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Chapter 13 - Uranus, Neptune, and Pluto
CHAPTER 13
THE OUTER SOLAR SYSTEM
CHAPTER OUTLINE AND LECTURE NOTES
1. Discoveries
Although nearly everyone realizes that the discoveries of both Uranus and Pluto were
accidental, I hadn’t fully realized until reading some of the references for this chapter that
good fortune also played a role in the discovery of Neptune. Leverrier had assumed an
orbital shape and period for the missing planet that would have led to a search in the wrong
part of the sky if the search had been conducted a little earlier or later than Galle’s
observations. Recent rediscovered documents concerning the discovery of Neptune make it
clear that the role of Adams, often cited as the co-discoverer of Neptune, has been
overstated.
2. Uranus and Neptune
Given the similarity in the masses, radii, and compositions of Uranus and Neptune, the
difference in the amounts of internal heat escaping into space is hard to understand. While
the layered model of Uranus described in the textbook might work to inhibit convection and
energy transport, I haven’t seen any discussion of what such layers consist of and how they
would develop.
The heat flow and convection in Neptune produce temperatures in Neptune’s atmosphere
comparable to those in the atmosphere of Uranus (Figure 13.6) and result in the beautiful
and interesting cloud features on Neptune. Voyager observations of these features have
been combined in a video included in the CD that accompanies this textbook.
3. Pluto
The demotion of Pluto from planet to dwarf planet had an interesting parallel during the
first half of the 19th century. The first four asteroids (see Chapter 15) were discovered
between 1801 and 1807. Then no more asteroids were found for almost 40 years. During
that interval the four asteroids (Ceres, Pallas, Juno, and Vesta) were generally considered to
be planets. Astronomy textbooks from that time list the four asteroids with the planets.
They had (and have) their own astrological/astronomical symbols. Just as Pluto’s status as
a planet was questioned after the discovery of numerous Pluto-like bodies in the outer solar
system, so was the four asteroids’ status as planets questioned after the avalanche of
asteroid discoveries began in the 1840s. Eventually, a new status (asteroid, or minor
planet) was invented for them just as dwarf planet recently was invented to deal with Pluto.
The fact that Pluto’s orbital period is 1.5 times that of Neptune prevents them from
getting very close to one another (Figure 13.20). The changing brightness and variability of
Pluto between the time it was discovered and the 1960s led to the first prediction that
Pluto’s rotation axis lies nearly in its orbital plane. The late Leif Andersson and I analyzed
the existing observations of Pluto in the early 1970s and wrote a paper predicting the
extreme tilt of Pluto’s axis. This prediction was later confirmed following the discovery of
Pluto’s satellite Charon.
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Chapter 13 - Uranus, Neptune, and Pluto
4. Planets Beyond Pluto
In addition to using the Voyager and Pioneer spacecraft to search for gravitational
perturbations due to unknown planets, astronomers have also used the spacecraft to search
for gravitational waves such as might be produced by a supernova within the Milky Way
Galaxy. No detections of either planets or gravitational waves have yet been made. The
discoveries of many bodies, too small to be planets, beyond Neptune have been made using
ground-based telescopes.
KEY TERMS
celestial mechanics — The part of physics and astronomy that deals with the motions of
celestial bodies under the influence of their mutual gravitational attraction.
Centaur – A small solar system body whose orbit lies between the orbits of Jupiter and
Neptune.
classical KBO – A Kuiper Belt Object with an orbital perihelion distance far enough from
Neptune that it does not experience large perturbations due to Neptune.
dwarf planet — A solar system body that does not orbit a planet and whose gravity is strong
enough to make it almost spherical yet that does not dominate the region near its orbit by
clearing the region of smaller bodies.
occultation — The cutoff of the light from a star when a solar system body passes between the
star and the observer
orbital resonance – A condition that occurs when two bodies have orbital periods that have the
ratio of small integers, such as 1:2, 2:3, etc.
resonant KBO – A Kuiper Belt Object in an orbital resonance with Neptune. Most resonant
KBOs are, like Pluto, in a 2:3 resonance with Neptune and orbit the Sun twice for every three
orbits of Neptune.
perturbation — A deviation of the orbit of a solar system body from a perfect ellipse due to the
gravitational attraction of one of the planets.
Scattered disk object – A trans-Neptunian body with a relatively large, highly eccentric orbit.
The perihelion distances of scattered disk objects are close enough to Neptune's orbit that they
will eventually experience an encounter with Neptune and move into the planetary system.
stellar
Trans-Neptunian Object (TNO) – A small, icy solar system body in an orbit beyond Neptune.
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
1. He had to see whether the new object moved with respect to the stars.
2. The motion of Uranus could not be explained by the gravitational attractions of the Sun and
known planets.
3. He calculated that there was an unknown planet near the position where other astronomers
later found Neptune.
4. They are similar in size and mass. They are different in the richness of cloud features in
their atmospheres and the extent to which they are self-luminous.
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Chapter 13 - Uranus, Neptune, and Pluto
5. Because the axis of Uranus lies in its orbital plane there are long periods of time when one
pole or the other nearly points at the Sun. During those times, one hemisphere is always in
daylight and the other in darkness.
6. One reason is that there is relatively little solar heating Uranus's distance. Another is that
the atmosphere of Uranus can hold enough heat that it takes longer than the planet's orbital
period to cool off.
7. Little heat flows out of Uranus to drive convection and cause atmospheric features.
8. Internal heat escaping from Neptune flows outward through the atmosphere, warming it.
9. Such models are consistent with the gravitational field measurements made by the Voyager
spacecraft.
10. They are similar in size and mass.
11. Their magnetic field axes are more tilted than the other planets and are also displaced from
their centers.
12. Their magnetic fields are offset from the centers of the planets.
13. The rings produced temporary drops in the light of stars when the rings passed between the
stars and the Earth.
14. The rings of Uranus are much narrower than the rings of Saturn.
15. Small dust particles are efficient at forward scattering and the regions between the rings
appear bright when viewed looking backward toward the Sun.
16. The rings of Neptune are very clumpy and some of the predicted occultations occurred
when an empty part of a ring passed in front of the star
17. At perihelion Pluto is closer to the Sun than Neptune is.
18. Because Pluto varies in brightness as bright and dim parts of its surface rotate into view.
19. Using Charon's orbital period and distance, Kepler's third law was used to calculate the
mass of Pluto.
20. A dwarf planet, unlike a planet, does not dominate the region near its orbit.
21. The scattered disk objects have orbits that are larger, more eccentric, and more inclined to
the ecliptic.
22. TNOs may have interacted gravitationally with Neptune, changing its orbit which in turn
resulted in changes in the orbits of the other outer planets.
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Chapter 13 - Uranus, Neptune, and Pluto
Problems
1. 8.9 m/s2, 11.7 m/s2
2. 586 years
Figure-Based Questions
1. 1 atmosphere
2. 45,600 km
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