1. INTRODUCTION
In our solar system, nine planets circle around our Sun. The
Sun sits in the middle while the planets travel in circular paths
(called orbits) around it. These nine planets travel in the same
direction (counter- clockwise looking down from the Sun's north
pole). The picture on the right shows the different paths and
positions of each planet
The solar system is made up of two parts:
1.-The inner solar system contains Mercury, Venus, Earth and
Mars. These four planets are closest to the Sun.
The outer solar system contains Jupiter, Saturn, Uranus,
Neptune and Pluto.
2.-The inner planets are separated from the outer planets by
the Asteroid Belt.
This picture below shows how big (and little!) the nine
planets are in relation to each other. Can you see tiny Mercury in
the very bottom, left-hand corner? Pluto is the teeny-tiny planet
in the very top, right-hand corner!
These nine planets are grouped in many different ways.
Two easy groupings are composition (what they are made up of)
and size.
COMPOSITION
ROCKY PLANETS (Mercury - Venus - Earth - Mars - Pluto)
The rocky planets are mostly made up of rock and metal. These
planets are very heavy and move slowly. They also do not have
rings and very few moons.
GAS PLANETS (Jupiter - Saturn - Uranus - Neptune)

2. The gas planets are mostly made up of gases (hydrogen and
helium). These planets are light for their sizes (just like a big air
balloon) and move quickly. They have rings and lots of moons.
SIZE
SMALL PLANETS (Mercury - Venus - Earth - Mars - Pluto)
The small planets have diameters less than 13000 km across.
Mercury and Pluto are sometimes referred to as lesser planets
because they are so tiny.
GIANT PLANETS (Jupiter - Saturn - Uranus - Neptune)
The giant planets have diameters greater than 48000 km. The
giant planets are sometimes also referred to as gas giants.
THE SUN
The Sun is personified in many mythologies. The Greeks
called it Helios. The Sun's name comes from the Romans who
called it Sol.
The Sun is a star - it is not a planet. Our Sun is just like
the stars we see in the night sky. The Sun is also the only star
we see during the daytime.
The Sun is so big that you could fit more than 1 million
earths inside of it! It measures more than a million kilometers
across.
The Sun's center is about 15 million degrees Celsius. The
Sun is so hot that planets millions of kilometers away can feel its
heat... That's *mighty hot*!
The Sun is about 75% hydrogen and 25% helium. This
changes slowly over time as the Sun converts hydrogen to helium
in its core (center).

3. The Sun's energy is produced by nuclear fusion reactions.
It generates 386 billion billion megawatts of energy!
MERCURY
In Roman mythology Mercury is the god of commerce,
travel and thievery. Mercury is also known as Hermes, the
messenger of the Gods, in Greek mythology. The planet probably
received this name because it moves so quickly across the sky.
Mercury is a small, rocky planet
Mercury has been visited by the Mariner 10 spacecraft.
Mariner 10 has mapped a little less than half (45%) of Mercury's
surface.
Scientists think that there may be *volcanic activity* on
Mercury. They are still studying information sent to Earth from
the Mariner spacecraft to make sure. The temperature on
Mercury ranges from 90 K to 700 K.
It was once believed that there was no water on Mercury,
but this turned out to be false. Recent radar information shows
evidence of ice at Mercury's north pole! The ice hasn't melted
because it is protected from the Sun's heat by shadows of some
craters.
Unlike many of our nine planets, Mercury has no moons.
VENUS
Venus is the Roman goddess of love and beauty. She is
known as Aphrodite in Greek mythology. The planet is so named

4. probably because it is the brightest of the planets known to the
ancients.
Venus is the second planet from the Sun, and the sixth
largest of all the nine planets.
Venus is a small, rocky planet blanketed in a thick layer of
yellowish clouds. These clouds are not made of water (like the
ones here on Earth). Instead, they are formed from a poison
called sulfuric acid.
Venus' surface is very hot - about 400 degrees Celsius!
Even though Venus is very cloudy, it's simply *too hot* for rain
to form.
The first spacecraft to visit Venus was Mariner 2 in 1962.
Venus has since been visited by more than 20 spacecraft in all so
far! Some of these visiting spacecraft include: Pioneer Venus,
Venera 7, Venera 9 and Magellan.
Venus has no moons.
EARTH
Earth is the only planet whose English name does not derive
from Greek or Roman mythology. The name comes from Old
English and Germanic language. There are, of course, hundreds
of other names for the planet in other languages.
Earth is a small, rocky planet which supports a variety of
life! As far as we know, Earth is unique from all other planets in
this respect.
Temperatures at the Earth's center (called the "core")
may be as high as 7500 K - That's hotter than the surface of
the Sun!
Besides being the most interesting and unique of all the
planets, our Earth is also the most beautiful. From space, the
Earth's white clouds, blue oceans and brown lands seem to shine
against the black backdrop of our solar system.

5. The Earth is the densest major body in the solar system.
This means that it's the most "compact" of all the planets. For
example, you have two loaves of bread, both of the same amount.
You smash one loaf of bread flat. Even though both loaves are of
the same amount, the smashed bread is *denser* because it is
more compact.
The Earth is 4.5 to 4.6 billion years old, but the oldest
known rocks are less than 4 billion years old. Rocks older than 3
billion years are rare. The oldest fossils of living organisms are
less than 3.9 billion years old!
The Earth is orbited by one moon.
MARS
Mars is the god of War. In Greek mythology, he is known as
Ares. The planet probably got this name due to its red color;
Mars is sometimes referred to as the "Red Planet." The name of
the month March derives from Mars.
Mars is a small, rocky planet which is cold and lifeless.
The first spacecraft to visit Mars was Mariner 4 in 1965.
Several others followed including the two Viking landers in 1976.
After a long break, Mars Pathfinder landed successfully on Mars
on July 4, 1997.
Mars has permanent ice caps at both poles made up mostly
of solid carbon dioxide. We know this as "dry ice."
Very strong winds and vast dust storms sometimes blow through
the entire planet for months!
Mars has two tiny moons which orbit very close to the
surface. Their names are Phobos and Deimos.
THE ASTEROID BELT
The Asteroid Belt is a region between the inner planets and
outer planets where thousands of asteroids are found orbitting
around the Sun.

6. Asteroids are chunks of rock and metal that orbit around
the Sun. Scientists think that they are loose material that never
formed into planets. The Main Asteroid Belt is located between
Mars and Jupiter.
The total mass of all the asteroids is less than that of the Moon.
There are 26 known asteroids larger than 200 km across.
We probably know 99% of the asteroids larger than 100 km
across. We've cataloged about half of the asteroids in the 10 km
to 100 km size range. Scientists still don't know much about the
smaller ones. It's thought there may be as many as a million 1 km
sized asteroids may exist.
Though they are rarely visible with the naked eye, many
asteroids are visible with binoculars or small telescopes.
JUPITER
Jupiter (a.k.a. "Jove") was the King of the Roman Gods and
the patron of the Roman state. To the ancient Greeks, he was
known as Zeus, ruler of the Greek Gods and Mount Olympus
Jupiter is a giant gas planet which is made up of about 90%
hydrogen and 10% helium.
Jupiter was first visited by the Pioneer 10 spacecraft in
1973. It was later visited by Pioneer 11, Voyager 1, Voyager 2
and Ulysses. The spacecraft Galileo is currently in orbit around
Jupiter and will be sending back data for at least the next two
years.
Jupiter is so big that you could cram 1,000 Earths inside of it!
That's *mighty big*!
It is thought that Jupiter's "Great Red Spot" is a storm of
swirling gas that has lasted for hundreds of years. Scientists
are still unsure as to how such a storm could last for so long.
Jupiter has 16 known moons! There are four large
"Galilean" moons, and 12 small ones.
SATURN

7. In Roman mythology, Saturn is the god of agriculture. He is
known as Cronus in Greek mythology. Cronus was the father of
Zeus (Jupiter). Saturn is the root of the English word
"Saturday."
Saturn is a giant gas planet which is made up of about 75%
hydrogen and 25% helium. It's most famous for its thousands of
beautiful rings.
Saturn was first visited by the Pioneer 11 spacecraft in
1979. Saturn has also since been visited by Voyager 1 and
Voyager 2.
Saturn's rings are made up mostly of water ice, but they
may also include rocky particles with icy coatings. The origin of
the rings of Saturn is currently unknown.
Saturn is made up of materials which are lighter than
water. If placed in a big pond, Saturn would float much like an
ice cube does in a glass of water.
Saturn has 18 known moons - more than any other planet!
There may very well be several small ones yet to be discovered.
URANUS
Uranus is the ancient Greek God of the Heavens - the
earliest supreme god. He was the father of Cronus (also known
as Saturn in Roman mythology) and of the Cyclopes and Titans
(predecessors of the Olympian gods).
Uranus is a giant gas planet which is made up of mostly
rock and various ices.
Uranus has been visited by only one spacecraft, Voyager 2
on Jan 24 1986.
Uranus spins differently from most planets. It seems to be
tilted "sideways" instead of right-side up. At the time of
Voyager 2's passage, Uranus' south pole was pointed almost
directly at the Sun.

8. Uranus has 15 known moons. Voyager 2 discovered 10 small
moons in addition to the 5 large ones already known. It is likely
that there are many more tiny moons within the rings.
NEPTUNE
In Roman mythology Neptune was the god of the Sea. He is
known as Poseidon in Greek mythology. The planet was probably
named after the sea gods because of its deep blue color.
Neptune is a giant gas planet which is most likely made up
of various "ices" and rock.
Neptune has been visited by only one spacecraft, Voyager
2 on Aug 25 1989. Almost everything we know about Neptune
comes from this one visit.
Depending on how far along Pluto is in its orbit (path
around the Sun), Neptune can be either the eighth or ninth
planet. Pluto's orbit is kind of wacky, and it sometimes crosses in
front of Neptune. When Pluto does this, Neptune is behind Pluto
- hence, it is the ninth planet for a short time.
Neptune has 8 known moons; 7 small ones and a large moon
called "Triton."
PLUTO
In Roman mythology, Pluto is the god of the underworld. In
Greek mythology, he is known as Hades, ruler of the underworld.
The planet received this name perhaps because it's so far from
the Sun that it is in perpetual darkness.
Pluto is a small, icy "dwarf planet". Scientists are still
unsure as to exactly what it's made of.
Pluto has not yet been visited by a spacecraft, but there's
one on the way!
Depending on how far along it is in its orbit (path around
the Sun), Pluto can be either the eighth or ninth planet. Pluto's
orbit is kind of wacky, and it sometimes crosses in front of

9. Neptune. When Pluto does this, Neptune is behind Pluto - hence,
it is the eighth planet for a short time.
Pluto has one moon. It's called "Charon."
Yikes! The astronomers have demoted Pluto so there are
now only eight planets; Pluto is no longer officially a planet. Now
they're calling it a "dwarf planet". Well, it's certainly a dwarf.
But planet or not, it's still out there and we can still try to find
out what it's made of and how it got there.