Download Solar System World Book at NASA A solar system is a group of

Solar System
World Book at NASA
A solar system is a group of heavenly bodies
consisting of a star and the planets and other objects
orbiting around it. We are most familiar with our
own solar system, which includes Earth, seven other
major planets, and the sun. Our solar system also
includes many smaller objects that revolve around
the sun, such as dwarf planets, meteoroids, and
comets; and a thin cloud of gas and dust known as
the interplanetary medium. More than 100 moons,
also called
satellites, orbit
the planets.
Besides the sun,
Earth, and Earth's
moon, many
objects in our
solar system are
visible to the
unaided eye.
These objects
include the
planets Mercury, An orbiting solar telescope known as
the Solar and Heliospheric
Venus, Mars,
Observatory (SOHO) studies the
Jupiter, and
sun's interior, its atmosphere, and
Saturn; the
the solar wind, a stream of
brightest
electrically charged particles that
asteroids; and
flow from the sun's surface. The
occasional comets European Space Agency launched
the telescope in 1995. Image credit:
and meteors.
NASA/ESA/Solar & Heliospheric
Many more
Observatory
objects in the
solar system can be seen with telescopes.
Since the 1990's, astronomers have discovered many
planets orbiting distant stars, though the planets
cannot be seen directly. By studying the masses and
orbits of these planets, astronomers hope to learn
more about solar systems in general. For example,
our own solar system contains four small, rocky
planets near the sun—Mercury, Venus, Earth, and
Mars—and four giant, gaseous planets farther out—
Jupiter, Saturn, Uranus, and Neptune. Astronomers
were surprised to find that other stars have giant,
gaseous planets in close orbits. For example, a planet
nearly the size of Jupiter orbits the star 51 Pegasi
closer than Mercury orbits our own Sun.
Our solar system
Solar System (ART 6.2)
The sun is the largest and most important object in
our solar system. It contains 99.8 percent of the solar
system's mass (quantity of matter). The sun provides
most of the heat, light, and other energy that makes
life possible.
The sun's outer
layers are hot and
stormy. The hot
gases and
electrically
charged particles
in those layers
continually
stream into space
and often burst
out in solar
eruptions. This
flow of gases and
particles forms
the solar wind,
which bathes
everything in the
solar system.
The sun is much larger than Earth.
From the sun's center to its surface,
it is about 109 times the radius of
Earth. Some of the streams of gas
rising from the solar surface are
larger than Earth. Image credit:
World Book illustration by Roberta
Polfus
Planets orbit the
sun in oval-shaped paths called ellipses, according to
a law of planetary motion discovered by German
astronomer Johannes Kepler in the early 1600's. The
sun is slightly off to the side of the center of each
ellipse at a point called a focus. The focus is actually
a point inside the sun—but off its center—called the
barycenter of the solar system.
The inner four planets consist chiefly of iron and
rock. They are known as the terrestrial (earthlike)
planets because they are somewhat similar in size
and composition. The four outer planets are giant
worlds with thick, gaseous outer layers. Almost all
their mass consists of hydrogen and helium, giving
them compositions more like that of the sun than
that of Earth. Beneath their outer layers, the giant
planets have no solid surfaces. The pressure of their
thick atmospheres turns their insides liquid, though
they may have rocky cores.
Dwarf planets are round objects smaller than planets
that also orbit the sun. Unlike a planet, a dwarf
planet lacks sufficient gravitational pull to sweep
other objects from the region of its orbit. As a result,
dwarf planets are found among populations of
Page 1 of 3
smaller bodies. The dwarf planet Ceres, for example,
orbits in a region of space called the Main Belt
between the orbits of Mars and Jupiter. Ceres shares
the Main Belt with millions of smaller asteroids.
wind then carries the gas outward, forming it into a
long tail.
Astronomers
divide comets
Other dwarf planets orbit primarily beyond Neptune
into two main
in a region of space known as the Kuiper belt. They
types, long-period
share this region with many smaller, icy, cometlike
comets, which
bodies. Together, these objects are known as the
take 200 years or
Kuiper belt objects (KBO’s). Compared to the
more to orbit the
planets, KBO’s tend to follow irregular, elongated
sun, and shortorbits. Dwarf planets of the Kuiper belt include
period comets,
Pluto and a larger body designated 2003 UB313.
which complete
their orbits in
Ganymede, a moon of Jupiter, has
fewer than 200
craters and cracks on its surface.
Moons orbit all the planets except Mercury and
Asteroids and comets that hit
years.
The
two
Venus. The inner planets have few moons. Earth has
types come from Ganymede made the craters. The
one, and Mars has two tiny satellites. The giant outer
two regions at the cracks are due to expansion and
planets, however, resemble small solar systems, with
edges of the solar contraction of the surface. Image
many moons orbiting each planet. Jupiter has at least
credit: NASA
system. Long63 moons. Jupiter's four largest moons are known as
period comets originate in the Oort
the Galilean satellites because the
cloud, a cluster of comets far
Italian astronomer Galileo
beyond the orbit of Pluto. The
discovered them in 1610 with one
Oort cloud was named after the
of the first telescopes. The largest
Dutch astronomer Jan H. Oort,
Galilean satellite—and the largest
who first suggested its existence.
satellite in the solar system—is
Short-period comets come from
Ganymede, which is even bigger
the Kuiper belt. Many of the
than Mercury. Saturn has at least
56 moons. The largest of Saturn's Pluto is so far from Earth that even powerful objects in the Oort cloud and the
Kuiper belt may be chunks of rock
moons, Titan, has an atmosphere telescopes reveal little detail of its surface.
and ice known as planetesimals
thicker than Earth's and a diameter The Hubble Space Telescope gathered the
light for the pictures of Pluto shown here.
left over from the formation of the
larger than that of Mercury.
solar system.
Uranus has at least 27 moons, and Image credit: NASA
Neptune has at least 13. The giant planets probably
have more small moons not yet discovered.
Asteroids are minor planets. Some have elliptical
orbits that pass inside the orbit of Earth or even that
of Mercury. Others travel on a circular path among
Many dwarf planets, asteroids, and other bodies also
the outer planets. Most asteroids circle the sun in a
have smaller moons. Pluto’s moon measures half
region called the asteroid belt, between the orbits of
Pluto’s diameter. 2003 UB313 has a smaller moon
Mars and Jupiter. The belt contains more than 200
around 1⁄8 its diameter.
asteroids larger than 60 miles (100 kilometers) in
diameter. Scientists estimate that there are more than
Rings of dust, rock, and ice chunks encircle all the
750,000 asteroids in the belt with diameters larger
giant planets. Saturn's rings are the most familiar,
than 3/5 mile (1 kilometer). There are millions of
but thin rings also surround Jupiter, Uranus, and
smaller asteroids. Astronomers have even found
Neptune.
several large asteroids with smaller asteroids
orbiting them.
Comets are snowballs composed mainly of ice and
rock. When a comet approaches the sun, some of the
ice in its nucleus (center) turns into gas. The gas
shoots out of the sunlit side of the comet. The solar
Solar System (ART 6.2)
Page 2 of 3
Meteoroids are
chunks of metal
or rock smaller
than asteroids.
When meteoroids
plunge into
Earth's
atmosphere, they
form bright
The asteroid Ida is about 35 miles
streaks of light
called meteors as (55 kilometers) long. It is one of
they disintegrate. thousands of asteroids in the
Some meteoroids asteroid belt, a region between the
reach the ground, orbits of Mars and Jupiter. Image
credit: NASA
and then they
become known as meteorites. Most meteoroids are
broken chunks of asteroids that resulted from
collisions in the asteroid belt. During the 1990's,
astronomers discovered a number of meteoroids that
came from Mars and from the moon. Many tiny
meteoroids are dust from the tails of comets.
Heliosphere is a vast, teardrop-shaped region of
space containing electrically charged particles given
off by the sun. Scientists do not know the exact
distance to the heliopause, the limit of the
heliosphere. Many astronomers think that the
heliopause is about 9 billion miles (15 billion
kilometers) from the sun at the blunt end of the
"teardrop."
Formation of our solar system
Many scientists believe that our solar system formed
from a giant, rotating cloud of gas and dust known
as the solar nebula. According to this theory, the
solar nebula began to collapse because of its own
gravity. Some astronomers speculate that a nearby
supernova (exploding star) triggered the collapse. As
the nebula contracted, it spun faster and flattened
into a disk.
The nebular theory indicates that particles within the
flattened disk then collided and stuck together to
form asteroid-sized objects called planetesimals.
Some of these planetesimals combined to become
the nine large planets. Other planetesimals formed
moons, asteroids, and comets. The planets and
asteroids all revolve around the sun in the same
direction, and in more or less the same plane,
Solar System (ART 6.2)
because they originally formed from this flattened
disk.
Most of the material in the solar nebula, however,
was pulled toward the center and formed the sun.
According to the theory, the pressure at the center
became great enough to trigger the nuclear reactions
that power the sun. Eventually, solar eruptions
occurred, producing a solar wind. In the inner solar
system, the wind was so powerful that it swept away
most of the lighter elements -- hydrogen and helium.
In the outer regions of the solar system, however, the
solar wind was much weaker. As a result, much
more hydrogen and helium remained on the outer
planets. This process explains why the inner planets
are small, rocky worlds and the outer planets, except
for Pluto, are giant balls composed almost entirely of
hydrogen and helium.
Other solar systems
Several other stars have disk-shaped clouds around
them that seem to be solar systems in formation. In
1983, an infrared telescope in space photographed
such a disk around Vega, the brightest star in the
constellation Lyra. This discovery represented the
first direct evidence of such material around any star
except the sun. In 1984, astronomers photographed a
similar disk around Beta Pictoris, a star in the
southern constellation Pictor.
By the early 2000's, astronomers had discovered that
more than 50 stars like our sun have planets orbiting
them. In almost all cases, they found only one planet
per star. All the planets found are probably gaseous
with no solid surface.
Contributor: Jay M. Pasachoff, Ph.D., Field Memorial
Professor of Astronomy and Director, Hopkins
Observatory of Williams College. Pasachoff, Jay M.
"Solar system." World Book Online Reference Center.
2004. World Book, Inc.
http://www.worldbookonline.com/wb/Article?id=ar51896
0.
Page 3 of 3