Pluto

1. Pluto • Most distant “planet” • Most of the time, but not all of the time! Its orbit can bring it inside of Neptune’s (as it did from 1979-1999) • Only “planet” not visited by a spacecraft • New Horizons, launched in Jan. 2006, will reach Pluto in 2015. • Pluto can be seen with an amateur telescope, but it is not easy! • About 14th magnitude • 6th magnitude is the limit for the naked eye • Pluto is about 1600 times dimmer than this Outer Worlds 4/19/07

2. Pluto: Basic Facts • Avg. distance to the Sun • 39.5 AU • Orbital Period • About 250 years • Retrograde rotation • Like Venus and Uranus • Eccentricity • 0.25 • larger than all of the planets Outer Worlds 4/19/07

3. Pluto: Basic Facts • Avg. density • About 1900 kg/m3 • Rock and ice • Diameter • 0.18 Earth Diameters • Not that well known (+/- 1%) • Mass • 0.0021 Earth Masses • Also not well known, although the combined mass of Pluto and Charon is well known • Tenuous atmosphere of N2 that is probably not in equilibrium Outer Worlds 4/19/07

4. Neptune and Pluto are in a 3:2 orbital resonance • Pluto’s orbital period is 1.5 times Neptune’s • Neptune is about 164 years • Pluto is about 249 years • This is a stable configuration and they will never crash into each other • Pluto is like other objects in this sense – comprising a class of objects called “Plutino’s” Outer Worlds 4/19/07

5. Charon: Pluto’s Moon • Discovered in 1978 by Jim Christy. • Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. • Charon is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). • Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon. Outer Worlds 4/19/07

6. Pluto and Charon are in an unusual resonance. • Charon has an orbital period equal to both its rotational period AND Pluto’s orbital period • From one side of Pluto, Charon just sits in the same place in the sky (never sets, never rises)! Outer Worlds 4/19/07

7. Is Pluto a planet ? • Since the discovery of thousands of objects orbiting the Sun, called Edgeworth-Kuiper objects (or Kuiper-belt objects, KBOs), it has been suggested that Pluto is simply one of these and is not a planet • Many KBO’s are in orbital resonances with Neptune (3:2 resonance  “Plutino”) • This is further complicated by the fact that the recently discovered Eris is larger than Pluto. • In 2006, Pluto was downgraded to a new class of objects called “dwarf planets” • But the debate rages on, and in all likelihood, Pluto will regain planet status at some point Outer Worlds 4/19/07

8. Eris Outer Worlds 4/19/07

9. The IAU’s decision in August 2006 • The IAU...resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way: (1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. • (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. • (3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". Outer Worlds 4/19/07