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

• Closest to the Sun

• Made of rock and metal

Mercury, Venus, Earth, Mars

• Only a few moons

Gas Giant Planets

• Biggest planets

• Lots of moons

• Made of gas (hydrogen, helium)

Jupiter, Saturn

Ice Giant Planets

• Medium-size

• Lots of moons

• Made of gas (hydrogen, helium) and also ice (methane, water, ammonia)

Uranus, Neptune

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, butall shapes and sizes

• Many have moons

1 Ceres 243 Ida & Dactyl25143 Itokawa

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 Tempel 1Halley

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 UB313 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– 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!

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 picsback 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 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

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 2x when Neptune orbits 3x

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