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Chapter 27 – Planets of the Solar System
The early models of our solar system placed the Earth in the center are referred to as
geocentric models. These models did not explain the apparent backwards motions, called
retrograde motion of some planets. Nicholas Copernicus is credited as developing a model with
the Sun at the center which is referred to as heliocentric.
Johannes Kepler developed three laws explaining planetary motion. His first law, called the Law
of Ellipses, states that the shape of planets orbits is an ellipse with Sun at one focal point. The
flatness of an ellipse is called eccentricity, which is calculated by dividing the distance between
the focal points by the major axis distance. Eccentricity ranges from 0.0 (a circle) to 1.0 (a
straight line).
The second law, called the Law of Equal Areas, states that a planet sweeps out equal areas in
equal amounts of time. This implies that the closer the planet is to the Sun, the faster it is
moving in its orbit, which is due to the greater influence of gravity closer to the Sun.
Kepler’s third law, the Law of Periods, states that the cube of a planet’s average distance from
the Sun is directly proportional to the square of its orbital period. This relationship is also true
for moons revolving around a planet.
There are eight planets revolving around one star in our solar system. The relative sizes, and
orbits (not to scale) are found in your text on page 888. Pluto is no longer considered a planet
(it is considered to be a dwarf planet), due to its orbit shape and size compared to objects near
it. The four inner planets – Mercury, Venus, Earth, and Mars – are referred to as “Earth –like”
or terrestrial. The four outer planets – Jupiter, Saturn, Uranus, and Neptune – are referred to
as “Jupiter – like” or jovian. Between the four inner planets and the four outer planets is the
asteroid belt, where most of our asteroids are found. Most comets are found in the outer parts
of the solar system called the Kuiper Belt.
The four inner planets have the following general characteristics: few or no moons, no rings, a
rocky iron core, small size, and a slow rotation. Mercury’s surface is similar to that of our
Moon, and has a tremendous range in temperatures due to a very thin atmosphere. Venus has
extremely high temperatures due to the runaway greenhouse effect of its dense carbon
dioxide atmosphere. It is known as Earth’s twin due to their similar size, composition, and
rotation. Earth is the only planet with liquid water. Mars has a reddish color due to the iron
oxides in its crustal rocks, and has the largest volcano in the solar system called Olympus Mons.
The outer planets have the following characteristics: many moons and rings, a composition of
mostly Hydrogen and Helium gases, large size, and fast rotations. Jupiter has a gigantic storm
called the Great Red Spot. Saturn has wide thin rings made of stone and ice. Uranus rotates on
its side with one pole pointing toward the Sun, and has a blue-green color due to the methane
in its atmosphere. Neptune has a large storm called the Great Dark Spot.