the legacy of Evan Pugh Professor Alex Wolszczan, who discovered the first known exoplanets in 1992 using surveys from ground-based instruments. “Kepler found thousands of planets,” Ford says. “Astronomers would love to learn more about all of them, but there is not enough telescope time. Since people are particularly interested in learning more about those that may resemble Earth, we plan to concentrate on characterizing planets in the habitable zones of their planetary systems.”
The habitable zone is a region within a solar system—a distance not too close and not too far from a sun—where a planet would have the conditions necessary to have liquid water on its surface, an important requirement for the existence of carbon-based life as we know it. James Kasting, Evan Pugh Professor of Earth Sciences, was one of the early developers of the concept. The planet’s surface temperature must be above the freezing point of water and below the boiling point. Other conditions also come into play, including the planet’s mass, rotation, and atmosphere. Among the Kepler exoplanets that have been analyzed so far, several dozen are considered to be in the habitable zone of their star.