How was the TRAPPIST-1 planetary system discovered?

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Extrasolar planet Artist's conception of the extrasolar planet HD 209458b, some 150 light-years from Earth.

The TRAPPIST-1 planetary system was discovered using transit photometry, a technique that identifies planets by measuring the periodic drops in starlight as they pass between their host star and a telescope.1

The discovery involved observations from both the TRAPPIST telescope located on Earth and the Spitzer Space Telescope. Through these efforts, astronomers identified seven Earth-sized planets orbiting the star, three of which are located within the habitable zone.1