1. What Is a Planet? Pluto and Its Place in the Solar System Dr. Matthew Tiscareno, Cornell University

2. What’s in the Solar System? Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto Asteroids Moons Comets Kuiper Belt

3. Earth Is a Planet Big Round Orbits the Sun Has a moon Made of rock & metal Atmosphere & life

4. Terrestrial Planets Closest to the Sun Made of rock and metal Mercury, Venus, Earth, Mars Only a few moons

5. Gas Giant Planets Biggest planets Lots of moons Made of gas (hydrogen, helium) Jupiter, Saturn

6. Ice Giant Planets Medium-size Lots of moons Made of gas (hydrogen, helium) and also ice (methane, water, ammonia) Uranus, Neptune

7. Pluto By far the smallest –Even 7 moons are bigger All other small planets are close to Sun Irregular orbit What is Pluto like??

8. Asteroids More than 100,000 known Made of rock & metal A few are round, but all shapes and sizes Many have moons 1 Ceres243 Ida & Dactyl25143 Itokawa

9. Comets Made of ice (water, etc) Dive close to Sun, produces “coma” and “tail” Most appear to come from well beyond the planets Hale-BoppTempel 1Halley

10. Kuiper Belt Orbits outside of Neptune A belt of objects, like Asteroid Belt Source of many comets

11. Kuiper Belt Made of ice Some quite big Many have moons (some are truly binary) This is where Pluto belongs!

12. Eris Bigger than Pluto! Nicknamed “Xena” until official name was approved Forced issue of defining a planet

13. Where Are These KBOs? Orbits are similar to Pluto’s These really are Pluto’s brothers and sisters Note: 2003 UB313 is the same as Eris

14. What Is a Planet? What is a continent? –Why is Australia a continent, but Greenland is just an island? What is a mountain? –How big would Connecticut Hill have to be before we call it a mountain? Main answer: It’s the word we’re used to using, and we haven’t thought a whole lot about exactly what it means

15. What Is a Planet? International Astronomical Union (IAU) is the authority on such matters Definition approved August 24 To be a planet, Pluto must: –Orbit the Sun –Be round –Clear the neighborhood around its orbit Leaves us with 8 “planets” If only “round” and “orbit Sun”, 12 or more CHECK! UH-OH! CHECK!

16. How Were Planets Discovered? Some we’ve always known about –Mercury, Venus, Mars, Jupiter, Saturn Uranus discovered with telescope 1781 Four asteroids discovered 1801-1807 –Ceres, Pallas, Juno, Vesta –They were called planets! Fifth asteroid discovered 1845, soon many –Re-classified, no longer planets Neptune predicted, then discovered 1846

17. Pluto’s History Clyde Tombaugh took pictures of the sky in pairs, looked for things that moved Night after night, he blinked pics back and forth One day in 1930, he found Pluto! He was 24 yr old

18. Pluto’s History Scientists originally thought that Pluto –Was as big as Earth –Had a unique orbit beyond Neptune We now know that neither of those is true Charon discovered 1978 –We found Pluto’s small mass First KBO discovered 1992, now thousands known

19. A Closer Look at Pluto 1,423 miles across –About half the size of the U.S.A. Made of ice, with a rocky core Ice is water and methane, even nitrogen is frozen! Even best pics still pretty fuzzy

20. Pluto’s Moons Charon is half as big as Pluto –Orbit only 8x bigger –Closest thing we know of to a “double planet” Both Charon and Pluto always keep the same face towards each other –Compare to Earth’s moon –One half of Pluto never sees Charon, the other half sees it always in the same place

21. Pluto’s Moons Nix and Hydra Two new moons discovered this year

22. Pluto’s Orbit 248 years to orbit Sun Elliptical (not circular) –Sometimes is closer to Sun than Neptune –Happened 1979-1999 Inclined (out of plane) –Never actually crosses Neptune’s orbit Orbits sun exactly 2x when Neptune orbits 3x

23. New Horizons Launch 1/19/06 Jupiter flyby 2/28/07 Pluto flyby July 2015 Will go on to one or more KBOs

24. Conclusions Solar system is more than just the planets! Asteroids, comets, KBOs are important Since 1992, we have learned that many other objects (KBOs) are similar to Pluto Since 2005, we have learned that Pluto isn’t even the biggest KBO (Eris) Pluto is one of the biggest and most interesting KBOs Continue to study it, learn about how the solar system works