THE SOLAR SYSTEM

For the next six weeks we will concentrate on trying to understand how the Solar System formed, how it has evolved since its formation, and its place in the Universe. In this context, at some point, we will address the questions of whether the Solar System and life is unique or whether there are other examples of these things in the Universe. sidebar -- it is only within your lifetimes that most of the detailed knowledge about the planets has become available. This is due primarily to the space program. You were born in a good time for planetary research.

The Solar System is located in the grouping of stars (galaxy) known as the Milky Way galaxy. The Milky Way is just one of many billions of galaxies in the Universe. The Milky Way galaxy is a spiral galaxy which contains roughly 100 billion stars and has a mass of roughly 10 - 100 billion M. The visible material of the Milky Way is contained in a thin rotating disk. The diameter of the disk is on the order 100,000 - 300,000 light years (1 light year = 6 trillion miles = 9.3 trillion kilometers) and has a thickness of several thousands of light years. The disk of the Milky Way is very thin.

The appearance of the Milky Way is quite striking (probably). In the center of the disk is a bulge out of which extends several spiral arms. The Solar System is located in one of the arms roughly two-thirds of the way out from the center of the disk. The Solar System orbits the center of our Galaxy with a speed of roughly 220 kilometers per second or 800,000 kilometers per hour!

Can you estimate the mass of our Galaxy?

Comment -- The Milky Way galaxy has another componet (in addition to the visible disk). There is a large spherical component to the Milky Way which is made up of nonluminous material ( Dark Matter). The Milky Way is dominated by this Dark Matter component. Perhaps 90 % of the Milky Way is nonluminous. This is also true for the Universe where as much as 99 % of the Universe may be nonluminous!!

The average mass of a star in the Milky Way galaxy is 0.3 M. The Sun is slightly larger than average, but it is not really that far out of line. It seems to be just 1 out of the hundred billion stars which make up the Galaxy. It is not atypical. Given such facts, it is not unreasonable to suspect that the Solar System is not unique. We will return to this issue at various times in the future.

The Solar System displays a seemingly incomprehensible amount of diversity. However, on a closer look, we see many regularities and patterns in the Solar System. It is these regularities and patterns which hold the key to our understanding of the mechanics of the formation of the Solar System.

DYNAMICAL REGULARITIES

I have already mentioned, at one time or another, all of the dynamical regularities of the Solar System. The principal ones are:

Some secondary regularities are:

The above general properties of the motions of the planets are quite significant. There are also some vexing properties which people also need to explain, e.g.,

DYNAMICAL REGULARITIES

Any successful theory for the origin of the Solar System must explain these facts.

PLANETARY REGULARITIES

The planets also show distinct regularities in that they can be divided up into three distinct classes: Terrestrials, Jovians, and Icy planets. In tabular form, we have

Planetary Properties. I

For a little bit of scale:

Mass of the Earth = 5.9742x10**24 kilograms

Mass of the Sun = 1.99x10**30 kilograms = 328,900 M

Diameter of the Earth = 12,756 kilometers

Diameter of the Sun = 1,392,000 kilometers = 109 D

So, adding up the masses of the planets (and including the asteroids which are < 0.001 M), we see that the Sun is > 99 % of the mass of the Solar System.

Random Comments

The regularities of the planets are clearly tied to how far they are away from the Sun. The Terrestrial planets are closer to the Sun than are the Jovian planets. In general, the Terrestrial planets are smaller and more dense than the Jovian planets. Let's concentrate on the density for a second (as this is an important point).

What do the observed properties of the planets imply?

  • Density

    Planetary Properties. II

    There is a clear distinction between the Terrestrials, Jovians, and Icy Planets in terms of their

    • distances from the Sun
    • masses and radii
    • interior compositions (densities)

    Comments--



    Jim Imamura
    Fri Oct 20 18:56:46 PDT 1995