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Unit 2 Lesson 6 Small Bodies in the Solar System
Bigger Is Not Better
Where are small bodies in the solar system?
• Scientists estimate that there are up to a trillion small bodies in the solar system. They lack
atmospheres and have weak surface gravity.
• The largest of the small bodies, the dwarf planets, are found in regions known as the asteroid
belt and Kuiper belt.
• The Kuiper belt is located beyond the orbit of Neptune. It contains Kuiper belt objects and
comets.
• Comets are also located in the Oort cloud, which is a region that surrounds the solar system
and extends almost halfway to the nearest star.
• Two other types of small bodies, asteroids and meteoroids, are located mostly between the
orbits of Venus and Neptune.
What are dwarf planets?
• A dwarf planet is a celestial body that orbits the sun and is round because of its own gravity.
• A dwarf planet does not have the mass to have cleared other bodies out of its orbit around the
sun.
• Five dwarf planets have been identified: Ceres, Pluto, Eris, Haumea, and Makemake.
KBOs
What are Kuiper belt objects?
• The Kuiper belt is a region of the solar system that begins just beyond the orbit of Neptune.
• The Kuiper belt extends outward to about twice the orbit of Neptune, a distance of about 55
astronomical units (AU).
• A Kuiper belt object (KBO) is any of the minor bodies in the Kuiper belt. They are made of
methane ice, ammonia ice, and water ice.
Pluto: From Planet to KBO
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Until 2006, Pluto was considered to be the ninth planet in the solar system.
Beginning in 1992, Kuiper belt objects began to be discovered beyond Neptune’s orbit, some
of which had similar size and composition as Pluto.
In 2006, Pluto was redefined as a “dwarf planet” by the International Astronomical Union.
Many large KBOs have satellites.
Pluto, the second-largest KBO, has Charon as its largest satellite.
Some KBOs and their satellites, such as Pluto and Charon, orbit each other.
KBOs
What do we know about comets?
• A comet is a small body of ice, rock, and dust that follows a highly elliptical orbit around the
sun.
• All comets have a nucleus that is composed of ice and rock. Most comet nuclei are between 1
km and 10 km in diameter.
• If a comet approaches the sun, solar radiation and heating cause the comet’s ice to change to
gas.
 A coma is a spherical cloud of gas and dust that comes off the nucleus.
 The ion tail of a comet is gas that has been ionized by the sun. This ion tail always points away
from the sun.
 A second tail made of dust and gas curves backward along the comet’s orbit. This dust tail can
be millions of kilometers long
 How does a comet change as it orbits the sun?
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Collisions between objects in the Kuiper belt produce fragments that become short-period
comets.
Short-period comets take fewer than 200 years to orbit the sun.
Short-period comets have short life spans. Every time a comet passes the sun, it may lose a
layer as much as 1 m thick.
Long-period comets come from the Oort cloud. They may take up to hundreds of thousands of
years to orbit the sun.
The Oort cloud is a spherical region that surrounds the solar system.
Comets can form in the Oort cloud when two objects collide, or when the gravity of a nearby
star sends an object into the inner solar system.
On the Rocks
What do we know about asteroids?
• An asteroid is a small, irregularly shaped, rocky object that orbits the sun.
• Most asteroids are located in the asteroid belt between the orbits of Mars and Jupiter.
• The asteroid belt contains hundreds of thousands of asteroids, called main-belt asteroids.
• Groups of asteroids are also located in the orbits of Jupiter and Neptune (called Trojan
asteroids) and in the Kuiper belt.
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Some asteroids are called near-Earth asteroids. These asteroids cross the orbits of Earth and
Venus.
Where are most asteroids located in the solar system?
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The composition of asteroids varies.
Some are rich in carbon.
Others are rocky, with cores of iron and nickel.
Some have a rocky core surrounded largely by ice.
Some asteroids appear to be piles of rock loosely held together.
Others contain economic minerals such as gold, iron, nickel, cobalt, and platinum.
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Burned Out
What do we know about meteoroids, meteors, and meteorites?
• A meteoroid is a rocky body, ranging in size from that of a sand grain to that of a boulder,
which travels through space.
• A bright streak of light that results when a meteoroid burns up in Earth’s atmosphere is called a
meteor.
• A meteorite is a meteoroid that reaches Earth’s surface without burning up.
• Meteoroids come from the asteroid belt, Mars, the moon, and comets.
• Most meteoroids that enter Earth’s atmosphere do not reach Earth’s surface. Many explode in
the upper atmosphere; others skip back into space.
• Large meteoroids that enter Earth’s lower atmosphere or strike Earth’s surface can be
destructive.
• Meteorites can be divided into three general groups. Stony meteorites, made of silicate
minerals, are the most common form.
• Iron meteorites, composed of iron and nickel, form a much smaller group of meteorites.
• Stony-iron meteorites, composed of silicate minerals, iron, and nickel, are the rarest group of
meteorites.