Table of Contents
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Mercury…………………………………………………….......……3
Venus ………………………………………………….……......……4
Earth ………………………………………….…………….......……5
Mars ………………..……………………………………….......……6
Jupiter ………………………………………………..…….......……7
Saturn ………………………………………………..…….......……8
Uranus ……………………………………………………........……9
Neptune ………………………………………………….......……10
Worksheet…………………………………………………………..11
Mercury
Mercury orbits the Sun every 87.97 Earth days at an
average distance of approximately 58 million km
(about 36 million mi), or 0.3871 astronomical unit
(AU). Mercury is the nearest point to the Sun
(perihelion) to 69.8 million km (43,380,000 mi/0.4667
AU) at its farthest point (aphelion). As a result,
sunlight is over 2.3 times stronger at perihelion than at
aphelion—during a single orbit Mercury receives as
much as 11 times the intensity of sunlight that Earth
does to a minimum of about 4.5 times. Mercury’s
orbital velocity is also about 46 percent faster at
perihelion than at aphelion. The planet’s orbit is tilted
7 degrees to the plane in which Earth orbits around
the Sun.
The point in Mercury’s orbit at which the planet is
closest to the Sun (perihelion) moves a tiny amount
every orbit, too much to be accounted for solely by the
gravitational influence of other planets. The
observation of these changes in Mercury’s perihelion
was one of the first confirmations of Einstein’s general
theory of relativity, which predicted such variation
due to the curvature of space caused by the enormous
mass of the Sun.
Like Earth and most other planets, Mercury turns
counterclockwise (west to east) when seen from its
north pole. Mercury’s axis is almost perfectly vertical,
unlike Earth’s axis, which is tilted 23.5 degrees. Radar
observations of Mercury show that it rotates only once
every 58.65 days, two-thirds of its period of revolution
around the Sun. As a result, only three rotations of the
planet occur during every two of its years. This
relationship is called a 3:2 spin-orbit resonance. It is
thought to be the result of differences in the pull of
the Sun’s gravity on Mercury as the planet moves
nearer and farther away in its orbit, an effect called
solid body tidal forces.
Venus
Venus (planet), second planet in distance from the Sun, but
the hottest planet in the solar system (hotter than Mercury).
Its hellish surface has broiling temperatures that make rocks
glow red under a crushing atmosphere that shrouds the
planet in thick layers of clouds. Venus is nearly the same size
as Earth, but takes 243 days to rotate on its axis in the
opposite direction. It also lacks a magnetic field and a moon.
Why conditions on Venus and Earth are so different remains
a major puzzle for planetary scientists. Venus circles the Sun
at a distance of 108 million km (67 million mi) in a little over
seven months (about 225 days). The planet was named for
Venus, the Roman goddess of beauty.
The thick cover of clouds around Venus meant that earlier
generations of astronomers using telescopes had little
information about conditions on the surface. Some
researchers speculated that Venus might be a lush tropical
world or an ocean planet drenched by thick rain clouds.
Other scientists predicted a dry desert swept by dust storms,
or with petroleum seas. The first clues that conditions on
Venus might be extremely hot came from microwave
observations in 1956. Earth-based radar studies in the 1960s
discovered the planet’s slow retrograde rotation. It would
take space probes to provide much more detailed
information.
Venus orbits the Sun at an average distance of about 108
million km (67 million mi), or 0.7233 astronomical units
(AU). An AU is equal to the average distance between Earth
and the Sun, or about 150 million km (93 million mi). Venus
is the nearest planet to Earth in distance at about 41 million
km (25.4 million mi) away at its closest approach.
Venus circles the Sun once every 224.7 days in a
counterclockwise direction, the same direction as the other
planets in the solar system. Its axis is nearly vertical and its
orbit is nearly circular so Venus does not experience seasons
the way Earth and Mars do because of their more tilted axes
and more elliptical orbits. Venus rotates very slowly, once
every 243 Earth days. Venus’s rotation is retrograde, which
means that the planet turns clockwise (from east to west) as
seen looking down on its north pole. Earth and most other
planets turn counterclockwise (from west to east).
Earth
Earth (planet), third planet in distance from the Sun in the
solar system, the only planet known to harbor life, and the
“home” of human beings. From space Earth resembles a big
blue marble with swirling white clouds floating above blue
oceans. About 71 percent of Earth’s surface is covered by
water, which is essential to life. The rest is land, mostly in
the form of continents that rise above the oceans.
Earth’s surface is surrounded by a layer of gases known as
the atmosphere, which extends upward from the surface,
slowly thinning out into space. Below the surface is a hot
interior of rocky material and two core layers composed of
the metals nickel and iron in solid and liquid form.
Unlike the other planets, Earth has a unique set of
characteristics ideally suited to supporting life as we know it.
It is neither too hot, like Mercury, the closest planet to the
Sun, nor too cold, like distant Mars and the even more
distant outer planets—Jupiter, Saturn, Uranus, Neptune, and
the tiny dwarf planet Pluto. Earth’s atmosphere includes just
the right amount of gases that trap heat from the Sun,
resulting in a moderate climate suitable for water to exist in
liquid form. The atmosphere also helps block radiation from
the Sun that would be harmful to life. Earth’s atmosphere
distinguishes it from the planet Venus, which is otherwise
much like Earth. Venus is about the same size and mass as
Earth and is also neither too near nor too far from the Sun.
But because Venus has too much heat-trapping carbon
dioxide in its atmosphere, its surface is extremely hot—
462°C (864°F)—hot enough to melt lead and too hot for life
to exist.
Although Earth is the only planet known to have life,
scientists do not rule out the possibility that life may once
have existed on other planets or their moons, or may exist
today in primitive form. Mars, for example, has many
features that resemble river channels, indicating that liquid
water once flowed on its surface. If so, life may also have
evolved there, and evidence for it may one day be found in
fossil form. Water still exists on Mars, but it is frozen in
polar ice caps, in permafrost, and possibly in rocks below the
surface.
Mars
Mars (planet), fourth planet in distance from the Sun in the solar
system. Mars is of special scientific interest because of its similarities
to Earth. It has an atmosphere with seasons and changing weather,
and its surface shows evidence of ancient water and volcanoes. The
length of its day and the tilt of its axis are similar to those of Earth.
Mars takes about two years to circle the Sun at an average distance of
228 million km (141.7 million mi). The possibility of life on Mars, now
or in the distant past, is one of the major questions in astronomy.
More space probes have been sent to Mars than to any other planet.
Mars is named for the Roman god of war. It is sometimes called the
red planet because it appears fiery red in Earth’s night sky, the result
of rusty, iron-oxide mineral dust that covers its surface.
Mars is a relatively small planet, with a diameter of about 6,794 km
(4,222 mi) or about half the diameter of Earth. Mars has about onetenth Earth’s mass. The force of gravity on the surface of Mars is
about three-eighths of that on Earth. Mars has twice the diameter and
twice the surface gravity of Earth’s Moon. The surface area of Mars is
almost exactly the same as the surface area of the dry land on Earth.
Mars is believed to be about the same age as Earth, having formed
from the same spinning, condensing cloud of gas and dust that
formed the Sun and the other planets about 4.6 billion years ago.
Mars has two moons, Phobos and Deimos, which are named after the
sons of the Roman god Mars. These tiny bodies are heavily cratered,
dark chunks of rock and may be asteroids captured by the
gravitational pull of Mars. Phobos orbits Mars once in less than one
Martian day, so it appears to rise in the west and set in the east,
usually twice each day. Deimos has the more ordinary habit of rising
in the east and setting in the west.
Mars appears as a fairly bright, red, starlike object in Earth’s night sky.
Because of the relative movements of Earth and Mars around the Sun,
Mars appears to move backward in the sky for a short time around
opposition, which is the time when the two planets are closest. As
Mars and Earth orbit the Sun, the distance between them varies from
about 56 million km (about 35 million mi) at their closest approaches
to about 375 million km (about 233 million mi) when the planets are
on opposite sides of the Sun. This change in distance causes the
apparent size of Mars to vary by more than a factor of 5 and its
brightness to vary by a factor of 25. Because the orbit of Mars is
elliptical and not circular, Earth and Mars approach each other more
closely during some orbits than others. For example, in late August
2003 Earth and Mars passed closer to each other than at any time
since 1924. The two planets will not get that close again until the year
2287.
Jupiter
Jupiter (planet), fifth planet from the Sun and the largest
planet in the solar system. The fourth brightest object in
Earth’s sky, after the Sun, the Moon, and Venus, Jupiter is
more than three times brighter than Sirius, the brightest
star. Due to its prominence in the sky, the Romans named
the planet for their chief god, Jupiter.
Jupiter orbits the Sun at an average distance of 778 million
km (484 million mi), which is about five times the distance
from Earth to the Sun. Jupiter’s year, or the time it takes to
complete an orbit about the Sun, is 11.9 Earth years, and its
day, or the time it takes to rotate on its axis, is about 9.9
hours, less than half an Earth day.
Unlike the rocky inner planets of the solar system (Mercury,
Venus, Earth, and Mars), Jupiter is a ball of dense gas and
has no solid surface. Jupiter may have a core composed of
rock-forming minerals like those trapped in comet ices, but
the core makes up less than 5 percent of the planet’s mass.
The force of gravity at the level of the highest clouds in
Jupiter’s atmosphere is about 2.5 times the force of gravity at
Earth’s surface.
Gas and clouds in Jupiter’s atmosphere travel at high speeds.
This phenomenon is not fully understood but it is related to
the planet’s high rate of rotation. These gases and clouds
travel faster at the equator than at higher latitudes. The
gases and clouds of the atmosphere are thrown outward as
the planet rotates, similar to the manner in which mud is
thrown outward from a spinning wheel. The balance
between gravity and this outward force, which is
proportional to the rotational speed of the atmosphere,
noticeably distorts the planet’s round shape. Higher speed at
the equator produces greater outward force, causing an
equatorial bulge, whereas lower speed at the poles gives
gravity the edge, leading to polar flattening. Jupiter’s
equatorial diameter is 143,000 km (89,000 mi), 6.5 percent
larger than the polar diameter of 133,700 km (83,000 mi).
The fact that Jupiter’s radius is 11.2 times larger than Earth’s
means that its volume is more than 1,300 times the volume of
Earth. The mass of Jupiter, however, is only 318 times the
mass of Earth. Jupiter’s density (1.33 g/cm3) is therefore less
than one-fourth of Earth’s density (5.52 g/cm3). Jupiter’s low
density indicates that the planet is composed primarily of
the lightest elements—hydrogen and helium.
Saturn
Saturn, sixth planet in order of distance from the Sun, and
the second largest in our solar system. Saturn’s most
distinctive feature is a giant system of rings that surrounds
the planet at its equator, stretching over twice the width of
the planet itself. The first person to see the rings was the
Italian scientist Galileo in 1610, using one of the earliest
telescopes. Space probes have greatly increased our
knowledge of Saturn, its rings, and its many moons. Flybys
by the Pioneer and the Voyager probes led to the Cassini
orbiter that began studying Saturn in detail in 2004. As seen
from Earth, Saturn appears as a yellowish object—one of the
brightest in the night sky. The planet is named for Saturn,
the Roman god of agriculture.
Saturn takes about 29.5 years to orbit the Sun at an average
distance of 1,435 billion km (891.5 million mi), or about 9.59
astronomical unit (AU). An AU is equal to the average
distance between the Earth and the Sun, or 150 million km
(93 million mi). Saturn rotates on its axis in about 10.5 hours
and is tilted at about 27°, giving the planet distinct seasons.
The diameter of Saturn is about 121,000 km (75,000 mi), and
its mass is equal to the mass of about 95 Earths, making it
the second largest planet in our solar system after Jupiter.
Saturn is 10 percent wider at its equator than at its poles and
has a more oblate (flattened sphere) shape than any other
planet.
Saturn's atmospheric constituents are, in order by mass,
hydrogen (88 percent) and helium (11 percent); and traces of
methane, ammonia, ammonia crystals, and such other gases
as ethane, acetylene, and phosphine comprise the remainder.
Voyager images showed whirls and eddies of clouds
occurring deep in a haze that is much thicker than that of
Jupiter because of Saturn's lower temperature. The
temperatures of Saturn's cloud tops are close to –176°C (285°F), about 27 degrees Celsius (49 degrees Fahrenheit)
lower than such locations on Jupiter.
The wind velocities in Saturn’s atmosphere change with the
planet’s seasons and are affected by the angle of the shadows
cast on the atmosphere by the planet’s rings. The Cassini
space probe found evidence that the velocity of winds at
Saturn’s equator has decreased from about 1,700 km/h (1,060
mph) to around 1,000 km/h (621 mph) since the early 1980s,
when the Voyager probes returned data about the planet.
Uranus
Uranus (planet), seventh planet in distance from the Sun, third
largest planet in diameter, and fourth largest in mass in the solar
system. Unlike other major planets, Uranus is tipped sideways on its
axis of rotation. It experiences extreme seasons, and its 13 rings and 27
known moons revolve around its equator nearly vertically to the
plane of its orbit around the Sun.
Because of its great size and mass, scientists classify Uranus as one of
the giant or Jovian (Jupiter-like) planets—along with Jupiter, Saturn,
and Neptune. Like more distant Neptune, Uranus is also classified as
an ice giant planet, mainly made of the ice-forming molecules water,
ammonia, and methane as a liquid mixture above what is thought to
be a rocky core. Its atmosphere is mainly hydrogen and helium, along
with methane gas that gives the planet a blue-green color.
Uranus looks like a star to the naked eye, but appears as a blue-green
disk through a large telescope—Uranus was the first planet
discovered by using a telescope. A flyby by the Voyager 2 space probe
in 1986 provided most of the information we have about the planet,
its rings, and its moons. Uranus is named after the god of the heavens
in Greek and Roman mythology.
Uranus orbits the Sun at an average distance of 2,860 million km
(1,780 million mi) in a period of 84 Earth years. The planet only
receives about 1/400th of the sunlight that Earth does. The diameter
of Uranus at its equator is 51,118 km (31,763 mi). The planet’s mass is
14.54 times greater than the mass of Earth, and its volume is 67 times
greater than that of Earth. The force of gravity at the surface of
Uranus is 1.17 times the force of gravity on Earth.
Uranus’s orbit varies from 2,740 million km (1,700 million mi ) to
3,000 million km (1,860 million mi) in distance from the Sun, with an
average distance of 2,860 million km (1,780 million mi), or 19.10
astronomical units (AU). An AU is equal to the average distance
between Earth and the Sun, or about 150 million km (93 million mi).
The orbit of Uranus traces out a flat region of space called the planet’s
orbital plane. The orbital plane of Uranus lies close to Earth’s orbital
plane. As a result, Uranus always crosses the same region of Earth’s
sky. Uranus takes 84 years to complete a single revolution around the
Sun, so a year on Uranus is 84 times longer than a year on Earth.
Uranus spins in place around its axis (an imaginary line that runs
down the middle of the planet) once every 17.25 hours (0.718 of an
Earth day), just as Earth spins once every 24 hours. The ends of the
axis mark the north and south poles of Uranus, just as Earth’s axis
marks the North Pole and the South Pole on Earth. Uranus rotates
about an axis (the way a plastic globe spins on a rod) that tilts 98°
into its orbital plane (the plane created by Uranus’s orbit around the
Sun). Another method is sometimes used to describe its rotation and
its axis. If the North Pole is considered the pole that projects above
the plane of its orbit, Uranus can be described as rotating in a
retrograde (clockwise) direction in -0.718 Earth days tilted at an angle
of 82.2° to the plane of its orbit.
Neptune
Neptune (planet), eighth planet in distance from the Sun,
fourth largest planet in diameter, and third largest in mass in
the solar system. Neptune’s gravity has a major influence on
the Kuiper Belt, a region of icy bodies in the outer solar
system that is a source of comets and includes the dwarf
planet Pluto, formerly counted as the ninth planet.
Because of its great size and mass, scientists classify Neptune
as one of the giant or Jovian (Jupiter-like) planets—along
with Jupiter, Saturn, and Uranus. Like Uranus, Neptune is
also classified as an ice giant planet, mainly made of the iceforming molecules water, ammonia, and methane as a liquid
mixture above what is thought to be a rocky core. Its
atmosphere is mainly hydrogen and helium, along with
methane gas that gives the planet a blue-green color.
Neptune orbits the Sun at an average distance of about 4,490
million km (about 2,790 million mi) in a period of 165 Earth
years and only receives about 1/900th the amount of sunlight
that Earth does. Neptune’s diameter at the equator is about
49,520 km (about 30,767 mi). Even though Neptune’s volume
is 72 times Earth’s volume, its mass is only 17.15 times Earth’s
mass.
Neptune has four rings and 13 known moons. The planet is
named after the sea god Neptune in Roman mythology.
Neptune orbits about 4,490 million km (about 2,790 million
mi), or 30 astronomical units (AU) from the Sun, beyond
Uranus. An AU is equal to the average distance between the
Earth and the Sun, or about 150 million km (93 million mi).
Neptune takes 164.79 years to complete a single revolution
around the Sun, so a year on Neptune is 164.79 times longer
than a year on Earth. The orbit of Neptune traces out a flat
region of space called the planet’s orbital plane. The orbital
plane of Neptune lies close to Earth’s orbital plane. As a
result, Neptune always crosses the same region of Earth’s
sky.
The planet spins around its axis once every 16 hours in a
counterclockwise direction, just as Earth spins once every 24
hours. The axis of rotation on Neptune tilts 29.6° into its
orbital plane (the plane created by Neptune’s orbit around
the Sun). This tilt gives Neptune almost Earthlike seasons.
(Seasons on Earth result from our planet’s 23.5° tilt into its
orbital plane.
WORKSHEET
Credits
 Colvin Georges Jr.
 Loryn Huff
 Eslyn Crandall
 Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft
Corporation. All rights reserved.