Six Regularities in the Solar System
that Are Not Demanded by Physical Laws
One of the senses in which the Solar System is a "system" is that its major components -- the Sun and the eight major planets -- share some properties that they don't have to share. These properties probably are the result of a common origin, and they probably are clues to details of that origin.
1) The planets all orbit around the Sun in about the same plane as the Sun's equator. The tilts of the planets' orbit planes to that of the Sun's equator are all less than ten degrees (and Pluto -- not a "major" planet, but still in most tables of solar system characteristics -- has an orbit that is tilted less than 20 degrees.) There is no physical law that demands that this be so, and the orbits of comets do not share this characteristic. It strongly suggests that the Sun and the planets formed from a disk of material.
2) The planets all orbit around the Sun in the same direction as the Sun spins. That direction is counterclockwise as seen from far above the Earth's or Sun's north pole. This regularity implies that the protoplanetary disk already had orderly dynamical characteristics before the planets formed.
3) The planets' orbits are all very nearly circular. More precisely, the planets' orbital eccentricities are all very small. The laws governing orbits of small bodies (like the planets) around a single very massive one (like the Sun) only demand that the orbits be elliptical, but allow any eccentricity. The orbits of comets and many asteroids, much smaller bodies than the eight major planets, are very "stretched out" with large eccentricities. That the planets' orbits are all so close to zero eccentricity implies that motions of most material within the protoplanetary disk had settled into this pattern of near-circularity before the planets formed.
4) Most of the planets' spin (rotation) axes are roughly parallel to the Sun's spin axis. Since the planets orbit the Sun in about the same plane as the Sun's equator, this also means that most of the planets' rotation axes are roughly perpendicular to their orbit planes. "Roughly" here means within 30 degrees. Earth's spin axis is tilted 23 1/2 degrees relative to a perpendicular to its orbit plane, for example, and Jupiter's axis tilt is about three degrees. Uranus is the big exception to this regularity; its axis lies almost in the plane of its orbit (that is, its axis tilt is about 90 degrees.) This is such a striking exception that many astronomers believe that Uranus started out with a more normal axis tilt and was subsequently disturbed by some cataclysmic event such as an enormous impact.
5) Most of the planets spin in the same direction as they orbit around the Sun. (The direction is counterclockwise as seen from above their north poles.) Uranus should be left out of consideration here, since which of its poles is "north" can't be firmly established due to its axis being essentially in the plane of its orbit. Venus, however, is a clear exception to this regularity. Its axis is tilted by less than five degrees, and yet it is spinning "backwards": clockwise as seen from above its north pole. As is the case with Uranus's odd axis tilt, this is such a striking abnormality that many astronomers invoke a catastrophic impact after formation to explain it.
6) The major satellite systems exhibit the above five regularities relative to their planet. They are, in a sense, miniature solar systems. ("Major satellite systems" refers to the larger moons of Jupiter, Saturn, and Uranus.) The "Galilean" satellites of Jupiter (Io, Europa, Ganymede, and Callisto) most strongly demonstrate this. All four of them orbit around Jupiter in Jupiter's equatorial plane, they orbit in the same direction that Jupiter spins, their orbits all have very small eccentricities, their spin axes are nearly parallel to Jupiter's, and they rotate in the same direction as they orbit around Jupiter. That we see similar regularities in these systems gave us confidence that we would see them in other solar systems once we became able to detect planets around other stars. This has not turned out to be the case in all instances, though: many of the first planets found around other stars have very eccentric orbits, for example.
This is a page of notes posted for a DeAnza College introductory astronomy class. Any questions or comments should be sent to its author, Sherwood Harrington.