Uranus and Neptune
- The outer two ``Gas Giants''
- Uranus was discovered by accident in 1781 -- making
it the first planet discovered (and named)
in the modern era.
- Neptune's position was predicted from the
shape of the orbit of Uranus by applying Newton's Laws. The story of its discovery
is one of international scandal and intrigue (at least scientifically).
- Both planets differ from Jupiter
and Saturn in their interior structure.
- Uranus and Neptune were not able to collect as much
hydrogen and helium gas from the solar nebula
- They thus have a higher density than Jupiter and Saturn
- They consist of water ice/rock cores covered by
an extremely thick hydrogen atmosphere.
- The two objects are often referred to as "ice giants" to
reflect their substantial ice cores.
- These two planets may have been "starved" of hydrogen
due to their slow formation. By the time they were large enough
to accumulate hydrogen the hydrogen had dissipated. Alternatively,
little hydrogen might have been available at their distance
in the nebula.
- The Blue-Green color arises from methane in the atmosphere (which is predominantly made of hydrogen and helium).
- Methane preferentially absorbs red light. Only the
blue light gets reflected.
- The clouds (for example ammonia) so common in views of Jupiter and Saturn and buried so deeply in the atmospheres of Uranus and Neptune that we do not see them from the outside.
- Frozen methane crystals produce the highest white clouds in the atmospheres of these planets.
- Like Jupiter, both objects have thin rings.
- Uranus has a rotation axis inclined more than 90 degrees to its orbital plane - it rotates "on its side."
- Could this excessive tilt be the result of a large impact which occurred during the final stages of the formation of Uranus?
- Neptune has one large satellite, Triton.
- Triton orbits Neptune backwards (opposite Neptune's rotation)!
- It has a thin nitrogen atmosphere
- Nitrogen frost forms on the surface near the poles during winter.
- It shows few craters implying recent resurfacing. The heat source is a mystery. Tidal heating could come into play if Triton was captured in its odd orbit in the last billion years.
- Small icy satellites orbit both objects. The smaller satellites
are particularly interesting.
Revised April 24, 2002