An Overview of the Solar System
Basics
The solar system consists of the Sun; the nine planets,
sixty eight (68) satellites of the planets,
a large number of small bodies
(the comets and asteroids), and the interplanetary medium.
The inner solar system contains the Sun,
Mercury,
Venus,
Earth and
Mars:
The planets of the outer solar system are
Jupiter,
Saturn,
Uranus,
Neptune and
Pluto:
The orbits of the planets are ellipses
with the Sun at one focus, though
all except Mercury and Pluto are very nearly circular.
The orbits of the planets are all more or less in the same
plane (called the ecliptic and defined by the plane of the
Earth's orbit). The ecliptic is inclined only 7 degrees from the plane
of the Sun's equator.
Pluto's orbit deviates the most from the plane of the ecliptic
with an inclination of 17 degrees.
The above diagrams show the relative sizes of the orbits
of the nine planets from a perspective somewhat above the ecliptic
(hence their non-circular appearance).
They all orbit
in the same direction (counter-clockwise looking down from above the Sun's north
pole); all but Venus, Uranus and Pluto also rotate in that same sense.
(The above diagrams show correct positions for October 1996 as generated by the
excellent Macintosh program
Starry Night;
there are also many other planetarium programs
available.)
The above composite shows the nine planets
with approximately correct relative
sizes (see another
similar composite
and a
comparison of the terrestrial planets
or Appendix 2 for more).
One way to help visualize the relative sizes in the solar system is to imagine
a model in which it is reduced in size by a factor of a billion
(1e9).
Then the Earth is about 1.3 cm in diameter (the size of a grape).
The Moon orbits about a foot away.
The Sun is 1.5 meters in diameter (about the height of a man) and
150 meters (about a city block) from the Earth.
Jupiter is 15 cm in diameter (the size of a large grapefruit) and 5 blocks away
from the Sun.
Saturn (the size of an orange) is 10 blocks away;
Uranus and Neptune (lemons) are 20 and 30 blocks away.
A human on this scale is the size of an atom; the nearest star would be over
40000 km away.
Not shown in the above illustrations
are the numerous smaller bodies that inhabit the solar system:
the satellites of the planets; the large number of
asteroids (small
rocky bodies)
orbiting the Sun, mostly between Mars and Jupiter but also elsewhere;
and the comets (small icy bodies) which
come and go from the inner parts of the solar system in highly elongated
orbits and at random orientations to the ecliptic.
With a few exceptions, the planetary satellites orbit in the same
sense as the planets and approximately in the plane of the ecliptic but
this is not generally true for comets and asteroids.
Classification
The classification of these objects
is a matter of minor controversy. Traditionally,
the solar system has been divided into planets (the big bodies
orbiting the Sun),
their satellites (a.k.a. moons, variously sized objects orbiting the planets),
asteroids (small dense objects orbiting the Sun) and comets
(small icy objects with highly eccentric orbits).
Unfortunately, the solar system has been found to be more complicated than
this would suggest:
- there are several moons larger than Pluto and two larger than Mercury;
- there are several small moons that are probably captured asteroids;
- comets sometimes fizzle out and become indistinguishable from asteroids;
- the Kuiper Belt objects and others like
Chiron don't fit this scheme well;
- The Earth/Moon and Pluto/Charon systems
are sometimes considered "double planets".
Other classifications based on chemical composition and/or point of origin
can be proposed which attempt to be more physically valid.
But they usually end up with either too many classes or too many exceptions.
The bottom line is that many of the bodies are unique; our present
understanding is insufficient to establish clear categories.
In the pages that follow, I will use the conventional categorizations.
The nine bodies conventionally referred to as planets are
often further classified in several ways:
- by composition:
- terrestrial or rocky planets:
Mercury, Venus, Earth, and Mars:
- The terrestrial planets are composed primarily of rock and metal
and have relatively high densities, slow rotation,
solid surfaces, no rings and few satellites.
- jovian
or gas planets: Jupiter, Saturn, Uranus, and Neptune:
- The gas planets are composed primarily of hydrogen and helium
and generally have low densities,
rapid rotation, deep atmospheres, rings
and lots of satellites.
- Pluto.
- by size:
- small planets: Mercury, Venus, Earth, Mars and Pluto.
- The small planets have diameters less than 13000 km.
- giant planets: Jupiter, Saturn, Uranus and Neptune.
- The giant planets have diameters greater than 48000 km.
- Mercury and Pluto are sometimes referred to as lesser planets
(not to be confused with minor planets which
is the official term for asteroids).
- The giant planets are sometimes also referred to as gas giants.
- by position relative to the Sun:
- inner planets: Mercury, Venus, Earth and Mars.
- outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto.
- The asteroid belt between Mars and Jupiter forms the boundary
between the inner solar system and the outer solar system.
- by position relative to Earth:
- inferior planets: Mercury and Venus.
- closer to the Sun than Earth.
- The inferior planets show phases like the Moon's when viewed from
Earth.
- Earth.
- superior planets: Mars thru Pluto.
- farther from the Sun than Earth.
- The superior planets always appear full or nearly so.
- by history:
- classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.
- known since prehistorical times
- visible to the unaided eye
- modern planets: Uranus, Neptune, Pluto.
- discovered in modern times
- visible only with telescopes
- Earth.
Pictures
Note: most of the images in The Nine Planets are not true color.
Most of them were created by
combining several black and white images taken thru various color filters.
Though the colors may look "right" chances are they aren't exactly
what your eye would see.
More General Overview
The Big Questions
- What is the origin of the solar system? It is generally agreed
that it condensed from a nebula
of dust and gas. But the details
are far from clear.
- How common are planetary systems around other stars?
There is now good evidence
of Jupiter-sized objects orbiting
several nearby stars. What conditions
allow the formation of terrestrial planets?
It seems unlikely that the Earth is totally unique but we still
have no direct evidence one way or the other.
- Is there life elsewhere in the solar system? If not, why is Earth
special?
- Is there life beyond the solar system? Intelligent life?
- Is life a rare and unusual or even unique
event in the evolution of the universe or is it adaptable,
widespread and common?
Answers to these questions, even partial ones, would be of enormous
value. Answers to the lesser questions on the pages that follow
may help answer some of these big ones.
Where to go next
... Introduction
... Overview
... Sun
... 
Bill Arnett; last updated:
2000 Sep 12
