1. Solar systems are groups of planets orbiting a central star. The
Sun is the star at the center of our Solar System, which is composed of nine
planets, innumerable asteroids orbiting primarily between Mars and Jupiter,
and comets that reside mostly in the outer reaches of the Solar System in the
Oort cloud and Kuiper Belt.
2. The inner planets of the Solar System are Mercury, Venus, Earth, and Mars.
They are similar in that they are composed of silicate minerals forming rocks
around iron and nickel cores. The outer planets of the Solar System are Jupiter,
Saturn, Uranus, Neptune, and Pluto. These planets are composed primarily of
hydrogen and water, perhaps surrounding small rock and iron cores (See Fig.
22.5 of your textbook).
3. Astronomical distances in the Solar System are measured in astronomical
units (A.U.). One A.U. is equal to the mean distance of the Earth from the
Sun, approximately 150,000,000 (150 million) kilometers. Pluto, the most distant
planet, is approximately 40 A.U. from the Sun.
4. Our Solar System is part of the Milky Way galaxy which in turn is part
of the Universe. Astronomers believe the Universe formed about 14 billion
6. Our Sun, the planets, and other celestial bodies in the Solar System are
believed to have coalesced from a very large, somewhat flattened rotating
mass of gas and dust about 4.6 billion years ago. The gas and dust from which
the Solar System formed is called the solar nebula. Similar nebulae (plural
of nebula) are very commonly observed throughout the Universe with telescopes,
and each star or planet is thought to have formed from the gravitational collapse
of a nebula or portion of a nebula.
7. The Moon is a satellite that orbits Earth, and is the only other body
in the Solar System on which humans have walked. The surface of the Moon is
thoroughly covered by craters, surface depressions formed by impacts with
meteors of various sizes. The fact that the Moon's surface shows so many craters
is evidence that many geologic processes found on Earth are not active on
the Moon. Brightly reflective areas of the Moon are known as lunar highlands
whereas dark areas are called maria. The difference in color of these two
lunar provinces is due to differences in the composition of rocks that form
them. Highland areas are formed of an intrusive igneous rock called anorthosite
while maria are composed of a volcanic rock called basalt.
8. Mercury is the planet closest to the Sun, and its surface is completely
covered with impact craters. It is too small and close to the sun to retain
an atmosphere. As a consequence, it is subject to broad temperature extremes.
The side of the planet facing the Sun can be as hot as 430 degrees Celsius,
while the side facing away from the Sun can be -173 degree Celsius.
9. Venus is the 2nd planet from the Sun. The surface of Venus is not visible
because it is cloaked by thick clouds in the atmosphere. Venus' atmosphere
is composed almost entirely of carbon dioxide, creating an extreme greenhouse
effect that raises the surface temperature of the planet to 480 degrees Celsius.
Radar can penetrate the thick clouds, and has provided scientists with very
detailed maps of the planetary surface revealing impact craters, volcanoes,
lava flows, and other tectonic features the look similar to some geologic
features on Earth.
10. Mars is the 4th planet from the Sun. Mars has a very thin atmosphere
composed mostly of carbon dioxide. The surface of Mars has many Earth-like
features including sand dunes, river channels, lake basins, canyons and gullies,
and volcanoes. Mars also has ice caps (composed of both water and carbon dioxide
ice) in both of its polar regions. Networks of valleys and channels across
the Martian surface suggest the presence of liquid water flowing across the
planet's surface during its early history.
11. Jupiter is the 5th planet from the Sun, and the largest planet in the
Solar System. Jupiter is composed primarily of hydrogen gas, but may have a
small solid core. One of Jupiter's moons (Io) is known to have active volcanoes.
Another moon, Europa, is believed to have a liquid water ocean beneath a surface
cover of ice.
12. Saturn is the 6th planet from the Sun, and the 2nd largest planet in
the Solar System. It is surrounded by a number of prominently visible rings.
The rings are composed of innumerable rock fragments orbiting the planet,
believed to represent fragments generated by collisions among many very small
moons or between a small moon and a comet whose orbit came too close to the
13. Uranus is the 7th planet from the Sun. Relatively little is known about
this planet because it is so far away. A few space vehicles have passed close
enough to Uranus to provide pictures of the planet. Its atmosphere is composed
mostly of hydrogen and methane. An unusual feature of Uranus is that its rotational
axis is inclined such that it rotates "on its side" relative to
the other planets.
14. Neptune is the 8th planet from the Sun, and like Uranus, has an atmosphere
composed mostly of methane and hydrogen. Pictures of Neptune show that there
are "storms" in the atmosphere similar to those observed in the
atmosphere of Jupiter.
15. Pluto is the most distant planet in our Solar System. Little is known
of this planet because it is so far away. It was discovered by astronomers
16. Asteroids are small rock bodies orbiting in a broad region between Mars
and Jupiter known as the asteroid belt. Asteroids are thought to be fragments
of small planets (planetesimals) that failed to coalesce into a larger planet
from the solar nebula due to the gravitational field of Jupiter.
17. Comets are small bodies in the farthest reaches of the Solar System that
are believed to be composed of ice of various compositions mixed with dust
and rocky debris. Some comets have highly eccentric orbits that bring them
through the inner Solar System where they can be illuminated by the Sun and
visible from Earth without the aid of telescopes. Some comet orbits are very
well known, making it possible to predict when they will be seen from Earth.