Now we know how to discover one planet—we measure the brightness of a star very carefully for a while, and if it dips in brightness for a few hours during a transit, we may have discovered a planet! But since the year 2000, thousands of planets have been discovered. How did astronomers find so many?
Most of the planets discovered so far have been found by the Kepler space telescope, which stared at 150,000 stars for 4 years, taking precise brightness measurements every half hour. More than 2,000 stars showed transits in the brightness measurements, revealing the presence of a whole zoo of planets that range in size from as small as Mercury to larger than Jupiter, and everything in between.
One of the biggest surprises that the Kepler space telescope discovered was that the most common size for a planet is in between the size of Earth and Neptune. Earth is the largest rocky planet in the solar system, and Neptune is the smallest gas planet in the solar system. There are no planets in our solar system between the size of Earth and Neptune, yet most of the planets found by Kepler fall in this size range.

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