In 2009, NASA launched the Kepler space observatory into an orbit around the sun with the express purpose of using the satellite to discover Earth-like planets orbiting other stars. The only instrument on board Kepler is a photometer, which measures the brightness of nearly 150,000 stars in the Milky Way. When the brightness of these stars dims slightly due to a planet passing in front of the star, scientists on Earth are able to determine quite a bit about that planet, such as its size and distance from the sun.
Photometry is a great way of quickly covering a lot of ground when searching for exoplanets, but it is still an indirect way of observing exoplanets. The ultimate goal is direct observation, which can give planetary scientists still more information about the planet being observed, such as its atmospheric composition.

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