Kepler-37b, along with two other planets, Kepler-37c and Kepler-37d, were discovered by the Kepler space telescope, which observes stellar transits. After observing transits of Kepler-37b, astronomers had to compare it with the size of the parent star.

Unsurprisingly, very small exoplanets, or planets orbiting other stars, are extremely hard to spot. But of the ones scientists have found, the smallest is Kepler 37-b. Discovered in 2013 by scientists using data from the Kepler space telescope, it is smaller than Mercury and about the same size as Earth's moon.

While it's possible to say the smallest exoplanet discovered so far is Kepler-37b, considerably smaller than Mercury and only a little bigger than our own Moon, we can't know for sure that there aren't billions of tiny planets out there that would make poor little Mercury look positively Jovian!