The $600 million Kepler mission launched in 2009 with a goal to survey a portion of the Milky Way for habitable planets.
From a vantage point 64 million miles from Earth, it scans the light from distant stars, looking for almost imperceptible drops in a star's brightness, suggesting a planet has passed in front of it.
It has discovered more than 1,000 planets. Twelve of those, including Kepler-425b, have been less than twice the size of Earth and in the habitable zones of the stars they orbit.
Missions are being readied to move scientists closer to the goal of finding yet more planets and cataloging their atmospheres and other characteristics.
In 2017, NASA plans to launch a planet-hunting satellite called TESS that will be able to provide scientists with more detail on the size, mass and atmospheres of planets circling distant stars.
The next year, the James Webb Space Telescope will go up. That platform, NASA says, will provide astonishing insights into other worlds, including their color, seasonal differences, weather and even the potential presence of vegetation.