Part 7: The Outermost Planets
John P. Pratt
Uranus & Neptune
- Uranus was discovered in 1781 accidentally by William Herschel when he was making a star map.
- Uranus had some irregular motions which indicated another planet farther out.
- Neptune was found in 1846 within an hour of beginning a search where Leverrier predicted.
- Both planets are blue, because of methane gas.
- Both have thick atmospheres of hydrogen and helium gas.
- Uranus has no cloud bands; Neptune has cloud belts and a Great Dark Spot.
- Uranus is featureless because it is cold, Neptune is producing heat, like Jupiter.
- Both are believed to have rocky cores.
Uranus Weird Axis of Rotation
- Its axis is lying nearly in the plane of the ecliptic.
- Thus it has one season with sun on one pole, then one with sun above equator, then other pole.
- The moons revolve around the equator, so they have the same experience.
- Both planets have ring systems. Those of Uranus were discovered during an occultation of a star.
- Uranus main ring is extremely narrow, kept so thin by two shepherd satellites.
- Neptune has two "arc-rings," where only segments of the ring are there.
- Five moons were known before Voyager, now the total is fifteen.
- All of the new discoveries are near to the planet, near the ring system.
- All are named after characters in Shakespeare's plays.
- Miranda, the smallest of the five main moons, is the most fractured and resurfaced.
- It might have been smashed by an impact and reassembled.
- Two moons were known before Voyager, now the total is seven.
- The five new satellites are all very close to the planet, near the ring system.
Triton: Neptune's Erupting Moon
- Triton, the largest moon, has a retrograde orbit, inclined at a high angle to Neptune's equator.
- It is a creamy colored, icy moon.
- It has very few craters, and must be very geologically active to resurface it.
- It has active, erupting geysers or volcanoes, with dark smoke rising from them.
- It has a circular orbit, suggesting that it has been tidally circularized which could have heated it.
- It was predicted because of perturbations of Neptune's orbit.
- It was discovered near where it had been predicted.
- When its mass was discovered to be less than our moon, we realized it's discovery had just been luck.
- It has a moon half of its own diameter.
Maybe not a Planet. The following are reasons not to classify it as a planet.
- It is much smaller than any other planet.
- It doesn't fit the trend of the giant planets.
- It doesn't follow Bode's rule, but actually comes inside of Neptune's orbit.
- Several other bodies that size are known to orbit the sun, such as Chiron.
- Thus, many think it should be classified as a double asteroid.