PTYS/ASTR 206Outer Worlds 4/19/07 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 ) Only “planet” not visited by a spacecraft –New Horizons, launched in Jan. 2006, will reach Pluto in Pluto can be seen with an amateur telescope, but it is not easy! –About 14 th magnitude 6 th magnitude is the limit for the naked eye Pluto is about 1600 times dimmer than this

PTYS/ASTR 206Outer Worlds 4/19/07 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

PTYS/ASTR 206Outer Worlds 4/19/07 Pluto: Basic Facts Avg. density –About 1900 kg/m 3 –Rock and ice Diameter –0.18 Earth Diameters –Not that well known (+/- 1%) Mass – Earth Masses –Also not well known, although the combined mass of Pluto and Charon is well known Tenuous atmosphere of N 2 that is probably not in equilibrium

PTYS/ASTR 206Outer Worlds 4/19/07 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”

PTYS/ASTR 206Outer Worlds 4/19/07 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.

PTYS/ASTR 206Outer Worlds 4/19/07 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)!

PTYS/ASTR 206Outer Worlds 4/19/07 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

PTYS/ASTR 206Outer Worlds 4/19/07 Eris

PTYS/ASTR 206Outer Worlds 4/19/07 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".