2. The solar system consists of the Sun, its nine
orbiting planets, and their moons as well as
asteroids, comets, and meteoroids. The planets are,
in order from the Sun: Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune, and Pluto.
The solar system probably formed from a cloud of
gas and dust that broke away from a larger cloud
about 4.6 billion years ago.
Gravitational forces caused the cloud to spin and
contract. The center, becoming densely packed and
extremely hot, formed the Sun. The outer material
formed a disk, which clumped into bodies that
collided and cooled to form the planets.
3.
4.
5. • VENUS
• Venus is the brightest object in
our sky, after the sun and moon.
Swirling clouds of sulfur and
sulfuric acid obscure Venus’s
surface and inhibited study of the
planet from Earth until technology
permitted space vehicles,
outfitted with probes, to visit it.
These probes determined that
Venus is the hottest of the
planets, with a surface
temperature of about 460° C
(about 860° F). Scientists believe
that a greenhouse effect causes
the extreme temperature,
hypothesizing that the planet’s a
thick clouds and dense
atmosphere trap energy from the
sun.
6.
7. • Mars (planet), one of the planets in the solar system, it is
the fourth planet from the Sun and orbits the Sun at an
average distance of about 228 million km (141.7 million
mi), or 1.524 astronomical units (AU).
AU is equal to the average distance between the
Earth and the Sun, or about 150 million km (93
million mi).
Mars is named for the Roman
god of war and is sometimes
called the red planet because it
appears fiery red in Earth’s
night sky.
8.
9.
10. Uranus’s blue-green color comes from the
methane gas present in its cold, clear
atmosphere. The dark shadings at the right
edge of the sphere correspond to the day-
night boundary on the planet. Beyond this
boundary, Uranus’s northern hemisphere
remains in a four-decade-long period of
darkness because of the way the planet
rotates. Scientists compiled this view of
Uranus from images returned from
Voyager 2 in 1986, when the probe was
9.1 million km (5.7 million mi) away from
the planet.
11. Neptune (planet), major planet in the solar system, eighth planet from the Sun
and fourth largest in diameter. Neptune maintains an almost constant distance,
about 4,490 million km (about 2,790 million mi), or 30 astronomical unit (AU)
from the Sun. An AU is equal to the average distance between the Earth and
the Sun, or about 150 million km (93 million mi).
Neptune revolves outside the orbit of Uranus and for most of its
orbit moves inside the elliptical path of the outermost planet Pluto
(see Solar System). Every 248 years, Pluto’s elliptical orbit
brings the planet inside Neptune’s nearly circular orbit for about
20 years, temporarily making Neptune the farthest planet from
the Sun.
12. Pluto is farther from the Sun than the other planets in the solar
system, although it occasionally moves in closer than Neptune due to
an irregular orbit.
The small, rocky, and cold planet takes 247.7 years to revolve around
the Sun. This artist's rendition depicts Pluto, foreground; its moon,
Charon, background; and the distant Sun, upper
