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ASTRONOMY 161
Introduction to Solar System Astronomy
Class 23
Uranus, Neptune, Pluto,
Eris, & Beyond
Wednesday, March 7
Uranus: Basic characteristics
Mass = 8.683×1025 kg (14.54 Earths)
Diameter = 51,118 km (4.00 Earths)
Density = 1,318 kg/m³
Sidereal rotation period = -0.718 d (17 h 14 min)
Albedo = 0.51 (Earth = 0.39)
Average distance from Sun = 19.19 A.U.
Neptune: Basic characteristics
Mass = 1.02×1026 kg (17.15 Earths)
Diameter = 49,528 km (3.83 Earths)
Density = 1,638 kg/m³
Sidereal rotation period = 0.671 d (16 h 6 min)
Albedo = 0.41 (Earth = 0.39)
Average distance from Sun = 30.07 A.U.
Pluto: Basic characteristics
Mass = 1.305×1022 kg (0.0021 Earths)
Diameter = 2,306 km (0.18 Earths)
Density = 2,000 kg/m³
Sidereal rotation period = -6.38d d (6 d 9 hrs)
Albedo = 0.49-0.66 (varies) (Earth = 0.39)
Average distance from Sun = 39.48 A.U.
Eris: Basic characteristics
Mass = ? (? Earths)
Diameter = 2,400 km (0.19 Earths)
Density = ? kg/m³
Sidereal rotation period = ?
Albedo = 0.86 (Earth = 0.39)
Average distance from Sun = 67.668 A.U.
Jupiter and Neptune: Key Concepts
(1) Uranus and Neptune are nearly identical in
their internal structure.
(2) The rotation axis of Uranus is tilted by
about 90 degrees, causing extreme seasons.
(3) Neptune has surprisingly strong storms,
driven by internal heat.
(4) Triton, the giant moon of Neptune, is a cold
world with nitrogen geysers.
Pluto ... and Beyond:
Beyond Key Concepts
(1) Pluto and its moon Charon are icy worlds that
resemble Triton.
(2) Eris, the troublemaker (Greek goddess of
strife).
(3) The Kuiper belt, beyond Neptune, contains
small, icy, Pluto-like objects.
(4) The icy Kuiper Belt Objects are leftover
planetesimals.
(5) Studies of the Outer Solar System continue.
URANUS
Discovered in 1781 by William Herschel.
NEPTUNE
Discovered in 1846, using Newton’s Laws.
Uranus and Neptune are quite large planets.
Radius of Uranus = 4.0 x radius of Earth
Radius of Neptune = 3.8 x radius of Earth.
Uranus: Revolution and Rotation
Orbital period = 84 years
Rotation period = 17 hours
Uranus is somewhat oblate due to its rotation.
Its equatorial diameter is 2.3% greater than its polar
diameter.
Neptune: Revolution and Rotation
Orbital period = 165 years
Rotation period = 16 hours
Neptune is somewhat oblate.
Its equatorial diameter is 1.7% greater than its polar
diameter.
(1) Uranus and Neptune are nearly
identical in their internal structure.
Uranus and Neptune are planetary “twins”, like Earth
and Venus. Uranus is 3% larger in radius but 15%
smaller in mass than Neptune.
1)
2)
3)
4)
Interiors of Uranus and Neptune
Gaseous atmosphere: hydrogen, helium, methane
Liquid outer layer: hydrogen, helium
Liquid or slushy mantle: water, ammonia
Solid core: rock, metal
(2) The rotation axis of Uranus is tilted by
about 90o, causing extreme seasons.
Axis tilts:
Jupiter = 3o
Saturn = 27o, Neptune = 30o
Uranus = 98o
The seasons of Uranus
Winter solstice (AD 1985):
north – perpetual dark, south – perpetual sun.
Vernal equinox (2006): sun rises once every 17 hours.
And so forth …
Near the winter solstice (1986), the atmosphere of
Uranus was boring:
(left: visible light)
(right: ultraviolet)
No storms; very faint belts & zones.
Heat flow from summery south to wintery north
smeared out storms.
As the vernal
equinox
approaches,
Uranus is
becoming stormy:
Heating of northern
hemisphere
produces storms,
made visible by
clouds of
methane.
Uranus has 27 known moons.
22 are small (< 200 km across) and irregular.
5 are mid-sized (400 => 1600 km in diameter) and
spherical. No giant moons.
(3) Neptune has surprisingly strong storms,
driven by internal heat.
Temperature at
Neptune’s cloud
level is 55 Kelvin
(-360o F).
This is as warm as
Uranus, although
Neptune is much
further from the
Sun.
Neptune’s
atmosphere
is being
warmed
from
below by
internal
heat.
This extra heat drives large circular storms, like
the “Great Dark Spot” seen above.
1985: Rings of Neptune discovered during a stellar
occultation.
Image obtained by the Voyager 2 spacecraft in 1989.
Neptune has 13 known moons.
12 are small (< 500 km across) and irregular.
One is the giant moon Triton.
Proteus
Triton
Nereid
(4) Triton: Neptune’s Frosty Moon
Surface
temperature =
38 Kelvin.
Covered with
“frost” at poles:
frozen methane,
frozen nitrogen.
Triton has craterfree plains;
perhaps the
frozen calderas
of extinct ice
volcanoes?
Density of Triton
= 2100 km/m3;
ice mantle over
rocky core.
Triton has geysers 5 miles high driven by vaporized
nitrogen.
Plumes are swept downwind in the very tenuous
nitrogen atmosphere.
Triton is on a tilted retrograde orbit around Neptune.
It has most likely been captured by Neptune.
Tidal forces are moving Triton toward Neptune. In
100 million years, Triton will enter the Roche limit.
Spectacular rings!
A farewell
to the
Solar
System:
Voyager 2
looks back
at Neptune
and
Triton.
PLUTO
Jan. 23, 1930
Jan 29. 1930
Discovered in 1930, by sheer luck.
Orbital period = 248 years
Rotation period = 6.4 days
ERIS (a.k.a. “Xena”)
Oct. 21, 2003
Discovered in 2005, systematic search.
Orbital period = 557 years
Rotation period = ??
Orbits of Neptune, Pluto and Eris
Do not worry – they will not collide.
The orbits are not in the same plane.
Pluto’s moon, CHARON, was discovered in 1978.
From the ground,
Charon looked
like a “lump”
on Pluto’s side.
Hubble Space Telescope
(1) Pluto and its moon Charon
are icy worlds that resemble Triton.
Diameter of Pluto =
2300 km
Diameter of Charon =
1200 km
Separation =
20,000 km
Pluto and Charon are tidally locked into “double
synchronous” rotation.
Rotation =
revolution =
6.4 days.
Very little is known about
the surface of Pluto and Charon.
Hubble Space Telescope images suggest white icecaps
are present, as well as dark patches (dusty?) and
light patches (clean ice?). Albedo varies.
Pluto and Charon have many properties in
common with Neptune’s moon Triton.
• Cold surfaces (about 40 Kelvin)
• Icy mantles and rocky cores (about
2000 kg/m3)
• Pluto has a thin atmosphere (like Triton);
Charon has none.
At last, a mission to Pluto: NEW HORIZONS
Launch:
January 2006
Jupiter flyby:
February 2007
Pluto flyby:
July 2015
Then onward into
the Kuiper belt.
(2) Eris (“Xena”), the troublemaker.
Discovered in 2005
by Mike
Brown and
collaborators.
It has a moon.
It is BIGGER than
Pluto!
Trouble!
Eris
and
Pluto:
Spectra
Q: Are Pluto and Eris planets?
A: That depends on how you define “planet”.
As of August 2006, new category of
“dwarf planet”.
The International Astronomical Union defines
"planet" as a celestial body that, within the
Solar System,
(a) is in orbit around the Sun;
(b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes
a hydrostatic equilibrium (nearly round)
shape; and
(c) has cleared the neighborhood
Planets: Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune
The International Astronomical Union defines a
"dwarf planet" as a celestial body that,
within the Solar System,
(a) is in orbit around the Sun;
(b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes
a hydrostatic equilibrium shape;
(c) has NOT cleared the neighborhood around its
orbit; and
(d) is not a satellite
Dwarf Planets (so far): Ceres, Pluto, Eris
(3) The Kuiper belt, beyond Neptune,
contains small, icy, Pluto-like objects.
The Kuiper belt lies close to the ecliptic plane,
and stretches from 30 A.U. to 50 A.U. from
the Sun.
Named after Gerard Kuiper, who predicted it
should be full of planetesimals.
Over 800 Kuiper belt objects (a subset of transNeptunian objects) have been discovered in
the belt since 1992.
Kuiper belt
Searches continue to reveal many objects
orbiting in the Kuiper belt.
Estimate: As many as 100,000 objects over
100 kilometers across.
Kuiper Belt Objects are a subset of
the Trans-Neptunian Objects.
(4) The icy Kuiper Belt Objects
are leftover planetesimals.
Kuiper Belt Objects have colors (and spectra)
consistent with them being icy.
They are probably planetesimals that formed
within the Kuiper belt.
The Kuiper belt Objects are scattered too
thinly for them to have accreted into a larger
body.
(5) Studies of the Outer Solar System
continue.
More objects like Pluto and Eris (or even bigger)
might exist. They are hard to find:
●
Very dim;
●
Very slow moving.
Perhaps more easily detected from their gravitational
influence?
From studies of Neptune’s orbit: No more Jovian
(massive) planets within 200 A.U. of the Sun.
TransNeptunian
Objects
searches
are
incomplete.
More Erislike objects
are likely
to be
discovered.
Sedna: Even further away!
Discovered in 2003 by Brown et al.:
Eccentricity: 0.855
Semi-major axis: 525.606 A.U.
Perihelion: 76.156 A.U.
Aphelion: 975.056 A.U.
Orbital period: 12050.32 years!
Might be added as a “dwarf planet”.
Sedna:
the most
distant
object
known in
the Solar
System.
Sedna
Case of Sedna's
Missing Moon
Solved
Few closing questions:
1) Name the major differences and similarities
between Uranus and the Earth.
2) Name the major differences between Uranus and
Neptune.
3) Is there magnetic field around Uranus?
4) Are there volcanoes on Neptune?
5) Are there quakes on Neptune?
6) Is there a ring around Uranus?
Few closing questions:
1) Does Pluto have an atmosphere?
2) Name the major differences between Pluto and
Eris.
3) What is Pluto made of?
4) Is there a ring around Pluto?
5) Does Pluto have a moon?
6) Can there be another large planet out there? (planet
“X”)