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.

Sun

Uranus

Neptune

gravitational pull

Distant Twins

• Uranus– 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– 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

• Mercury 0° tidal lock to sun

• Venus 177° reversed tidal lock to sun

• Earth 23°

• Mars 25°

• Jupiter 3°

• Saturn 27°

• Uranus98° How does one explain this?

• Neptune 30°

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