Expert: Courtney Seligman - 11/20/2008

Question
QUESTION: What is supporting the solar system; I mean, what is down it?
Quisiera saber si los demas planetas del sistema solar tienen inclinacion de su eje imaginario de rotacion y si esta, es en la misma direccion del de la Tierra.
Muchas gracias.Thanks a lot.

ANSWER: Nothing is "supporting" the Solar System, as there is no "down" in space, save for the direction to the nearest large mass.

In the case of the Solar System, that large mass is the Sun; so everywhere in the Solar System, things moving toward the Sun are heading downward (and going faster, as they head toward it, because its gravitational pull is pulling them forwards), and things moving away from the Sun are heading upward (and going slower, as they move away, because its pull is pulling them backwards).

The above paragraph assumes that you are far enough away from any planet that its gravitational pull is small, compared to that of the Sun. But if you were very close to a planet, as one of its moons would be, then the planet's gravity would determine the direction "down", and moving closer to the planet would be downward, and moving away from it would be upward, as in the case of the Earth.

As the Sun moves around our Galaxy, all the other stars are so far away that their gravities have no significant effect on anything in the Solar System, so all the planets follow the Sun, moving around it the same as if it were stationary. Neither they nor the Sun "falls" in any direction, save for the fact that they are orbiting our Galaxy, once every 150 to 200 million years. In a human lifetime, short as it is, that motion is absolutely uniform and unchanging straight-line motion.

Courtney Seligman

---------- FOLLOW-UP ----------

QUESTION: I would like to know if there's any matter, force or whatever keeping the solar system as it is, and floatting in the space.
Thank.

Answer
No, there isn't; at least, not in the sense I think you mean.

Your question suggests that if something weren't keeping the Solar System as it is, it would somehow "fall" to some other location. But nothing falls, in space or anywhere else, without some force to make it do so. Near the Earth, the Earth's mass exerts a gravitational force which causes things to fall toward it or orbit it, and near the Sun, the Sun's mass exerts a gravitational force which causes things to fall toward it or orbit it. But there is nothing in the essentially empty space within eighty trillion miles of the Solar System to make it "fall" or otherwise move, in any way whatsoever.

In such an empty space, an object wouldn't "fall" in any direction. It would sit where it is, without any force needed to keep it there. If there were other objects nearby, their gravity might cause it to move toward them, but if they were far enough away (as, for example, the stars are), then their mutual gravitational forces would be too small to cause any noticeable change in the object's motion. If it were stationary relative to its distant neighbors, it would remain that way. If it were moving in some direction relative to them, it would continue in that direction, without any change in its speed or direction, unless it happened to pass close enough to one of them to experience a significant gravitational interaction.

Our Solar System is like the above-mentioned object. It is so far from anything else that it can remain stationary, or move uniformly, relative to the distant stars. Its straight-line motion through space does slowly bend, over many millions of years, because of the combined gravitational force of the several hundred billion stars between us and the center of the Milky Way Galaxy. But other than that relatively small effect, there is no change in its motion over periods as long as several millions of years. The gravity of the Sun keeps the planets moving around the Sun, while the Solar System moves through the Galaxy; but no force is required to keep the Solar System (or the Sun) on its nearly uniform path.

Courtney Seligman