The major planets are the largest satellites of the Sun, with a few exceptions.  Some of the moons are larger
than some of the planets.

Jupiter, Saturn, Uranus, and Neptune are classified as giant planets or Jovian planets.  The other planets are
classified as terrestrial planets.

The giant planets are physically very different than the terrestrial planets.

1.  The giant planets have extensive atmospheres with a small solid core, whereas the terrestrial planets have
      shallow atomospehres and the bulk of the planet is solid.

2.  The giant planets form a more homogeneous class in terms of their physical properties than do the terrestrial
     planets.  For example, there is a big difference between Pluto and the Earth.

3.  The gaint planets have many more moons than the terrestrial planets.

A.    PHYSICAL CHARACTERISTICS OF THE PLANETARY BODIES

Terrestrial-like Planets (Mercury, Venus, Earth, Mars, The Moon, some of Jupiter's moons
and most of the minor planets.  Some astronomers include the TNOs):

1. Relatively near the Sun, therefore relatively warm surfaces and atmospheres.

2. Somewhat small masses, and therefore, weak surface gravities.

3. Comparatively small diameters

4. High average density (approximately 3 gms/cm³).

5. Thin, shallow atmospheres of CO₂, NO₂, 0₂, N₂, H₂O (oxidized state).

6. Bulk of planet is rock (silicate material) and a metallic or rocky core

Giant or Jovian Planets (Jupiter, Saturn, Uranus, Neptune)

1. Far from the Sun and therefore, colder atmospheres (< -100C)

2. Large in mass with strong gravities

3. Large overall diameters

4. Low average densities ( approximately 1 gm/cm³)

5. Deep, dense atmospheres of H₂, He, CH₄, NH₃, and other H compounds (reduced state).

6. Most of planet comprised of various layers of gas, liquid, and ice. A rocky core may exist.
    For Jupiter, the core may be solid hydrogen.
 

Icy Planetary-like Bodies (Pluto, Eris, & most of the moons of planets beyond 5.0 AU,
    and the TNOs)

Comets  are not considered to be planetary bodies, though they have many of the characteristics below.

1. Very far from the Sun, and therefore, very cold.

2 Small mass

3. Small size

4. Low density

5. No atmosphere or very shallow atmosphere

6. Composed of a great amount of ice (frozen gases such as H₂O, CO₂, and NH₃) and a core made of  rock.

B.    COMPARATIVE CHEMICAL COMPOSITIONS

    The stars, including the Sun, have chemical compositions that are 70% H, 28% He, and all the other elements
amount to about 2%.

On the other hand, the terrestrial major and minor planets are composed mostly of O, Si, Mg, Al, and Fe.

    Pluto, its moon Charon, Eris, and the TNOs, have relatively large amounts of various ices (H₂O, CO₂,
and NH₃) in their structure.   Comets also consist of a relatively large amount of various frozen gases or ices,
but they are not planetary-like bodies.

    The giant or Jovian planets (Jupiter, Saturn, Uranus, and Neptune) have chemical compositions that are
similar to that of the Sun.  But they are not stars, because they have insuffient masses to generate energy by TNF.
 

Actually brown dwarfs that have masses greater than 13 - 17 Jupiter masses may innitiate some
limited amount of TNF for a short time.  However, this does not qualify them to be true
stars in the usual sense.

The IAU considers bodies with masses less than 13 Jupiter masses to be massive planets.

C.  Exoplanets

        Exoplnets are planetary bodies that have been found revolving in orbit  around another star
other than the Sun.  More than 1000 exoplanets have been discovered so far.  Most of these exoplanets
are very massive planets.  At first, only very massive exoplanets were discovered by detecting small changes
in the motion of a star in its orbit around the center of the galaxy.  These small changes were produced by
the gravitational interaction between the star and the planet.  Now exoplanets are being detected  by
photometric means.  That is, as a planet happens to transit its host star, it bloicks out some of the light of the
star.  Using very sensitive photodetectors, such very small dips in the star's light curve may be interpreted
to reveal the presence of an orbiting planet.  The satellite telescope Kepler has been in orbit around the Earth
now for about 2 years doing just that.  To date it has discovered hundreds of exoplanets.
 

D.  THE PLANETARY SATELLITES OR MOONS

    Below is a table of the number of satellites for each of the major planets:

                    Planet          No. of Satellites
                    Mercury                  0
                    Venus                     0
                    Earth                       1
                    Mars                       2
                    Jupiter                   64 (>250 very small moons)
                    Saturn                   61
                    Uranus                   27
                    Neptune                13
                    Pluto                       3

    Total  number of good sized moons:   170  as of 2011

    Source:  Observer's Handbook 2012, pg. 24, University of Toronto Press.

    The total number of moons may actually be greater than several hundred if we include some of the newly
discovered small objects orbiting the giant planets, especially Jupiter.

    At least two minor planets are also known to have a satellite.

    Two planetary satellites are larger than the major planet Mercury, and 7 planetary satellites are larger than
Pluto.   See the diagram below.

    The satellites of Earth, Mars and some of those belonging to Jupiter and Saturn consist mostly of rocky
material. Most of the larger moons of the outer planets contain a large amount of ice in their structure. This
is because of the very low temperatures found far from the Sun.

    Most of the planetary satellites (moons) belong to the giant planets.

    All the giant planets possess ring systems that are comprised of small bodies moving in orbits very close
together around their host planet. Each of these small bodies is actually a satellite also or a very small moon.
The rings of Saturn are the most conspicuous because a large number of the bodies are covered with ice.
 

E.    PLANETARY SURFACE FEATURES (TERMINOLOGY)

  1.    Craters:   Most are the result of meteorite impacts, although a few  are volcanic in origin.

    2.    Caldera:    A large opening in the top of a volcano that may contain several vents

    3.    Plain:    A relatively flat region of large expanse.  The maria of the Moon are lava plains.

    4.    Ridge.:  A pressure fold in a lava plain caused by cooling and shrinking.

    5.    Rays:   Bright streaks emanating from an impact crater.  In reality, they are radial bands of small,
                        secondary craters produced by material ejected from a larger impact site falling back on
                        the surface.

    6.    Rills:   Channel like grooves forming complex patterns on a planetary surface.  They appear to be
                       similar to meandering river gorges and may actually have formed by running water a long
                        time ago.   Some are chains  of collapse craters caused by sinking  along a fault  line.

    7.    Regolith:    A granular or powder like material forming a layer on the surface of a planetary body.
                            This material is produced by numerous small meteorites striking the hills, mountians,
                            and othr outcroppings of rock on  the surface.

F.    THE LUNAR SURFACE (Read corresponding chapter  in the textbook)

   The surface of the Moon consists of apparent dark and bright regions.  However, the far side of the Moon
has fewer and less distinct dark areas, as shown in the image below. The far side of the Moon was first imaged
by a Soviet Union spacecraft in 1961.

    Early telescopic observers saw that the dark regions were relatively smooth looking with no or few
craters.  Hence they concluded these regions were bodies of water which they named the Maria (plural)
of the Moon or seas.  One sea is called a Mare, such as Mare Serenitatis.

    We now know there is no water on the Moon and the maria are actually basins filled with solidified lava.
The maria are dark because they are smooth.  The basins were formed by large objects colliding with the
Moon about 3.9 billion years ago.  Notice on a photograph of the Moon that the maria are  roughly circular
in outline with some overlapping one another.

    The ages of the maria range from 3.1 to 3.9 billion years, as determined from radioactive dating of the
samples of rock obtained from the maria by the Apollo Astronauts during the 1970s.

    The bright regions of the Moon are called the Highlands.  These regions are actually higher in elevation
than the maria.  The highlands are bright because they are heavily cratered from an intense meteorite
bombardment that was in progress as the surface of the Moon cooled to form a solid  crust.

    Lunar rocks retrieved from the Highlands range in age from 4.0 billion to 4.6 billion years.

    The heavily cratered highlands of the Moon, which have not been covered with lava, indicate that
the Moon was subject to an intense meteorite bombardment prior to 4.0 billion years ago.

    Since there is much less cratering of the maria than the highlands and the maria are about a billion years
younger than the highlands, means the intense meteorite bombardment that produced the cratering of the
highlands had ended prior to the formation of the maria.   However, there is evidence to indicate the Moon
and other planets were subjected to what is being called "the Late Heavy Bombardmen" at about 3.9 billion
years ago.  It was during this interval that several large bodies bombarded the Moon to produce the Mare
basins and produced the lava flows that filled these basins thereby destroying any older craters that were
on these areas of the Moon's surface.

    The Moon's surface has essentially remained unchanged for the last billion years or more.
 

Photo of the Lunar Excursion Module that carried two astronauts down
to the surface from the command module (courtesy of NASA).

The Moon probably formed by the accretion of material ejected into orbit around the Earth as a result of a
collision between the very young Earth and another planetary body about the size of Mars.  This happened
about 4.55 billion years ago. A computer similation of this event is shown below.

The diagram below displays in graphical form the chronology of the evolution of the Lunar surface.
In this figure, the intensity of the cratering rate is plotted versus time since the Moon formed.
A meteorite colliding with the surface cannot result in a crater visible today unless the Lunar surface
has cooled and become solid.  The meteoroid bombardent abates simply because material is being
consumed.  The more massive meteoroids are used up faster the less massive ones so that only the
smaller meteoroids are striking the Lunar surace after a sufficiently long time.  Though not shown in
the diagram, the age of the crater Kepler is about 780 million years.

Diagram courtesy of Addison-Wesley