The Outer Planets

Uranus, Neptune and Pluto

Uranus

• About 4 times diameter of Earth• 3 billion km (1-3/4 billion miles) from Sun• 84 Years to Circle Sun• Rotates in 16 hours• Five large satellites (500-1500 km), ten

smaller ones• Nine narrow rings

Uranus

Uranus Seen From Beyond

Uranus Shows Little Detail

Oberon

Titania

Umbriel

Ariel

Miranda

Neptune• About 4 times diameter of Earth- a bit

smaller than Uranus• 4.5 billion km (2.8 billion miles) from Sun• 165 Years to Circle Sun• Rotates in 18 hours• One large satellite (2700 km), seven smaller

ones• Four narrow rings

Neptune: A Twin of Uranus?

No!

The Great Dark Spot

Neptune and Triton

Neptune’s Rings

Triton

Pluto• At 2300 km, Pluto is the smallest planet• Has most elliptical orbit: ranges from 4.4 to

7.4 billion km from Sun (2.8-4.5 billion miles)

• Actually crosses orbit of Neptune; closer to Sun than Neptune until 2009

• Orbits in 248 years, 1.5 times Neptune• Because of orbital tilt and resonance, cannot

collide with Neptune

Pluto and Charon• Pluto’s moon Charon is almost half as big

as Pluto (1100 km)• Orbits only 20,000 km away• Pluto and Charon always keep same face to

each other (rotation locked)• Pluto rotates, and Charon revolves, in 6.4

days

Pluto and Charon

Why We Have to Go There

Plutinos and the Kuiper Belt• The Kuiper Belt is an outer ice asteroid belt,

probably the source of most comets• Over 50 are now known orbiting beyond

Neptune, some beyond Pluto• Pluto is probably just the biggest of these

objects (not really a planet?)• Several dozen have periods similar to

Pluto’s - 250 years - and have been dubbed “plutinos.”