AY 101 Lecture Notes - Lecture 10: Compressible Flow, Solar Wind, Momentum
Introduction to the Solar System
The solar system consists of the Sun, nine planets, some 60 or so moons, and assorted
minor materials (asteroids, meteoroids, comets, dust, and gas). All of these objects are
tiny in comparison to the distances that separate them. Imagine the solar system scaled
down such that distances to the planets could be spaced along a 10‐kilometer hiking trail.
On such a scale the Sun would be represented by a ball only 2.3 meters in diameter.
Origin and Evolution of the Solar System
The solar system was formed 4.6 ± 0.1 × 10 9 years ago. Astronomers have recognized
a number of observable facts about the solar system that are not otherwise the result of
obvious physical laws (for example, Kepler's Laws of Planetary Motion, which are the
direct result of the nature of gravity). But the foundation of science assumes that every
observable property must result from some cause. These features must therefore be the
direct result of how the solar system formed. The following list outlines these observable
facts:
• All planetary orbits lie nearly in a single plane; in other words, the solar system is
flat (the orbit of Pluto is an exception).
• The Sun's rotational equator is in the same flat plane.
• Planetary orbits are nearly circular (exceptions are Mercury and Pluto).
• The planets and Sun all revolve in the same direction, that is, a motion that is
west to east across the sky as viewed from Earth (what astronomers refer to
as direct motion).
• The Sun and planets all rotate in the same direction with obliquities (the tilt
between the equatorial and orbital planes) generally small (exceptions are
Venus, Uranus, and Pluto).
• Planets and most asteroids have similar rotational periods (exceptions are
Mercury, Pluto, and Venus).
• Planets are regularly spaced (this is often expressed in the form of a simple
mathematical progression, known as Bode's law).
• The major moons in planetary satellite systems resemble the solar system on a
smaller scale (circular orbits, uniform direction of revolution, in a flat plane with
regular spacing).
• Most angular momentum (∼ mass × velocity × orbital radius) of the solar system
is in the planets (99.8%), whereas most of the mass of the solar system is in the
Sun (99.8%). This may be expressed alternatively as a question: Why does the
sun rotate so slowly?
• Differences in chemical composition exist throughout the solar system, with
dense, metal‐rich (terrestrial) planets found close to the sun, but giant, hydrogen‐
rich (gas) planets only in the outer part of the solar system. In addition, the
chemical composition of meteorites, while similar, is not identical to all known
planetary and lunar rocks.
• Comets exist in a much larger, spherical cloud surrounding the solar system.
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Document Summary
The solar system consists of the sun, nine planets, some 60 or so moons, and assorted minor materials (asteroids, meteoroids, comets, dust, and gas). All of these objects are tiny in comparison to the distances that separate them. Imagine the solar system scaled down such that distances to the planets could be spaced along a 10 kilometer hiking trail. On such a scale the sun would be represented by a ball only 2. 3 meters in diameter. The solar system was formed 4. 6 0. 1 10 9 years ago. Astronomers have recognized a number of observable facts about the solar system that are not otherwise the result of obvious physical laws (for example, kepler"s laws of planetary motion, which are the direct result of the nature of gravity). But the foundation of science assumes that every observable property must result from some cause. These features must therefore be the direct result of how the solar system formed.