Download The Inner Planets

Inner Planets, Outer Planets, the Asteriod Belt and the Kuiper Belt
The Inner Planets
The inner planets, or terrestrial planets, are the four planets closest to the Sun: Mercury, Venus,
Earth, and Mars. Figure below shows the relative sizes of these four inner planets.
This composite shows the relative sizes of the four inner planets. From left to right, they are Mercury, Venus, Earth, and Mars.
Unlike the outer planets, which have many satellites, Mercury and Venus do not have moons, Earth
has one, and Mars has two. Of course, the inner planets have shorter orbits around the Sun, and they
all spin more slowly. Geologically, the inner planets are all made of cooled igneous rock with iron
cores, and all have been geologically active, at least early in their history. None of the inner planets
has rings.
The Outer Planets
The four planets farthest from the Sun are the outer planets. Figure below shows the relative sizes
of the outer planets and the Sun. These planets are much larger than the inner planets and are made
primarily of gases and liquids, so they are also called gas giants.
This image shows the four outer planets and the Sun, with sizes to scale. From left to right, the outer planets are Jupiter, Saturn,
Uranus, and Neptune.
The gas giants are made up primarily of hydrogen and helium, the same elements that make up most
of the Sun. Astronomers think that hydrogen and helium gases comprised much of the solar system
when it first formed. Since the inner planets didn’t have enough mass to hold on to these light gases,
their hydrogen and helium floated away into space. The Sun and the massive outer planets had
enough gravity to keep hydrogen and helium from drifting away.
All of the outer planets have numerous moons. They all also have planetary rings, composed of dust
and other small particles that encircle the planet in a thin plane.
Summary

The four inner planets have slower orbits, slower spin, no rings, and they are made of rock and metal.

The four outer planets have faster orbits and spins, a composition of gases and liquids, numerous moons,
and rings.
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The outer planets are made of hydrogen and helium, so they are called gas giants.
The Asteroid Belt
The Asteroid Belt is a region between the inner planets and outer planets (betenn Mars and Jupiter) where thousands of
asteroids are found orbiting around the Sun.
More than 7000 asteroids have been discovered. Several hundred more are discovered each year. There are
undoubtedly hundreds of thousands more that are too small to be seen from the Earth.
Asteroids are huge chunks of rock, and can be metal, too. They were formed with the rest of the Solar System around
5,000 million years ago. There have been about 4,000 asteroids recorded, but astronomers think there are over 10,000
asteroids that are over 62 miles across, and greater numbers of small ones. In 1801 a man named Piazzi discovered the
first asteroid. He thought it was a small planet, and named it "Ceres." Soon, other astronomers realized that it was not a
planet, and they called it an "asteroid" which means "like stars."
Asteroids circle around the Sun like other planets do.
Lots of asteroids have an oval-shaped orbit. They orbit the Sun between Mars and Jupiter. They call this ring of asteroids
"The Asteroid Belt". Normally they never come near our home planet, but still, very rarely, some of their orbits come
very close to The Earth. When their diameter reaches a certain point, they are called Killer Asteroids. Like our moon, The
Earth has some craters. These craters are caused by asteroids entering our atmosphere and striking the surface of the
Earth. Not all asteroids make it to the surface, though. They burn up while they are still in the atmosphere.
There are many different kinds of asteroids, but these are the main kinds:
Carbonaceous asteroids= These are common, stoney, and very dark, like coal.
Silicaceous asteroids= These can contain metal, and are very bright and stoney.
Metallic asteroids= These could possibly be the cores of much larger bodies.
The Kuiper Belt
The Kuiper Belt is a region of space in our solar system, shaped more like an ellipse than a circle,
which is similar to an asteroid belt. While the asteroid belt is mostly metal and rock, the Kuiper Belt is
composed almost entirely of icy chunks of various substances.
The Kuiper Belt is a disc-shaped region of icy objects beyond the orbit of Neptune -- billions of
kilometers from our sun. Pluto and Eris are the best known of these icy worlds. There may be
hundreds more of these ice dwarfs out there.
References:
http://www.ck12.org/earth-science/Inner-versus-Outer-Planets/lesson/Inner-versus-Outer-Planets/
http://kids.nineplanets.org/asteroid.htm
http://library.thinkquest.org/J002725/asteroidbelt.html
http://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs
http://www.universetoday.com/32515/kuiper-belt/