Pluto (w/Charon), Eris (w/Dysnomia) and Ceres
A young Kansas farmer, working a telescope by feel in the dark of a wooden dome on a frozen Arizona mesa in 1930, discovered Pluto, the ninth planet of our Solar System. Clyde W. Tombaugh, who had taught himself to be an astronomer, became the first 20th century man to discover a planet when he found Pluto.
Pluto's moon Charon was discovered in 1978.
Then, in 1987, NASA scientists at the Jet Propulsion Laboratory, Pasadena, California, reported observations that showed Pluto has a substantial atmosphere.
Most distant, least understood. Pluto – most distant, smallest and least understood of the Sun's known planets -- recently has been the closest that mysterious body has come to Earth since 1740. As it swung within 2.8 billion miles, astronomers tried their best to unlock some of its secrets.
Pluto is 3.6 billion miles from the Sun. With a diameter of only 1,370 miles, Pluto is the smallest planet, smaller even than Earth's airless Moon. Pluto is so far away and so small that little was learned about it for decades.
Pluto demoted. For decades, we counted Pluto among the nine largest bodies of the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
However, in 2006, the International Astronomical Union, for the first time, created a scientific definition for the word planet and thereby demoted the former major planet Pluto to the lesser status of dwarf planet. It was a sad moment for many astronomers.
At the same time, it promoted the distant, deep-space object Eris and the big asteroid Ceres to dwarf planet. The IAU's action made Eris the largest known dwarf planet. It's larger than Pluto.
More at hXXp://www.spacetoday.org/SolSys/KuiperBelt/Quaoar.html