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Transcript
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 – (1979-1999) 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