* Although it is not really accurate to say that Uranus, Neptune, and Pluto were discovered by accident, fortuitous circumstances did lead to the discovery of each of the outer planets.
* Uranus and Neptune are so similar in mass, diameter, and rotation
rate that they are often called "twin planets." Their
axial tilts, however, are very different. Neptune's is similar
to the Earth's, but Uranus's is so greatly tilted that its pole
lies almost in its orbital plane.
* Uranus is a nearly featureless blue sphere, whereas Neptune
shows banded structure, bright clouds, and storm systems. The
difference between the appearances of the two planets is caused
by the lack of convection in the atmosphere of Uranus and the
presence of convection in Neptune's atmosphere.
* The interiors of Uranus and Neptune probably consist of rocky
cores surrounded by thick shells of a fluid mixture of rock and
ice. On top of that are relatively thin envelopes rich in hydrogen
and helium. The two planets apparently were less effective than
Jupiter and Saturn in gathering thick envelopes of hydrogen and
helium while they formed.
* Neptune is significantly self-luminous, whereas Uranus is not.
It seems likely that Uranus, like Neptune, has a hot interior
but that heat is unable to escape easily.
* The magnetic fields of Uranus and Neptune are about 100 times
stronger than the Earth's magnetic field but about 100 times weaker
than Jupiter's. Moreover, Uranus's and Neptune's magnetic fields
are tilted with respect to their rotation axes, and offset from
the centers of the planets. The current theory is that the magnetic
fields are generated by currents in the icy shells of Uranus and
Neptune rather than in their cores.
* The ring system of Uranus consists of thin rings of large particles
separated by regions filled with fine dust. Neptune has two thin
rings and a broad inner ring, all made of very small particles.
The arcs in Neptune's outer ring may be caused by the gravitational
influences of Neptune's satellites.
* Pluto and its satellite, Charon, are locked in synchronous orbits
with periods of 6.4 Earth days. The orbital period and distance
of Charon made it possible to show that Pluto is smaller and less
massive than our Moon. The density of Pluto suggests that it is
made of a combination of ice and rock.
* It was once thought that Pluto was an escaped satellite of Neptune.
Now, however, it seems unlikely that Pluto could have been ejected
from Neptune's satellite system without separating it from Charon
and giving Pluto escape velocity from the solar system.
* Despite a number of searches and attempts to find gravitational
perturbations, no solar system planets beyond Pluto have been
found. There may be additional planets, but if they are as dim
and as small as Pluto, they may escape detection for a long time.
celestial mechanics, perturbation, stellar occultation
1. How did William Herschel know that Uranus is a planet and not a star? he saw it move among the stars from night to night
2. Which of the following planets occasionally has its poles pointed
almost directly at the sun? Uranus
3. What is remarkable about the magnetic field of Uranus? it is
tipped at a large angle with respect to the axis of rotation
4. What is remarkable about the rotation of Uranus? its rotation
axis lies in its orbital plane
5. Which of the following statements is a good description of
the appearance of Uranus in the Voyager images.nearly featureless
greenish ball
6. Which of the following planets was discovered because of its
gravitational effect on the orbits of other planets? Neptune
7. What is unusual about the obits of Neptune and Pluto? Pluto
is sometimes nearer the sun than is Neptune
8. What property of Pluto could be determined only after the discovery
of its satellite, Charon? its mass
9. What is meant by the statement that Pluto's rotation is synchronous?
it always keeps the same face turned to its satellite
10. How are the Pioneer and Voyager spacecraft most likely to
be used to detect trans-Plutonian planets? because their orbits
may be altered by them