Dwarf planet
With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies (asteroids and comets). For almost 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates. It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had one-fifth the mass of Earth's Moon. Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets.
Kepler-11f
Kepler-11f is, at 2.3 times the mass of Earth, the least massive of the six planets discovered in the orbit of Kepler-11, although the planet's mass may range from 1.1 to 4.5, or from approximately that of Earth's mass to that of Kepler-10b, a rather large confidence interval. Its radius is the second smallest of the six planets discovered in the system at 2.61 times the radius of Earth. Kepler-11f has a density of about 0.7 g/cm, comparable to that of the Solar System's least dense planet, Saturn. Kepler-11f is the fifth planet from Kepler-11, orbiting its host star every 46.68876 days at a distance of 0.25 AU. Its orbital eccentricity is unknown. In comparison, Mercury orbits the Sun every 87.97 days at a distance of 0.387 AU. Kepler-11f has an orbital inclination of 89.4°; it can be seen almost edge-on with respect to Earth. Its surface equilibrium temperature is 544 K, over twice the surface equilibrium temperature of Earth and about two-thirds the surface temperature of Venus.