Kepler predicted that Venus' diameter is almost a quarter of the Sun's diameter, so Mercury should have a tenth of the Sun's diameter. Pierre Gassendi saw the true size of Mercury in 1631 during its transit across the Sun, and was surprised by how small it was as it looked like a sunspot
Astronomers Discover the Last Three Planets NASA’s Kepler Space Telescope Observed Before Going Dark
Kepler’s last planet discoveries: two new planets and one single-transit candidate from K2 campaign 19 | Monthly Notices of the Royal Astronomical Society