Asteroids, Comets, and Pluto (Most of) The Rest of the Solar System

Is there another planet? After Uranus’ discovery, many astronomers thought there must be a planet between Mars and Jupiter New object discovered b/w Mars and Jupiter (1801) Second object discovered b/w Mars and Jupiter (1802) Third object discovered (1804) …

The Asteroid Belt ~50,000 asteroids discovered Most orbit between Mars and Jupiter –Why wouldn’t they combine to form a planet? Hint: Saturn’s rings

Asteroids Range from 500 miles across to “sand grains” All add up to less than half the Moon’s mass / size Made of stone, metal, or both

Comets Seen occasionally throughout history Halley noticed that one could be seen every 76y –It last appeared in 1986; when will it be back? Originate very, very far away from the Sun –Why then do some comets have short periods?

Comets Cont. Small nucleus - “Dirty Snowball” Tail – material vaporized by Sun’s heat –If material is vaporized each time a comet passes close to the Sun, what happens to the comet after a long time?

Discovery of Pluto Search for “Planet X” b/c Discovered on photos (1930) Large moon, Charon, discovered (1978) –Charon is half Pluto’s size! Diameter = 1,440 mi Density = 1.8 g/cm3 –Likely mixture of ice and rock

Pluto vs. Outer Planets Much smaller than the outer planets Pluto has no hydrogen or helium “Gas Giant” – except no gas or giant

Pluto vs. Inner Planets Density = 1.8 g/cm3 vs. 5 g/cm3 –Must have a different composition; more ice, less rock Size = less than ½ diameter of Mercury Distance from Sun: Why’s an inner planet out there?

Pluto vs. All Planets Orbit tilted 17° to ours Very eccentric orbit –( ) Pluto was closer to the Sun than Neptune! Because of its size, composition, and unusual orbit, Pluto has never really fit in as a planet

Is there more beyond Neptune? Computer simulations of Nebular Theory predicted a disk of debris beyond Neptune New object discovered beyond Neptune (1992) Second object discovered beyond Neptune (1993) Third object discovered… –Does this story sound vaguely familiar?

The Kuiper Belt Like asteroid belt, contains many small bodies made of ice and rock Several large bodies discovered: –Quaoar (2002) –Sedna (2003) –Eris (“Xena”) (2005) Eris is larger than Pluto! –Why might that be significant?

Dwarf Planets Either Eris is a planet, or Pluto isn’t! IAU voted to define planets as an object that has cleared the neighborhood around its orbit (Aug 2006) –Why would this disqualify Pluto? Any object that has not cleared its neighborhood but is large enough to be round is a Dwarf planet –Currently, 3 objects; likely, more will follow soon

The “New” Solar System