Uranus and Neptune:
1)How did Uranus and Neptune come to be discovered? William Hershel discovered
Uranus by accident (in 1757). Uranus did not quite track the way astronomers thought—
it seemed to disobey Newton’s laws of motion. Then a few astronomers suggested that
this motion could be caused by gravitational effects from another planet. They used
Newton’s laws to estimate where that planet should be and then found it and named it
Neptune.
2)What gives Uranus its distinctive bluish-green color? Methane in its atmosphere.
3)Why are the clouds on Neptune much more visible than those on Uranus? Because
Neptune has internal heating—astronomers believe it is still contracting gravitationally-and consequently has a more dynamic atmosphere. This gives it more clouds and more
weather.
4)Are Uranus and Neptune merely smaller versions of Jupiter and Saturn? No. Jupiter
and Saturn have similar composition as the Sun, but Uranus and Neptune have a different
composition. They have higher percentages of rock and they contain large areas of water
and ammonia.
5)Why are the rings of Neptune and Uranus so difficult to see? Because they are very
thin (less than 10 miles wide) and they are as dark as lumps of coal.
6) Do the Moons of Uranus show any signs of geologic activity? Some of the moons of
Uranus have regions with cracks, ice-lava flows, and no craters. These are all indications
that there was geologic heating in their past. Astronomers believe some of the moons
were tidally heated.
7)Could astronomers in antiquity have seen Uranus? Yes, Uranus is bright enough to be
seen with the naked eye when it is at opposition. Why was it not recognized as a planet?
It is very faint and it moves very slowly across the celestial sphere (about 4 degrees per
year).
8)Why was the discovery of Neptune rated as a great triumph of science while those of
Uranus and Pluto were not? Because astronomers predicted the existence of Neptune and
its location after applying Newton’s laws to the motion of Uranus. Both Uranus and
Pluto were found more or less by accident.
9)Why is it so difficult to see features in the atmosphere of Uranus? Because Uranus has
little internal heating so it doesn’t have much atmospheric activity.
10)Describe the seasons on Uranus. Since Uranus is spinning on its side it has the ost
severe seasons. In the summer on the north pole, the southern hemisphere gets no light.
The opposite happens in winter on the north pole. Also, since the orbital period is about
84 years, the seasons last 21 years each.
11)Why are Uranus and Neptune distinctly blue-green in color while Jupiter and Saturn
are not? Because Uranus and Neptune have significant amounts of methane in their
atmosphere; this is what gives them the color. Jupiter and Saturn do not have much
methane in their atmospheres.
12) Briefly describe the evidence that Uranus was struck by a large planet-like object in
its past. Uranus’ rotational axis is tilted 98 degrees and its magnetic field is out of line
with its rotational axis. Both of these could indicate a violent collision in its past.
13) Why do astronomers think that Triton was captured by Neptune? First it is in a
retrograde orbit—it is the only large moon in the solar system that does this. Second, it is
orbiting Neptune on a path that is tilted at 23 degrees to the plane of Neptune’s equator.
It is the only large moon in the solar system that is doing this. These are both strong
indicators that Triton was captured.