Outer Planets, Page 6: [Previous Page] [Next Page] [Class Home Page] Textbook pages 228 - 240

The Outer Solar System: Things that go bump in the night..

In studying the three outer planets, we came across several strange characteristics, not entirely predicted from our model of solar system formation. These include:
• Uranus' axis is almost parallel to the ecliptic (it's poles can face the Sun!).
• Triton's (Neptune's largest moon) orbit is retrograde (backwards).
• Triton and Pluto have extremely similar properties.
• Pluto's orbit crosses within Neptune's.
• Pluto is locked in a 3:2 resonance with Neptune.

The History of Uranus
Uranus probably formed about where it is found today, though it was not able to grow as large as Jupiter and Saturn because the outer solar nebula had less material at such distances. Uranus was unable to capture as much H and He. Uranus is only 15x Earth's mass.

It seems likely that Uranus experienced a large collision, late in its formation, which altered its spin axis, nearly lining it up with the ecliptic (the plane of the Sun-planets). Uranus seems to have a lower mass than we'd expect (lower even than its further sibling Neptune) which may have been partially lost during the collision. Moreover, such a collision may have stirred up its central core, quickly releasing Uranus's internal heat, leaving it the cold world it is today.

The History of Neptune
Neptune developed like the other gas giants, but like Uranus, it was unable to gain mass quickly enough and therefore not able to become massive enough to capture as much H and He as Jupiter and Saturn did, leaving it a mere 17x Earth Mass. Perhaps because of its slightly higher mass (and density), Neptune does have a source of internal heat energy, which drives its atmospheric winds and storms.

While Neptune itself does not show any unusual characteristics, its moons suggest there have been cataclysmic encounters in the past, in particular Nereid, with its highly eccentric orbit, and Triton, currently in a retrograde (moving backwards) orbit.

The Possible Shared History of Neptune and Pluto
The unusual nature of the orbits of Pluto (crossing Neptune's orbit) and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have been a satellite of Neptune's. This theory seemed unlikely once Charon was discovered (it would have been difficult for Pluto to have maintained its moon if ejected from Neptune's orbit). A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected to the outer solar system. Like the Earth's Moon, Charon may have formed as a result of a collision between Pluto and another body.