2 Pluto 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
3 Pluto No spacecraft has yet visited Pluto. Adaptive optics have imaged part of Pluto’s surface.
4 Ice 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.
5 Charon 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.
6 Classifying 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.
7 Kuiper 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
8 Kuiper 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.
9 Planets 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