What Is a Planet Pluto and Its Place

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

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

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

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

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

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

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

Asteroids • More than 100, 000 known • Made of rock & metal • A few are round, but all shapes and sizes • Many have moons 1 Ceres 25143 Itokawa 243 Ida & Dactyl

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

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

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

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

Where Are These KBOs? • Orbits are similar to Pluto’s • These really are Pluto’s brothers and sisters • Note: 2003 UB 313 is the same as Eris

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

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 CHECK! – Be round CHECK! – Clear the neighborhood around its orbit UH-OH! • Leaves us with 8 “planets” • If only “round” and “orbit Sun”, 12 or more

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

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

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

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

Pluto’s Moons • Charon is half as big as Pluto – Orbit only 8 x 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

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

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 2 x when Neptune orbits 3 x

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

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