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Chapter 24
Uranus, Neptune,
and the Dwarf Planets
Guidepost
Two planets circle the sun in the twilight beyond Saturn.
You will find Uranus and Neptune strangely different from
Jupiter and Saturn, but recognizable as planets. As you
explore you will also discover a family of dwarf planets,
which includes Pluto, which will give you important clues to
the origin of the solar system. This chapter will help you
answer five essential questions:
• How is Uranus different from Jupiter?
• How did Uranus, its rings, and its moons form and
evolve?
• How is Neptune different from Uranus?
• How did Neptune, its rings and its moons form and
evolve?
• How are Pluto and the dwarf planets related to the origin
of the solar system?
Guidepost
As you think about the search for worlds in the outer
solar system, you will be able to answer a
fundamental question about how science works:
• Why are nearly all truly important scientific
discoveries made by accident?
As you finish this chapter, you will have visited all of
the major worlds in our solar system. But there is
more to see. Vast numbers of small rocky and icy
bodies orbit among the planets, and the next chapter
will introduce you to these fragments from the age of
planet building.
Outline
I. Uranus
A. The Discovery of Uranus
B. The Motion of Uranus
C. The Atmosphere of Uranus
D. The Interior of Uranus
E. The Rings of Uranus
F. The Moons of Uranus
G. A History of Uranus
II. Neptune
A. The Discovery of Neptune
B. The Atmosphere and Interior of Neptune
C. The Rings of Neptune
D. The Moons of Neptune
E. The History of Neptune
Outline (continued)
III. The Dwarf Planets
A. The Discovery of Pluto
B. Pluto as a World
C. The Family of Dwarf Planets
D. Pluto and the Plutinos
The Discovery of Uranus
Chance discovery by William
Herschel in 1781,
while scanning the sky
for nearby objects with
measurable parallax:
discovered Uranus as
slightly extended object,
~ 3.7 arc seconds in
diameter.
The Motion of Uranus
Very unusual
orientation of rotation
axis: Almost in the
orbital plane.
Possibly result of
impact of a large
planetesimal during
the phase of planet
formation.
Large portions of the
planet exposed to
“eternal” sunlight for
many years, then
complete darkness
for many years!
19.18 AU
97.9o
The Atmosphere of Uranus
Like other gas giants: No surface.
Gradual transition from gas phase to fluid interior.
Mostly H; 15 % He, a few % Methane, ammonia and water vapor.
Optical view from
Earth: Blue color due
to methane,
absorbing longer
wavelengths
Cloud structures only visible after artificial
computer enhancement of optical images
taken from Voyager spacecraft.
The Structure of Uranus’ Atmosphere
Only one layer of Methane clouds
(in contrast to 3 cloud layers on
Jupiter and Saturn).
3 cloud layers in
Jupiter and Saturn
form at relatively high
temperatures that
occur only very deep in
Uranus’ atmosphere.
Uranus’ cloud layer is
difficult to see
because of thick
atmosphere above it.
Also shows belt-zone
structure
 Belt-zone cloud structure must be dominated by
planet’s rotation, not by incidence angle of sun light!
Cloud Structure of Uranus
Keck Telescope images of Uranus show clear
variability of the cloud structures
 Possibly
due to seasonal changes of the
cloud structures.
The Interior of Uranus
Average density ≈ 1.29 g/cm3  larger portion
of rock and ice than Jupiter and Saturn.
Ices of water,
methane, and
ammonia,
mixed with
hydrogen and
silicates
The Magnetic Field of Uranus
No metallic core  no magnetic field was expected.
But actually, magnetic field of ~ 75 % of Earth’s
magnetic field strength was discovered:
Offset from
o
center: ~ 30 % Inclined by ~ 60
Possibly due to dynamo in
against axis of
of planet’s
liquid-water/ammonia/methane
rotation.
radius!
solution in Uranus’ interior.
Magnetosphere with weak radiation belts; allows
determination of rotation period: 17.24 hr.
The Magnetosphere of Uranus
Rapid rotation and large inclination deform
magnetosphere into a corkscrew shape.
UV images
During Voyager 2 flyby: South pole pointed towards sun; direct
interaction of solar wind with magnetosphere  Bright aurorae!
The Rings of Uranus
Rings of Uranus and Neptune are similar to Jupiter’s rings.
Confined by shepherd moons; consist of dark material.
Apparent motion of
star behind Uranus
and rings
Rings of Uranus were
discovered through
occultations of a
background star
The Rings of Neptune
Ring material must
be regularly resupplied by dust
from meteorite
impacts on the
moons.
Interrupted between
denser segments (arcs)
Made of dark
material,
visible in
forwardscattered
light.
Focused by small shepherd
moons embedded in the
ring structure.
The Rings of Neptune (2)
Two newly
discovered rings
orbit Uranus far
outside the
previously
known rings.
Outermost ring
is only visible
as a segment,
following the
small moon
Mab.
The Moons of Uranus
5 largest moons are
visible from Earth.
10 more discovered
by Voyager 2; more
are still being found.
Dark surfaces,
probably ice
darkened by dust
from meteorite
impacts.
5 largest moons all tidally locked to Uranus.
Interiors of Uranus’s Moons
Large rock cores surrounded by icy mantles.
The Surfaces of Uranus’s Moons (1)
Oberon
Old, inactive, cratered surface,
but probably active past.
Long fault across the surface.
Dirty water may have flooded
floors of some craters.
Titania
Largest moon
Heavily cratered surface, but no
very large craters.
Active phase with internal melting
might have flooded craters.
The Surfaces of Uranus’s Moons (2)
Umbriel
Dark, cratered surface
No faults or other signs of
surface activity
Ariel
Brightest surface of 5 largest moons
Clear signs of geological activity
Crossed by faults over 10 km deep
Possibly heated by tidal interactions
with Miranda and Umbriel.
Uranus’s Moon Miranda
Most unusual of the 5 moons detected from Earth
Ovoids: Oval groove patterns, 20 km high cliff near the equator
probably associated with
convection currents in the Surface features are old; Miranda is
no longer geologically active.
mantle, but not with impacts.
Neptune
Discovered in
1846 at position
predicted from
gravitational
disturbances on
Uranus’s orbit by
J. C. Adams and
U. J. Leverrier.
Blue-green color
from methane in
the atmosphere
4 times Earth’s
diameter; 4 %
smaller than
Uranus
The Atmosphere of Neptune
The “Great
Dark Spot”
Cloud-belt structure with high-velocity
winds; origin not well understood.
Darker cyclonic disturbances,
similar to Great Red Spot on
Jupiter, but not long-lived.
White cloud features of methane ice crystals
The Moons of Neptune
Two moons (Triton and Nereid) visible from Earth;
6 more discovered by Voyager 2
Unusual orbits:
Triton: Only satellite in the
solar system orbiting
clockwise, i.e.
“backward”.
Nereid: Highly eccentric orbit;
very long orbital period (359.4 d).
The Surface of Triton
Very low temperature
(34.5 K)
 Triton can hold a
tenuous atmosphere
of nitrogen and some
methane; 105 times
less dense than
Earth’s atmosphere.
Surface composed of ices:
nitrogen, methane, carbon
monoxide, carbon dioxide.
Possibly cyclic nitrogen
ice deposition and revaporizing on Triton’s
south pole, similar to
CO2 ice polar cap
cycles on Mars.
Dark smudges on the nitrogen ice surface,
probably due to methane rising from below
surface, forming carbon-rich deposits when
exposed to sun light.
The Surface of Triton (2)
Ongoing surface
activity: Surface
features probably
not more than 100
million years old.
Large basins might
have been flooded
multiple times by
liquids from the
interior.
Ice equivalent of greenhouse effect may be one of the
heat sources for Triton’s geological activity.
The Discovery of Pluto
Discovered 1930 by C. Tombaugh.
Existence predicted from orbital disturbances of Neptune,
but Pluto is actually too small to cause those disturbances.
Pluto as a World
Virtually no surface
features visible from Earth.
~ 65 % of size of Earth’s
Moon.
Highly elliptical orbit;
coming occasionally closer
to the sun than Neptune.
Orbit highly inclined (17o)
against other planets’ orbits
 Neptune and Pluto
will never collide.
Surface covered with nitrogen ice; traces of frozen methane
and carbon monoxide.
Daytime temperature (50 K) enough to vaporize some N
and CO to form a very tenuous atmosphere.
Pluto’s Moon Charon
Discovered
in 1978;
about half
the size and
1/12 the
mass of
Pluto itself.
Tidally
locked to
Pluto.
Hubble Space Telescope image
Pluto and Charon
Orbit highly inclined against
orbital plane.
From separation and orbital period:
Mpluto ~ 0.2 Earth masses.
Density ≈ 2 g/cm3
(both Pluto and Charon)
 ~ 35 % ice and 65 % rock.
Large orbital inclinations 
Large seasonal changes on
Pluto and Charon.
The Origin of Pluto and Charon
Probably very different history than neighboring Jovian planets.
Older theory:
Pluto and Charon formed as moons of Neptune,
ejected by interaction with massive planetesimal.
Mostly abandoned today since
such interactions are unlikely.
Modern theory: Pluto and
Charon members of Kuiper
belt of small, icy objects (see
Chapter 25), caught in orbital
resonances with Neptune
(“Plutinos”).
Collision between Pluto and Charon may have caused the peculiar
orbital patterns and large inclination of Pluto’s rotation axis.
Because of its different origin from the planets, Pluto is
no longer considered a planet, but is the prototype of a
new class of “Dwarf Planets”.
The Family of Dwarf Planets
• Small, rocky bodies which originated in the Kuiper Belt.
• Later captured into shorter-period orbits in the realm of
the regular planets.
• Three dwarf planets clearly identified so far:
Pluto
Eris
Ceres
The ``10th planet’’:
Discovered in 2004
The largest asteroid
4 % larger than Pluto
900 km diameter
27 % more massive
than Pluto
70 % further away
from the sun than
Pluto
Typical asteroid
orbit between
Mars and Jupiter