1. Pluto?
Pluto is still included in your book as a planet, but has recently been demoted to a dwarf planet
We will discuss some of the characteristics of Pluto today and compare them to the other planets
With your current knowledge of the planets, you can also judge whether Pluto is 'worthy' of planetary status

2. Properties of Pluto Semimajor Axis 39.5 A.U. Orbital Eccentricity
0.249
Orbital Period
248 years
Rotational Period
-6.4 days
Mass
1.27 x 1022 kg
x Earth
Radius
1137 km
0.18 x Earth
Average Density
2.06 g/cm3
0.374 x Earth
Surface Gravity
0.66 m/s2
0.067 x Earth
Surface Temperature K
~300K for Earth
Atmosphere
Methane?

3. Discovery of Pluto
As was the case with Neptune, it was believed that an unseen planet was still tugging on the others
The lead searcher for this undiscovered planet was Percival Lowell, who spent over a decade looking _
Surprisingly, Pluto is too small to have any gravitational effects on Uranus or Neptune, and was found essentially by chance

4. Pluto's Orbit _
At perihelion, Pluto is only A.U. from the Sun, while at aphelion, Pluto is A.U. away
This orbit actually brings Pluto inside of Neptune's orbit for a period of time
The two will never collide, however, because the planets share a 3:2 resonance
Pluto orbits the Sun 3 times in the time it take for 2 revolutions of Neptune

5. Observations of Pluto
Ground-based image
Pluto's huge distance from the Sun and its very small size make it hard to observe _
Ground-based measurements of the Pluto-Charon system require the best telescopes on Earth
Hubble Space Telescope has provided the most detailed images of Pluto and Charon to date
HST

6. Pluto and Charon Pluto and Charon are very similar in size
Pluto diameter ~ 2270 km
Charon diameter ~ 1300 km
The two bodies are tidally locked, so that the same side constantly faces the other body _
Recently, Pluto and Charon's orbit was aligned with Earth so that many eclipses between the two were visible

7. More Moons for Pluto
In 2005, astronomers using the Hubble Space Telescope discovered two more moons in orbit around Pluto
The two small bodies are named Nix and Hydra _

8. Pluto's Surface
Direct measurements of Pluto's surface have not been made, but it assumed to be an ice/rock mixture
Spectroscopic studies have revealed the presence of methane _
The 'image' at right is an HST model based on multiple observations

9. Visiting Pluto
The first probe to visit Pluto and Charon will be the New Horizons spacecraft
New Horizons was launched in January of this year _
This craft will provide the first real images of Pluto and Charon's surface

10. What is Pluto?
Pluto is far from the Sun, has a very odd orbit, and exhibits characteristic more like a moon than a planet
A combination of factors has led astronomers to believe that Pluto may be a member of the Kuiper belt
No matter what Pluto actually is, its title as a planet has been stripped by the International Astronomical Union ?

11. What Defines a Planet? _ _

12. The Troublemakers
Note: All images are artist conceptions

13. The IAU Definition
The definition of a planet as of August 2006 is: _
A dwarf planet fulfills conditions 1&2. All other objects are small Solar System bodies.