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

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 0.0021 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 40 - 60 K ~300K for Earth Atmosphere Methane?

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

Pluto's Orbit _ At perihelion, Pluto is only 29.66 A.U. from the Sun, while at aphelion, Pluto is 49.31 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

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

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

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 _

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

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

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 ?

What Defines a Planet? _ _

The Troublemakers Note: All images are artist conceptions

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.