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Ice Giants
Amateur Discovery
• Classical astronomy
observed 5 planets.
– Uranus slow
– Looked like a star
• Uranus was discovered in
1781 by William
Herschel.
– Confused for a comet
– 84 year period
Mathematical Discovery
• Uranus didn’t quite match Kepler’s laws.
– 60 years of its 84 year revolution
• Mathematicians calculated the gravitational pull needed by
an unknown planet, and then found it - Neptune in 1849.
Uranus
gravitational pull
Sun
Neptune
Distant Twins
• Uranus
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Seventh planet
20 AU from Sun
4 times Earth size
15 times Earth mass
84 year revolution
17 hour day
Water and iron core
Temperature -190 °C
• Neptune
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Eighth planet
30 AU from Sun
3.8 times Earth size
17 times Earth mass
165 year revolution
16 hour day
Water and iron core
Temperature -200 °C
Axial Tilt
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Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
0°
177°
23°
25°
3°
27°
98°
30°
tidal lock to sun
reversed tidal lock to sun
How does one explain this?
Major Collision
• To be tilted on the side Uranus must have had a collision.
• The colliding object must have been huge to deflect a
planet the size of Uranus (14 times the mass of Earth).
before collision
after collision
Uranian Clouds
• Uranus has the brightest
clouds in the solar system.
– Different on the north
and south sides
– Long time to see both
sides
• Clouds have “ices”.
– methane, ammonia, and
water
Neptunian Winds
• Methane makes clouds blue.
• There are massive clouds in the
atmosphere and it has the fastest
winds of any planet.
– Great Dark Spot