What is the solar system? The solar system is the sun and any objects in orbit around the sun or in orbit around an object orbiting the sun. Most prominent of these objects are the planets. The nine planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. In addition to the planets are many objects of smaller size but also in orbit around the sun. These objects include the asteroids and comets. In addition there is dust and debris orbiting the sun, but we shall consider this insignificant and uninteresting.

How big is a solar system? If we consider only the planets, then the extent of the solar system is approximately 39 astronomical units. This would be the orbit of Pluto. However, if we consider comets, then the solar system is a much larger body. Cometary orbits may be on the order of 100,000 astronomical units. This is about one and one-half light years, or roughly one-third the distance to the nearest stars. To gain some insight into the size of the solar system consider the tip of your nose as the sun. Imagine the Earth to be one inch from your nose. Pluto would be a little more than one yard from your nose. The farthest comets would be more than one and one-half miles away.

Where did the planets, asteroids, and comets come from? Consider 3 main theories for their origin.

The first theory is the captures theory. Under this theory, the planets and other objects came within the gravitational pull of the sun. They were then unable to escape the sun's gravitational attraction. Under this theory however, it would be likely that the orbits of the planets would be in random orientation. However, we know that all the planets orbit the sun in roughly the same plane and in the same direction. Therefore this theory can be discarded.

A second theory had the planets spinning off the sun during the sun's evolution. This would explain why all the planets orbit in roughly the same plane. However, if this theory were correct then all the planets would be of the same composition. But we know this is not the case. The inner planets are made of rocky material while the outer planets are mostly gaseous. Therefore we discard this theory also.

Notice the importance of a theory explaining current observations. This is in addition to predicting future observations.

The final theory states that a disk of material formed around the protosun. This disk would form along the solar equator. It is within this disk that the planets would form. The fact that all the planets orbit in roughly the same plane supports this theory. In addition, the planets near the sun have a different composition and structure than the planets far from the sun. This would seem to make sense, as the nearer planets formed closer to the heat of the sun. This would also explain why all the planets orbit the sun in the same direction.

In order to further support this theory, astronomers search among newly formed stars for disks in which planets may be forming. One of the first successful views of such a system was Beta Pictoris. The images below show the disk surrounding this star in which planets may be forming.

How do the planets form in such a disk? The dust and debris in the disk will collide with each other and in so doing will begin to stick together either by gravitational attraction or by static electricity. As time goes on the pieces sticking together grow larger in size accumulating more and more of the nearby material. These larger chunks are referred to as planetesimals. Eventually, these planetesimals will grow larger to become the planets.

In the case of the asteroids the debris never formed into a large enough planetesimal to form a full planet. In fact, if all the asteroids were put together to form a single body it would not be as large as our Moon. However, their small size leads to another interesting twist in the theory for the solar system formation.

The asteroids, being smaller, formed completely, very early in the solar system history. Therefore they give us a clue as to what conditions existed before the solar system formed. Chemical evidence from meteorites, which generally originate from asteroids, indicates that a supernova explosion occurred shortly before formation of our solar system began. It is considered very likely therefore, that this supernova explosion triggered the formation of our solar system.

Finally, let us discuss some terms used in describing the planets and their position in the solar system or with respect to the Earth. Mercury and Venus have orbits inside the orbit of the Earth. These two planets are therefore referred to as inferior planets. The other six planets have orbits outside the orbit of the Earth, they are referred to as superior planets. The diagrams below show the orbit of the Earth and the orbit of either an inferior planet or a superior planet. Note the terms accompanying the diagrams. These terms are used in the following discussion.

When a planet orbits the sun, the time it takes to make one complete orbit is referred to as the sidereal period of the planet. In contrast to this, is the period required to align with the Earth in a specific configuration and repeating that configuration (for example, opposition to opposition). This period is referred to as the synodic period. The synodic and sidereal periods are different because of the motion of the Earth. By the time Earth, for example, returns to the far left position in the figure above (one sidereal period), Mars has moved. Therefore it takes a little while longer for Earth to catch up and align with Mars again. They will not align on the far left hand side either. So the opposition will occur at another position in the planets' orbits around the sun. For Earth, this means another time of year. So if opposition occurs in February one year, it will occur during a different month the next year.

In order to view the planets from Earth, we wish to have the planet as far away from the sun, as we view it, as is possible. For any inferior planet this best viewing occurs during greatest elongation. This is because the planetary position is at the largest angle away from the sun as it will get. For a superior planet this best viewing occurs during opposition. Because the orbits of the planets are ellipses and not circles some oppositions are better than other oppositions. You may therefore come across the terms least favorable opposition and most favorable opposition. The diagram below shows the true orbit of Mars and the Earth and points out least and most favorable opposition. Note that the orbits are closer together near most favorable opposition and farther apart near least favorable opposition.

Keep these terms and alignments in mind as we discuss the planets further in this course.

This page was last updated on 08/25/04.