The outer planets Uranus, Neptune, and Pluto were unknown to ancient astronomers. Uranus was discovered in the eighteenth century, by chance. Neptune was discovered after mathematical calculations of Uranus's slightly non-Keplerian orbit revealed the presence of an eighth planet.
At opposition, Uranus is barely visible to the unaided eye. It appears as a pale green disk through a telescope. Neptune cannot be seen with the naked eye, but a telescope shows it as a tiny bluish disk. Today we know the giant planets Uranus and Neptune mainly through data taken by Voyager 2. Small, remote Pluto has not been visited by a spacecraft, and our knowledge of it stems from painstaking Earth-based observations from Earth and the Hubble Space Telescope.
The masses of the outer planets are determined from measurements of their orbiting moons. The radii of Uranus and Neptune were relatively poorly known until the Voyager 2 flybys in the 1980s. Uranus and Neptune have similar bulk properties; their densities imply large, rocky cores making up a greater fraction of the planets' masses than in either Jupiter or Saturn.
Surface features are barely discernible on Uranus, but computer-enhanced images from Voyager 2 revealed atmospheric clouds and flow patterns moving beneath Uranus's haze. Neptune, although farther away from us, has atmospheric features that are clearer because of warmer temperatures and less haze. The Great Dark Spot on Neptune had many similarities to Jupiter's Red Spot. It disappeared in 1994.
For unknown reasons, Uranus's spin axis lies nearly in the ecliptic plane, leading to extreme seasonal variations in solar heating on the planet as it orbits the Sun.
Unlike the other jovian planets, Uranus has no excess heat emission. The source of Neptune's excess energy, like that of Jupiter's, is most likely heat left over from the planet's formation.
Both Uranus and Neptune have substantial magnetospheres. Voyager 2 discovered that the magnetic fields of both planets are tilted at large angles to the planets' rotation axes. The reason for this is not known.
All but two of Uranus's moons revolve in the planet's equatorial plane, almost perpendicular to the ecliptic, in circular, synchronous orbits. Like the moons of Saturn, the medium-sized moons of Uranus are made up predominantly of rock and water ice. Many of them are heavily cratered and in some cases must have come close to being destroyed by the meteoritic impacts whose craters we now see. The strange moon Miranda has geological features that suggest repeated violent impacts in the past.
Neptune's moon Triton has a fractured surface of water ice and a thin atmosphere of nitrogen, probably produced by nitrogen "geysers" on its surface. Triton is the only large moon in the solar system to have a retrograde orbit around its parent planet. This orbit is unstable and will eventually cause Triton to be torn apart by Neptune's gravity.
Uranus has a series of dark, narrow rings, first detected from Earth by stellar occultation —their obscuration of the light received from background stars. Shepherd satellites are responsible for the rings' thinness. Neptune has three narrow rings like Uranus's and one broad ring, like Jupiter's. They were discovered by Voyager 2. The dark coloration of both the rings and the moons of the outer giant planets may be due to radiation darkening, whereby exposure to solar high-energy radiation slowly causes a dark hydrocarbon layer to build up on a body's icy surface.
Pluto was discovered in the twentieth century after a laborious search for a planet that was supposedly affecting Uranus's orbital motion. We now know that Pluto is far too small to have any detectable influence on Uranus's path.
Pluto has a moon, Charon, whose mass is about one-sixth that of Pluto itself. Studies of Charon's orbit around Pluto have allowed the masses and radii of both bodies to be accurately determined. Pluto is too small for a terrestrial planet. Its properties are far more moonlike than planetlike. Most astronomers believe that it is probably the largest (or nearest) member of a class of icy asteroids found in the outer solar system.
1. Uranus was discovered by Galileo. (Hint)
2. After the discovery of Uranus, astronomers started looking for other planets and quickly discovered Neptune. (Hint)
3. Since its discovery Uranus has completed just two and a half orbits of the Sun.
4. Neptune has a smaller radius than Uranus and therefore a smaller mass and density. (Hint)
5. The rotation rates of Uranus and Neptune are almost identical. (Hint)
6. During the northern summer of Uranus, an observer near the north pole would observe the Sun high and almost stationary in the sky. (Hint)
7. Both Uranus and Neptune have a layer of metallic hydrogen surrounding their central cores. (Hint)
8. Relative to the size of the planet, the rocky cores of Uranus and Neptune are larger than the cores of Jupiter and Saturn. (Hint)
9. The system of moons around Uranus have orbits that share the tilt of the planet. (Hint)
10. Uranus has no large moons. (Hint)
11. The surfaces of the largest Uranian moons are darker than similar moons of Saturn. (Hint)
12. Triton's surface has a marked lack of cratering, indicating significant amounts of surface activity. (Hint)
13. Pluto's moon, Charon, is more than half the size of Pluto. (Hint)
14. Pluto is larger than Earth's Moon. (Hint)
15. Astronomers have firm evidence that Pluto is an escaped moon of Neptune. (Hint)
1. Uranus is about _____ more distant from the Sun than is Saturn.
2. Uranus's rotation axis is almost _____ to the ecliptic plane. (Hint)
3. Whereas the abundance of hydrogen and helium for the jovian planets remains more or less constant among the four planets, the relative abundance of _____ increases with increasing distance from the Sun. (Hint)
4. The planet _____ is blue-green in color, and virtually featureless. (Hint)
5. The planet _____ is dark blue in color, with white cirrus clouds and visible storm systems. (Hint)
6. The _____ of both Uranus and Neptune are highly tilted relative to their rotation axis and significantly offset from the planets' centers. (Hint)
7. The Uranian moon that shows the greatest amount of geological activity and disruption over time is _____. (Hint)
8. Voyager 2 discovered six of the _____ known moons of Neptune. (Hint)
9. Triton's orbit is unusual because it is _____. (Hint)
10. Nereid's orbit is unusual because it has a high orbital _____. (Hint)
11. Like only one other moon in the solar system, Triton has an _____. (Hint)
12. Nitrogen geysers are found on _____. (Hint)
13. The orbit of Pluto is locked into a 3:2 resonance with _____. (Hint)
14. Overall, Pluto is most similar to which object in the solar system? _____. (Hint)
15. The radii of Pluto and Charon were determined accurately by observing a fortuitous series of _____during the late 1980s. (Hint)
1. How was Uranus discovered? (Hint)
2. Why did astronomers suspect an eighth planet beyond Uranus? (Hint)
3. What is unusual about the rotation of Uranus? (Hint)
4. What is responsible for the colors of Uranus and Neptune? (Hint)
5. How are the interiors of Uranus and Neptune thought to differ from those of Jupiter and Saturn? (Hint)
6. What is odd about the magnetic fields of Uranus and Neptune? (Hint)
7. What is unique about Miranda? Give a possible explanation for this uniqueness.
8. How does Neptune's moon system differ from those of the other jovian worlds? What do these differences suggest about the origin of the moon system? (Hint)
9. What is the predicted fate of Triton? (Hint)
10. The rings of Uranus are dark, narrow, and widely spaced. Which of these properties makes them different from the rings of Saturn? (Hint)
11. How do the rings of Neptune differ from those of Uranus and Saturn? (Hint)
12. How was Pluto discovered? (Hint)
13. How were the mass and radius of Pluto determined? (Hint)
14. In what respect is Pluto more like a moon than a jovian or terrestrial planet?
15. Why was the discovery of Uranus in 1781 so surprising? Might there be similar surprises in store for today's astronomers?
1. Calculate the time between successive closest approaches of (a) Neptune and Uranus (b) Pluto and Neptune. Compare your answers with Neptune's sidereal orbital period.
2. What is the gravitational force exerted on Uranus by Neptune, at closest approach? Compare your answer with the Sun's gravitational force on Uranus.
3. What is the angular diameter of the Sun, as seen from Neptune? Compare it with the angular diameter of Triton. Would you expect solar eclipses to be common on Neptune? (Hint)
4. From Wien's law, at what wavelength does Pluto's thermal emission peak? In what part of the electromagnetic spectrum does this lie? (See More Precisely 3-2 and Figure 3.11.)
5. Assuming a spherical shape and a uniform density of 2000 kg/m3, calculate how small an icy moon of one of the outer planets would have to be before a fastball pitched at 40 m/s (about 90 mph) could escape. (Hint)
6. Add up the masses of all the moons of Uranus, Neptune, and Pluto. (Neglect the masses of the small moons—they contribute little to the result.) How does this sum compare with the mass of Earth's Moon, and with the mass of Pluto? (Hint)
7. Astronomers on Earth are observing the occultation of a star by Uranus and its rings (see Figure 13.18). It so happens that the event is occurring when Uranus is at opposition, and the center of the planet appears to pass directly across the star. Assuming circular planetary orbits and, for simplicity, taking the rings to be face-on, calculate (a) how long the 90-km-wide Epsilon ring takes to cross the line of sight, and (b) the time interval between the passage of the Alpha ring and that of the Epsilon ring. (Hint: Most of Uranus's apparent motion is actually due to Earth's motion in its orbit around the Sun.)
8. What would be your weight on Pluto? On Charon?
9. How close is Charon to Pluto's Roche limit? (Hint)
10. What is the round-trip travel time of light from Earth to Pluto (at a distance of 40 A.U.)? How far would a spacecraft orbiting Pluto at a speed of 0.5 km/s travel during that time?
1. The major astronomy magazines Sky and Telescope and Astronomy print charts showing the whereabouts of the planets in their January issues. Consult one of these charts, and locate Neptune and Uranus in the sky. Uranus may be visible to the naked eye, but binoculars make the search much easier. (Hint: Uranus shines more steadily than the background stars.) With the eye alone, can you detect a color to Uranus? Through binoculars?
2. The search for Neptune requires a much more determined effort! A telescope is best, but high-powered binoculars mounted on a steady support will do. If you can see both planets through a telescope—and they will remain close together on the sky for the rest of this century—contrast their colors. Which planet appears bluer? Through a telescope, does Uranus show a disk? Can you see that Neptune shows a disk, or does it look more like a point of light?
