Planet
The smallest known planet is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) slightly higher than that of the Moon.
Atira asteroid
In great part because of the search methods used to look for asteroids, there are currently no known asteroids with orbits contained within Venus (Q = 0.728 AU) or Mercury's (Q = 0.467 AU, e.g. vulcanoids). As of January 2019, the asteroid with the smallest known aphelion is , with an aphelion of 0.773 AU.