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Earth Science
Chapter 28 – Our Solar System
Chapter Overview
Section 1: Formation of the Solar System
1. A Collapsing Interstellar Cloud
Stars and planets form from interstellar clouds, which exist in space between the stars.
These clouds consist mostly of hydrogen and helium gas with small amounts of other
elements and dust. Gravity slowly draws matter together until it is concentrated enough to
form stars and planets.
• Collapse accelerates
The collapse of an interstellar cloud begins slowly but gradually accelerates until a flat
disc is formed with the majority of the mass in the center of the disk
• Matter condenses
The Sun formed when the center of the rotating disk reached a temperature and
pressure high enough to fuse hydrogen into helium. Within the disk, the temperature
varied greatly, from fairly high near the new-born Sun to much colder the farther from the
center.
⇒ The inner planets formed from elements with high melting points, while the outer
planets are formed from more volatile elements.
2. Planetesimals
Planetesimal – any object ranging in size from one kilometer to hundreds of kilometers in
diameter. They formed early in the solar systems history, and are the building blocks of
planets.
• The gas giants formed by the merging of icy planetesimals that contain mostly lighter
elements (hydrogen and helium) and hydrogen-rich compounds (methane and
ammonia).
• The terrestrial planets formed from planetesimals that were composed primarily of
elements that resist vaporization, the heavier elements and metal-rich compounds.
• The leftover material, called debris, formed asteroids, comets, and other icy/rocky bodies
that are found in the asteroid belt between Mars and Jupiter, in orbits in the plane of the
solar system beyond Neptune, and in a shell that surrounds the solar system.
3. Modeling the Solar System
Ancient astronomers assumed that the Sun, planets, and stars orbited a stationary Earth.
However, as more observations were done the geocentric model did not fit the data that was
collected.
• In 1543, Polish scientist Nicolaus Copernicus proposed that the Sun lie at the center of
the solar system and the planets orbited around it. This proposal helped to resolve some
of the issues that could not be explained by the geocentric model.
Section 2: The Inner Planets
Terrestrial planets – the four inner planets of the Solar System that share the common
characteristics of density and having solid, rocky surfaces.
1. Mercury
The smallest of the terrestrial planets, Mercury has very little atmosphere and a heavily
cratered surface.
2. Venus
Venus is nearest in size to Earth, but has an extremely thick atmosphere composed
primarily of carbon dioxide. Surface conditions are extreme in terms of temperature and
pressure.
3. Mars
Mars is often referred to as the red planet because of its reddish surface color. Mars has a
thin atmosphere of mostly carbon dioxide. The surface of Mars shares many features in
common with Earth. There are shield volcanoes (extinct) and a deep canyon that indicate
that tectonic activity once occurred on Mars, and there are erosional features—dried river
and lake beds, gullies, and runoff channels—that suggest that liquid water once existed on
the planet.
Section 3: The Outer Planets
The four outer planets are very different from the small, rocky inner planets. Jupiter and Saturn,
the two largest planets in the solar system, are considered gas giants, while Uranus and
Neptune are called ice giants. The outer planets are very different from the terrestrials with
regards to size, surface features, rings, and the number of moons
1. Jupiter
Jupiter is by far the largest planet in the solar system with a diameter 11 times larger than
Earth’s. Jupiter has no solid surface, and observations of the top of its atmosphere show
dynamic flow patterns and large storms that last years. The Great Red Spot is a giant vortex
(about three Earth diameters) of a storm that has lasted for hundreds of years.
2. Saturn
Saturn is slightly smaller than Jupiter, and is much lighter. In fact, if could find a large
enough bathtub, Saturn would float on the surface of the water. Saturn’s most prominent
feature is its complex ring circling the planet along its equator. The rings consist of ice
particles that range in size from microscopic to house sized chunks.
3. Uranus
Uranus is 4 times larger and 15 times heavier than Earth. Its blue appearance is caused by
the presence of methane in its atmosphere. The oddest feature of the planet is that is axis of
rotation is tilted so much (nearly 90O) that its north pole almost lies in its orbital plane.
4. Neptune
Neptune is smaller than Uranus but shares a similar blue color. Unlike Uranus, though,
Neptune atmosphere shows distinctive clouds and belts much like Jupiter and Saturn.
Section 4: Other Solar System Objects
1. Dwarf Planets
Dwarf planets are objects that are spherical, orbit the Sun, but have not cleared their orbital
path of smaller debris. Pluto is an example of a dwarf planet. Most dwarf planets are found
in the Kuiper Belt, a region in the solar system that lies beyond the orbit of Neptune.
2. Small Solar System Bodies
• Asteroids – small rocky bodies that vary in diameter and have pitted, irregular surfaces.
Most are found between the orbits of Mars and Jupiter.
• Kuiper Belt objects- bodies of ice and rock that lie beyond the orbit of Neptune.
3. Comets
Comets are small icy bodies that have highly eccentric orbits around the Sun., They range in
size from 1 to 10 kilometers and most originate in the Oort Cloud, a body of millions of icy
pieces that form a shell around the solar system.