Solar System
Sun – we’ll talk about this in Part 4
Planets
Moons of the planets
Asteriods
Plutoids
Kuiper Belt objects
Comets
Meteors, meteoroids, meteorites
Planets
Basic Information
Motions: inclinations to the ecliptic,
directions, “inferior” and “superior”
Types: terrestrial and giant
Water-ice clouds, polar ice, polar
regions, and geological features can be
seen in this full-disk image of Mars.
A true-color image of Jupiter taken by the
Cassini spacecraft. The Galilean moon
Europa casts a shadow on the planet's cloud
tops.
Basic Information
How many planets are there?
To answer the first question, we have to ask
the question: how do we define a planet?
Basic Info
How do we define a planet?
The first definition was for the objects in
the sky that “moved” relative to the
background starts. Based on this definition,
there were five planets when we were
limited to our own vision: Mercury, Venus,
Mars, Jupiter, and Saturn.
Basic Info
After the telescope was invented, we realized
that earth was a planet (6). We also found
two others: Uranus (7) and Neptune (8).
Later, a ninth planet, Pluto, was found.
Recently, we have located lots of other things
that move relative to the background star:
asteroids, comets, and most recently Kuiper
belt objects.
Basic Info
This large number of objects has called into
question the definition of a planet. The
actual definition of a planet is still under
dispute since most don’t want asteroids and
comets to be included, and most don’t want
the Kuiper belt objects to be included also.
This brings up the problem of Pluto – it
seems to belong to the Kuiper belt group,
but it has traditionally been the ninth planet
for quite a while.
Basic Info
Because of the disagreements among many
astronomers, for the purposes of this class, I
will accept either the traditional nine
planets:
Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, Neptune, and Pluto;
or I will accept that Pluto does not belong and
that there are only eight.
Motions of the Planets:
Inclinations to the ecliptic
The orbit of the earth around the sun defines a plane
in space called the ecliptic plane. All of the
planets orbit close to this plane, but not exactly on
it. The furthest off from this plane are the Plutoids
Eris (44o off) and Pluto (17o off), with Mercury (7o
off) next. The next furthest is Venus at only 3o off.
Pluto at 17o
Mercury at 7o
ecliptic
Motions of the Planets:
Direction of orbit, spin, moons
When viewed from a location above the North Pole,
all of the planets orbit around the sun in a
counterclockwise direction.
When viewed from a location above the North pole,
6 of the 9 planets rotate (spin) in a
counterclockwise direction; one rotates almost on
its side (Uranus); one rotates clockwise – but very
slowly (Venus),; and the other, Pluto, rotates at an
angle of 120o.
All of the major moons have orbits that are close to
the equators of their planets and go in the same
direction as the planet’s rotation except Triton
around Neptune.
Basic Information:
Order & Distance
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
*Pluto
*Eris
radius of orbit in
106 miles / A.U
36
67
93
142
484
888
1,783
2,800
3,673
.387
.723
1.00
1.52
5.20
9.54
19.2
30.1
39.5
38-97
period in
size compared rotation length of
days years
to earth
period
“day”
88 d
0.38
225 d
0.95
1y
1.00
1.88 y 0.53
5.20 y 11.2
29.5 y
9.45
84.0 y
4.01
164.8 y
3.88
248.6 y
0.18
560 y
0.19
tilt of
axis
58 d 176 d
2o
-243 d 117 d 177.3o
24 h
23.5o
24 h+39m
25.2o
9 h+55m
3.1o
10 h+14m
26.7o
-16 h+30m
97.8o
19 h+6m
29.6o
-6.39 d
118o
Motions of the Planets
Since the planets move around the sun at different
speeds in different orbits, the planets will appear at
different places in the sky at different times but
always near the ecliptic.
Whenever a planet is a “morning star”, that is, it
appears in the morning, it will be “west” of the
sun regardless of whether it is in the eastern or
western part of the sky.
Whenever a planet is an “evening star”, it will be
“east” of the sun regardless of whether it is in the
east or west.
“Inferior” Planets
An “inferior” planet is one who’s orbit is inside the earth’s
orbit. The inferior planet moves faster around its orbit
than the earth since it is closer to the sun and so has to
move faster to prevent it from falling into the sun.
The diagram below is from the perspective as being viewed
from above the North pole. The orbit of the planet, the
earth, and the earth’s spin are all counterclockwise when
viewed from above the North pole.
Maximum Eastern elongation – planet is “east” of the sun in the evening
evening
midnight
noon
Inferior
conjuction
Superior
conjunction
morning
Maximum Western elongation – planet is “west” of the sun in the morning
“Inferior” Planets
Maximum elongation for Mercury is 28o. This means it rises
or sets at most about two hours before or after the sun.
Maximum elongation for Venus is 48o. This means it rises or
sets at most about 3 hours before or after the sun.
48o
28o
Phases for the “inferior” planets
When the “inferior” planet is near superior conjunction, most
of the visible side is in daylight. When the planet is near
inferior conjunction, most of the visible side in in darkness
– leading to a crescent shape when viewed through a
telescope.
Inferior
planet
Inferior conjunction
earth
Superior conjunction
“Superior” planets
A “superior” planet is one who’s orbit is outside the
earth’s orbit. The superior planet moves slower
around its orbit than the earth since it is further
from the sun and so has to move slower to prevent
it from escaping out into space.
The diagram on the next slide is from the perspective
as being viewed from above the North pole. The
orbit of the planet, the earth, and the earth’s spin
are all counterclockwise when viewed from above
the North pole.
Note that the planets will always have most of their
visible surface in daylight no matter where the
planet is in its relative orbit.
“Superior” planets
Western Quadrature – planet is “west” of the sun in
the morning by 90o.
midnight
evening
morning
90onoon
Superior
planet
opposition
conjunction
90o
Eastern Quadrature – planet is “east” of the sun in
the evening by 90o.
Choose a planet
There is a lot of info about each of the planets in your
text and on the web. You will be responsible for
writing a short descriptive paragraph about one
terrestrial type planet (Mercury, Venus, or Mars)
and one giant type planet (Jupiter, Saturn, Uranus,
or Neptune).
Your paragraph might include info about the planet’s:
atmosphere (if any), surface features, temperature
extremes, magnetic fields, rings (if any), and major
moons.
http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys
Terrestrial Planets
basic information
Planet:
Mercury
Venus
Temperature range
-279 to 801 F
Ellipse: (million miles)
29 to 43
66.8 to 67.7 128 to 154
Atmosphere:
~ none
90X earth’s
mainly CO2
Volcanos
none active
Moons:
none
~ 875 F
extensive
turnover of surface
none
Mars
-195 to 70
0.7% of earth’s
mainly CO2
large ones
two small ones
Deimon & Phobos
Moons
This photo illustration shows selected moons
of our solar system at their correct relative
sizes to each other and to Earth.
Major Moons (r > 1,000 km)
in the Solar System
Mercury: no moons
Venus:
no moons
Earth:
The Moon (r = .27*Rearth)
Mars:
two minor moons, Phobos and Deimos
Jupiter:
Io (r = .28*Rearth), Europa (r = .25*Rearth),
Ganymede (r = .41*Rearth), Calliso (r = .38*Rearth)
Saturn:
Titan (r = .40*Rearth)
Uranus:
several minor moons
Neptune: Triton (r = .21*Rearth)
*Pluto:
one minor moon, Charon
*Eris:
one minor moon, Dysnomia
Major Moons
There is more and more information being
collected on the major moons. Read the text
or go on-line to find out information about
any or all of these major moons.
http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys&Display=Moons
Note: Pluto has a diameter of 2,274 km, or .18*Rearth
which means all 7 of the major moons are bigger
than Pluto. Mercury has a diameter of 4,880 km, or
.38*Rearth, so Ganymede and Titan are both a little
bigger than Mercury and Callisto is about the same
size as Mercury!
Colonization?
You will be responsible for picking a planet or
major moon to colonize. You should be
able to write a short paragraph detailing the
reasons for your choice. Some criteria you
should consider are: closeness to earth,
surface (gas, liquid, or rock; stable or
volatile), temperature ranges, atmosphere
(amount and type), existence of water or
other useful materials.
Asteriods
This picture of Eros, the first of an asteroid taken from an
orbiting spacecraft, is a mosaic of four images obtained by
NASA's NEAR mission immediately after the spacecraft's
insertion into orbit.
Asteroids: basic info
Asteroids are “minor planets” that orbit the sun
(rather than objects that orbit planets as moons).
They are much smaller than the planets, with the
largest being Ceres at about 1,000 km in diameter,
(remember that the diameter of the Earth is about 12,800
km) and only another five or so having diameters
over 300 km. There are roughly 200 or so with
diameters over 100 km. There may be on the
order of a million with a diameter of kilometer or
more. The total mass of all the asteroids is
probably less than the mass of the smallest planet,
Pluto.
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids
Asteroids: orbits
The main group of asteroids, including Ceres, is in
the “asteroid belt” between Mars and Jupiter with
orbits roughly between 2.2 and 3.5 AU.
Some of the asteroids have very elliptical orbits, and
some - called the Apollo asteroids - come inside
earth’s orbit. We think only a few dozen of the
earth-crossing asteroids have sizes greater than 1
km.
Another group of asteroids called the Trojan
asteroids follow and lead Jupiter by about 60o in
its orbit around the sun.
Asteroids
Trojan
belt
Apollo
Earth
Jupiter
Mars
Trojan
belt
Asteroids
On the previous slide, the size of the orbits is more
or less to scale. However, the size of the sun,
planets and asteroids are NOT to scale.
The sun should be about 1% of the earth-sun distance.
Jupiter should be about 1/10 the diameter of the sun.
Earth should be about 1/10 the diameter of Jupiter.
Mars should be about ½ the diameter of the Earth.
The biggest asteroids should be about 1/10 the size of Mars.
Dwarf Planets
Size Comparisons Date: 11 Dec 2010
A size comparison of dwarf planets Eris, Pluto, and Ceres (artist's concepts); Pluto's moon
Charon (artist's concept); Earth's Moon; and Earth.
Image Credit: NASA
Plutoids and Dwarf Planets
According to the International Astronomical Union (IAU):
“Plutoids are celestial bodies in orbit around the sun at a
distance greater than that of Neptune that have sufficient
mass for their self-gravity to overcome rigid body forces so
that they assume a hydrostatic equilibrium (near spherical)
shape, and that they have not cleared the neighborhood
around their orbit.”
There are two named Plutoids right now: Pluto and Eris.
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Dwarf
Kuiper Belt
The Kuiper Belt is an area beyond Neptune
from about 30 AU out to about 50 AU that
has at least 70,000 small objects with
diameters greater than 100 km. (Recall that
Pluto has a diameter of about 2,300 km and the
earth has a diameter of about 12,800 km.
Note that Pluto and Eris are Kuiper belt objects.
Pluto has an orbit with a radius of about 40 AU,
but it is very elliptical and it varies between about
30 and 50 AU. Eris has an even more elliptical
orbit that varies between 38 and 97 AU.)
http://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs
Comets
http://www2.jpl.nasa.gov/com
et/hyakutake/soho2.html
Comets are objects composed of dust and ice that
come from the far reaches of the solar system on
extremely elliptical orbits. They appear to come
from the Kuiper Belt (for relatively short period
comets) and an area known as the Oort Cloud (for
much longer period comets). The Oort Cloud is
hypothesized to be a spherically shaped area (not
confined to the ecliptic plane like most everything else)
about 50,000 AU from the sun.
Comets
Comet Kohoutek
This color photograph of the comet Kohoutek was taken by members of the lunar and planetary
laboratory photographic team from the University of Arizona. They photographed the comet from the
Catalina observatory with a 35mm camera on January 11, 1974. (Courtesy NASA)
In their elliptical orbits, comets move very fast and
spend very little time when they come near the sun
(due to the strong gravity near the sun), and they
move very slowly and spend a very long time
when they are far from the sun (due to the very
weak gravity far from the sun). Near the sun they
can be very bright, but far from the sun they are
extremely hard to see even in powerful telescopes.
See the following website
http://www.solarviews.com/eng/comet.htm
Comets – structure
Far from the sun (several AU’s and farther), the
comet is just a dirty snowball.
When the comet comes closer to the sun, the heat
from the sun evaporates some of the ice and snow
which forms a “coma” that surrounds the
“nucleus”. The coma and nucleus are called the
“head” of the comet. The gas and dust in the
coma reflect sunlight and so the comet gets
brighter. As the comet gets even closer, the gas
absorbs some of the sun’s light and fluoresces (reemits the light) and becomes brighter still.
Comets – structure (cont.)
As the comet approaches the sun, the gas and dust
that were evaporated form a “tail”. This tail can
have three parts:
The “dust” tail tends to follow the comet in its orbit.
The “hydrogen” envelope tends to be pushed by the
sun’s radiation somewhat behind the head. This
hydrogen envelope is not visible from the earth’s
surface since our atmosphere absorbs its light –
but it has been seen by spacecraft.
The “ion” tail is pushed straight behind the comet by
the sun’s radiation.
Meteors
Meteors are grains of dust and small “rocks” that fall
through the atmosphere and glow due to the heat of
air resistance as they fall.
Meteorites are the remains that reach
the earth’s surface.
Meteroids are the material of meteors
in space (before they become
meteors). Some of this material
meteorite found on
comes from dust from comets, some The
Mars by Opportunity
from asteroids.
rover in 2005.
See the website:
http://csep10.phys.utk.edu/astr161/lect/meteors/showers.html