mercury venus earth mars jupiter saturn uranus neptune pluto

An Overview of the Solar System


Orbits

The solar system consists of the Sun; the nine planets, more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are probably also many more planetary satellites that have not yet been discovered.)

The inner solar system contains the Sun, Mercury, Venus, Earth and Mars:

inner solar system

The main asteroid belt (not shown) lies between the orbits of Mars and Jupiter. The planets of the outer solar system are Jupiter, Saturn, Uranus, Neptune and Pluto:

outer solar system

The first thing to notice is that the solar system is mostly empty space. The planets are very small compared to the space between them. Even the dots on the diagrams above are too big to be in proper scale with respect to the sizes of the orbits.

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 planetarium program Starry Night; there are also many other similar programs available, some free.)

Sizes

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 everything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter (the size of a grape). The Moon would be about 30 cm (about a foot) from the Earth. The Sun would be 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be the size of an atom but 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; 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; and the many small icy bodies beyond Neptune in the Kuiper Belt. 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

Hardcopy

The New Solar System
Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.

The Compact NASA Atlas of the Solar System
This 'road map' of the solar system is the definitive guide for beginning planetary science.

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: 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 officially categorized as planets are often further classified in several ways:

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

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

Home ... Intro/FAQ ... Overview ... Sun ... Data

Bill Arnett; last updated: 2004 Feb 19 The most recent version of this page can be found at http://www.nineplanets.org/overview.html