1. Dwarf Planets

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.
Neptune
Sun
Pluto

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 Pluto
The 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 Belt
Gerard 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 1992
Many times the mass of the asteroid belt
Pluto
Kuiper Belt
Neptune’s orbit

8. Kuiper Belt Objects
Hundreds of Kuiper Belt Objects (KBOs) have been found.
Sizes from 50 to 2400 km across.
Eris is the largest KBO
Astronomers estimate 100,000 KBOs larger than 100 km.

9. Planets Redefined
The discovery of Eris forced astronomers to create a better definition of a planet (2006).
In orbit around the Sun
Sufficient mass to assume a nearly round shape
Cleared the neighborhood around its orbit
A dwarf planet was defined as a new category.
Not cleared the neighborhood around its orbit
Not a satellite of a planet

10. History of Planets
Scientific American