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The Major Planets
Eight major or classical planets are currently recognized by the International Astronomical Union
(IAU), the body that gives official names to objects in the solar system. The planets are
commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the
outer planets (Jupiter, Saturn, Uranus, and Neptune). The inner planets are small and are
composed primarily of rock and iron. The outer planets are much larger and consist mainly of
hydrogen, helium, and ice. Pluto, historically counted as the ninth planet, does not belong to
either group, and was reclassified as a dwarf planet by the IAU in 2006. Some astronomers have
objected to the reclassification of Pluto and the new IAU definition of a planet. Others feel
strongly that classifying Pluto as one of the largest members of a family of similar icy bodies
orbiting the Sun beyond Neptune provides a much better explanation of Pluto’s existence.
Mercury
The smallest of the rocky or terrestrial planets that include Venus, Earth, and Mars, Mercury has
a global magnetic field, but only a trace of an atmosphere and no moons of its own. It is the
second hottest planet after Venus. Mercury circles the Sun every 88 Earth days at an average
distance of 58 million km (36 million mi) and takes 59 days to turns on its axis. It retains an
ancient cratered surface that has changed little since the formation of the solar system, making
the planet of special interest to planetary scientists. Mercury was named for the fleet-footed
messenger of the gods in Roman mythology.
Mercury’s diameter is 4,879 km (3,032 mi), about 40 percent the diameter of Earth or about 40
percent wider than the Moon. Mercury’s volume and mass are about one-eighteenth that of
Earth. Mercury’s mean density, 5.4 g/cm³, is nearly as great as that of Earth and is higher than
that of any of the other planets. The force on the planet’s surface is about one-third of that on
Earth’s surface or about twice the surface gravity on the Moon and about the same as the
surface gravity on Mars, which is larger than Mercury but less dense. Two moons in the solar
system—Jupiter’s Ganymede and Saturn’s Titan—are also larger than Mercury but are much less
dense and hence have lower gravity (about the same as the Moon).
Venus
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.
Except for the Sun and the Moon, Venus is the brightest object in the sky. It is often called the
morning star when it appears in the east at sunrise, and the evening star when it is in the west
at sunset. In ancient times the evening star was called Hesperus and the morning star
Phosphorus, Eosphoros, or Lucifer.
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Earth
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.
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.
Moon
Telescopes have revealed a wealth of lunar detail since their invention in the 17th century, and
spacecraft have contributed further knowledge since the 1950s. Earth’s Moon is now known to
be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually
no atmosphere, and is lifeless. The Moon shines by reflecting the light of the Sun. Although the
Moon appears bright to the eye, it reflects on average only 12 percent of the light that falls on it.
This reflectivity, called albedo, of 0.12 is similar to that of coal dust.
The diameter of the Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of
Earth. The Moon’s mass is only 1.2 percent of Earth’s mass. The average density of the Moon is
only three-fifths that of Earth, and gravity at the lunar surface is only one-sixth as strong as
gravity at sea level on Earth. The Moon moves in an elliptical (oval-shaped) orbit around Earth at
an average distance of 384,403 km (238,857 mi) and at an average speed of 3,700 km/h (2,300
mph). It completes one revolution in 27 days 7 hours 43 minutes. For the Moon to go from one
phase to the next similar phase—as seen from Earth—requires 29 days 12 hours 44 minutes.
This period is called a lunar month. The Moon rotates once on its axis in the same period of time
that it circles Earth, accounting for the fact that virtually the same portion of the Moon (the
“near side”) is always turned toward Earth.
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Mars
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 one-tenth 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.
Phobos
Phobos orbits Mars at an average distance of only 9,378 km (5,827 mi), closer to its planet than
any other moon in the solar system. In fact, the moon is so close to the planet that tidal forces
caused by Mars’s gravity are slowly dragging the moon down. Phobos spirals inward about 1.8 m
(about 6 ft) per century. Around 50 million years from now Phobos will be so close to Mars that
the moon will either break apart, forming a ring around the planet, or crash into the Martian
surface. Because it is so near Mars, Phobos completes its nearly circular orbit every 7.65 hours,
whizzing around the planet three times each day. As seen from the surface of Mars, Phobos
crosses the Sun’s disk about 1,300 times a year. The moon is tidally locked, which means that it
keeps the same face toward Mars at all times, just as Earth’s Moon keeps the same face toward
Earth. Phobos therefore rotates once per orbit around Mars.
Deimos
Deimos orbits Mars at an average distance of 23,460 km (14,580 mi), completing an orbit once
every 1.26 Earth days. The moon’s orbit is almost circular and is only slightly tilted relative to the
Martian equator. Deimos rotates once in exactly the same amount of time that it completes one
orbit, keeping one face toward Mars at all times, just as Earth’s moon shows only a single face as
seen from Earth’s surface.
Jupiter
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.
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Io
Io circles Jupiter in a slightly elliptical (oval-shaped) orbit over Jupiter’s equator at an average
distance of 422,000 km (262,200 mi). It completes an orbit once every 1.769 Earth days and
rotates once in the same period. Io is slightly larger than Earth’s moon, with a radius of about
1,820 km (about 1,130 mi). Like Earth’s moon, it ranks among the ten largest moons in the solar
system.
Europa
Europa rotates once in exactly the same amount of time that it takes to complete one orbit
around Jupiter and therefore keeps a single face toward the planet at all times. The moon’s
slightly elliptical (oval-shaped) orbit parallels Jupiter’s equator.
Europa is slightly smaller than Earth’s moon, with a radius of 1,565 km (972 mi). Like Earth’s
moon, it ranks among the ten largest moons in the solar system.
Ganymede
With a radius of 2,634 km (1,637 mi), Ganymede is even larger than the planets Pluto and
Mercury. The moon is only half as massive as Mercury, however, because about half of
Ganymede’s composition is frozen water, which has a lower density than Mercury’s rock and
iron.
Ganymede orbits Jupiter at an average distance of 1.07 million km (665,000 mi), completing an
orbit once every 7.155 Earth days. The moon is tidally locked, meaning that it rotates only once
per orbit and keeps the same face toward Jupiter at all times. Ganymede’s nearly circular orbit
parallels Jupiter’s equator.
Callisto
Callisto completes an orbit and rotates in the same amount of time, once every 16.69 Earth
days. Its nearly circular orbit parallels Jupiter’s equator.
Callisto is spherical and is the third-largest moon in the solar system. The moon has a radius of
2,403 km (1,493 mi), making it nearly the same size as the planet Mercury. Since Callisto consists
mostly of low-density water ice, however, the moon is only one-third as massive as rocky,
metallic Mercury. Callisto’s interior is probably not differentiated into a rocky core surrounded
by lighter icy material, like that of the other three large moons of Jupiter—Io, Europa, and
Ganymede. Instead, scientists believe that the entire moon is a mixture of rock and ice, with the
percentage of rock in the mixture increasing toward the moon’s center.
Saturn
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.
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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.
Mimas
Mimas orbits Saturn at a distance of about 186,000 km (about 116,000 mi) about once every 23
hours. Mimas’s orbit is circular and tilted only 1.5° relative to Saturn’s equator.
Mimas has a flattened or oblate shape, and measures about 397 km (about 246 mi) across, or
about one-ninth the diameter of Earth’s moon. Its rapid rotation around Saturn causes its
diameter to bulge nearly 10 percent larger at the equator than at the poles. Mimas’s internal
structure and composition are unknown, but measurements of its density cause planetary
scientists to theorize that it is made almost entirely of water ice, with very little rock.
Titan
Scientists think Titan may provide important clues about conditions on the primitive Earth, and
even how life began. Titan is spherical, measuring about 5,150 km (about 3,200 mi) in diameter,
larger than the planet Mercury. It is the second largest moon in the solar system, after Jupiter’s
moon Ganymede. Saturn’s first moon to be discovered, Titan was found in 1655 by Dutch
astronomer Christiaan Huygens. A space probe named for Huygens landed on the surface of
Titan in 2005. The moon is named after the Titans, a family of giants in Greek mythology.
Sun
The Sun is a huge mass of hot, glowing gas. The strong gravitational pull of the Sun holds Earth
and the other planets in the solar system in orbit. The Sun’s light and heat influence all of the
objects in the solar system and allow life to exist on Earth.
The Sun is an average star—its size, age, and temperature fall in about the middle of the ranges
of these properties for all stars. Astronomers believe that the Sun is about 4.6 billion years old
and will keep shining for about another 7 billion years.
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