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 from Voyager
The Rings of Uranus
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
Pluto Has Three Moons
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
• Hundreds are now known orbiting beyond
Neptune, some beyond Pluto
• Pluto is just the biggest of these objects
and not really a planet
• Several dozen have periods similar to
Pluto’s - 250 years - and have been dubbed
“plutinos.”