2Pluto Neptune’s orbit also didn’t quite match Kepler’s laws. In the late 1800’s Lowell predicted a ninth planet.It was discovered in 1929 as a faint star that moved slightly each day.Pluto’s orbit is sometimes inside Neptune’s.NeptuneSunPluto
3Pluto No spacecraft has yet visited Pluto. Adaptive optics have imaged part of Pluto’s surface.
4Ice Ball Pluto is small (seven moons are larger). Pluto has low density.60% stony core.40% frozen gas: nitrogen, carbon monoxide, methane, and water.Some ice can vaporize when Pluto is at its closest point to the sun. This forms a thin atmosphere.
5Charon Pluto’s moon Charon is almost as big as Pluto. The pair can be viewed as a double planet and they are tidally locked to each other.Charon has more water ice on the surface than Pluto.
6Classifying PlutoThe inner planets (Mercury, Venus, Earth, Mars) are rocky.The outer planets (Jupiter, Saturn, Uranus, Neptune) are gas giants.Pluto doesn’t fit in; it’s icy mixed with rock and carbon.
7Kuiper BeltGerard Kuiper suggested in the 1940’s that there was a ring of icy objects from the early solar system beyond the orbit of Pluto.Confirmed in 1992Many times the mass of the asteroid beltPlutoKuiper BeltNeptune’s orbit
8Kuiper Belt ObjectsHundreds of Kuiper Belt Objects (KBOs) have been found.Sizes from 50 to 2400 km across.Eris is the largest KBOAstronomers estimate 100,000 KBOs larger than 100 km.
9Planets RedefinedThe discovery of Eris forced astronomers to create a better definition of a planet (2006).In orbit around the SunSufficient mass to assume a nearly round shapeCleared the neighborhood around its orbitA dwarf planet was defined as a new category.Not cleared the neighborhood around its orbitNot a satellite of a planet