The first confirmation of a planet outside of our solar system happened in 1992. Since then, exoplanets have become almost commonplace; NASA's Kepler mission has confirmed more than 2,000 to date. But how? By staring at one spot, unceasingly, for more than three years. The telescope aboard the Kepler spacecraft monitors a single region in the Cygnus and Lyra constellations, waiting for signs of a transit—that is, the moment a planet passes in front of its home star. When this happens, all sorts of things can be calculated. The time it takes the planet to pass tells us the size of its orbit and its mass, and the dip in brightness that the planet creates tells us its size. Comparing how hot the star is to how far the planet orbits tells us the planet's temperature. The most exciting discoveries are planets that orbit in their star's so-called "Goldilocks zone"—not too hot, not too cold—and are less than twice the size of Earth, which suggests that they probably have solid surfaces. With these elements in place, those planets have the potential for sustaining life.
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Key Facts In This Video
The first confirmed exoplanets were found orbiting a pulsar in 1992. 02:03
Hot Jupiters are gas giants that orbit very close to their star. 04:30
During a transit, a planet passes in front of its star, letting Earth astronomers detect a dip in the star's brightness. The amount of light blocked tells you the planet's size. 05:52