Expert: Courtney Seligman - 4/16/2011

Question
Do scientists know yet what started the planetary straight line movement and do scientists know what caused the sun to have a gravitational pull on the planets? Thanks.

Answer
We know how gravity works (in a simple way, as described by Newton, and an equally simple but different way, as described by Einstein). But why gravity exists is not known, and probably cannot be known. That's just the way things are. Science is good at describing what happens in nature, and in many cases, figuring out how various things interact. But we can only make unprovable guesses about why the laws of physics are what they are.

The original motion of the planetary bodies is presumably due to the way the Solar System formed. A cloud of gas contracted (or was compressed) to a smaller size. As it did, the random motions of the atoms and molecules within the cloud averaged out to a certain direction (it doesn't make any difference what that direction was, and odds are it was completely random). As it continued to contract, the "net" motion of the cloud caused it to become a rotating disk of gas and dust surrounding the central star (the Sun) forming in its center. The planets built up from collisions of the dust grains rotating around the Sun, and as a result, ended up with about the same motion as the surrounding material. Since the gas and dust in the so-called Solar Nebula was all heading around the Sun in the same direction in nearly the same plane, the planets all ended up with orbits close to that plane, and in the same direction as its rotation.

If you want a more detailed discussion of the way this "planetesimal accretion" of dust in the Solar Nebula produced the planets, you should find it useful to refer to my web page on the origin of the solar system, at http://cseligman.com/text/ssevolve/ssorigin.htm