Most stars have planets orbiting them. These stars are called host stars, and exoplanets are what we call planets that orbit other stars, outside of our own solar system. If we view an exoplanet system from the correct angle, the exoplanets will appear to eclipse their host star as they move around the star on their orbits. If we measure the brightness of the star with excellent precision when it is eclipsing its star, we can observe a transit event, which is described in detail in this Frontiers for Young Minds article. During a transit, a planet blocks out some starlight, and astronomers on Earth see a dip in the total amount of starlight, which typically lasts for a few hours.
TRAPPIST-1 is a very small, red star located 40 light years away, in the constellation Aquarius. In fact, TRAPPIST-1 is about as small as a star could be—if it were a bit smaller, it would not even be considered a star, because it would not be big enough to produce its own light by fusing hydrogen into helium in its core. TRAPPIST-1 is about as big as the planet Jupiter, but it weighs 80 times as much as Jupiter.

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