  Discovering other worlds the size of our own has been a long-held dream of
astronomers. The transiting planets Kepler-20e and Kepler-20f, which belong to
a multi-planet system, hold a very special place among the many groundbreaking
discoveries of the Kepler mission because they finally realized that dream. The
radius of Kepler-20f is essentially identical to that of the Earth, while
Kepler-20e is even smaller (0.87 R[Earth]), and was the first exoplanet to earn
that distinction. Their masses, however, are too light to measure with current
instrumentation, and this has prevented their confirmation by the usual Doppler
technique that has been used so successfully to confirm many other larger
planets. To persuade themselves of the planetary nature of these tiny objects,
astronomers employed instead a statistical technique to "validate" them,
showing that the likelihood they are planets is orders of magnitude larger than
a false positive. Kepler-20e and 20f orbit their Sun-like star every 6.1 and
19.6 days, respectively, and are most likely of rocky composition. Here we
review the history of how they were found, and present an overview of the
methodology that was used to validate them.
