If we call T the length of the year and multiply T times itself (T*T) the result is called "T squared" and it is represented as T². Call the radius of a planets orbit R and multiply it times itself three times, R*R*R. The result is called "R cubed" and it is represented as R³. The computations are given below:

As you notice the ratios of T² to R³ are all close to 1.0. If all of the numbers were precisely correct the ratios would all have been exactly 1.0 This means that for each planet the squared of the length of its year is equal to the cube of the radius of its orbit. This is called Kepler's Law. From Kepler's Law if you know how far a planet is from the sun you can tell how long it takes for that planet to go around the sun.

Kepler's Law is exact and can be derived from Newton's laws of the motion of bodies. There is another "law" concerning the planets but it is only approximate and cannot be derived from other laws. It is often called Bode's Law because it was popularized by Bode, but it was actually discovered by Titius. It is a rule or formula for finding the orbit radiuses of the planets. The Bode-Titius Rule goes like this:

Take the series of numbers,

and multiply each by 0.3 to get:

To these numbers add 0.4 to get:

Now compare these numbers with the radiuses of the orbits of the planets (relative to the Earth's orbit radius):

The Bode-Titius Law gives a pretty fair approximation of the radiuses of the orbits of the planets. It appears to fail between Mars and Jupiter but that is where there are many asteroids and where they would have combined to form a planet if Jupiter was not so close by. The Law fails to give the right figure for Neptune but Pluto fits the value given by the law quite well. Astronomers think that Pluto is not a true planet but an escaped moon of one of the planets. As remarkable as the Bode-Titius Law is for predicting the orbit radiuses of the planets there is no explanation of the law in terms of other laws of physics.

In April of 2004 a planet-like object of approximately 1000 km in diameter was identified. Its distance is now approximately 86 A.U., not far from the figure of 77 A.U. predicted by the Bode-Titius "Law." Its orbit is quite eccentric so its distance from the sun may range from 75 A.U. to 100 A.U. over the course of its 10,000 Earth year revoltution about the sun. Tentative it is being called Sedna, after an Arctic goddess of the sea. It seems like a good choice, in that all planets except Venus and Earth were named after male Roman gods.

If an explanation for the Bode-Titius "Law" were to exist it might be in terms of the size of a radius zone for a planet within which no other planet could form because of the disruptive effects of the gravitational attraction of that planet. There is such a zone for a planet within which a satellite cannot form because of the stresses produced by the gravitational attraction of the planet. This zone is defined by the Roche Limit.

Incidentally, formulas analogous to the Bode-Titius Law applies to the radiuses of the orbits of the satellite systems of the planets Jupiter, Saturn, Uranus and Neptune. To see this analysis click Here.