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What Is a Planet?
Pluto and Its Place in the Solar System
Dr. Matthew Tiscareno, Cornell University
What’s in the Solar System?
Kuiper Belt
Earth Is a Planet
• Big
• Round
• Orbits the
• 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
Gas Giant Planets
• Biggest
• Lots of
• Made
of gas
Jupiter, Saturn
Ice Giant Planets
Uranus, Neptune
• Medium-size
• Lots of moons
• Made of gas
helium) and also
ice (methane,
water, ammonia)
• By far the smallest
– Even 7 moons are
• All other small planets
are close to Sun
• Irregular orbit
What is Pluto like??
• 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
• Made of ice (water, etc)
• Dive close to Sun, produces
“coma” and “tail”
• Most appear to come from
well beyond the planets
Tempel 1
Kuiper Belt
• Orbits outside of
• A belt of objects,
like Asteroid Belt
• Source of many
Kuiper Belt
• Made of ice
• Some quite big
• Many have moons
(some are
truly binary)
This is where
Pluto belongs!
• Bigger than Pluto!
• Nicknamed “Xena” until official name was
• 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 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
• 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
• Two new
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
• Pluto flyby
July 2015
• Will go on to
one or more
• 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