Expert: Courtney Seligman - 7/21/2011

Question
Why all the planets and stars revolve around everything in an elliptical orbit rather circular? If you say, the planets are not moving at constant speed. Why? Big bang expelled everything out each at certain constant speed. right? If I'm wrong, please explain me. I'm waiting for your reply...

Answer
The expansion of the Universe that started with the Big Bang is not related to the orbits of planets or stars. It only affects the overall expansion of space, and has significant effects only over distances of hundreds of millions of light years. Within regions smaller than that, gravitational interactions between galaxies, or between stars within galaxies, or between planets going around one of those stars, are far far larger than any expansion effect, and the expansion of the Universe has no influence on their orbital motions.

As to whether those orbital motions are circular or elliptical, that is a matter of chance. If a planet happens to have a certain speed, called the circular orbital velocity, and is headed exactly perpendicular to the star it is orbiting, it will follow a circular orbit. And in fact, every planet has a speed equal to the circular orbital velocity at some point in its orbit. It is just that usually, at that time, the planet is headed a little upwards or downwards, relative to the direction to the star. As a result, the planet gets a little closer (gradually speeding up as it does so), and after a while, a little further away (gradually slowing down as it does so). The shape of the orbit that it follows can be close to a circle, but if it is headed up or down at an angle of even a millionth of a degree at the time it has the circular velocity, the orbit will technically be an ellipse, and not a circle.

Actually, any elliptical orbital shape is, all other things being equal, more or less equally likely. It is just that if you use, say, five decimal place accuracy to describe the shape of an orbit, then there is only one value (0.00000) that represents a perfectly circular orbit, and 99,998 numbers (from 0.00001 to 0.99999) that represent some kind of elliptical orbit. If there are several large planets in a planetary system, their mutual gravitational interactions skew the numbers closer to zero than to one, so the chance of a non-circular orbit is only a few thousand times greater than a roughly circular orbit, and nearly circular orbits are probably not that uncommon. In fact in our solar system we have three planets -- Venus, the Earth and Neptune -- which have orbits which are less than 2% different from a perfectly circular orbit. However, even though the difference is small, it is still there, and can be significant, at least in terms of miles.

As an example, the Earth's orbit is the same as a circular orbit to within 1.7% accuracy. That is, our distance from the Sun is only 1.7% smaller when we are closest to it in early January, and 1.7% bigger when we are furthest from it in early July (I know this is backwards from the seasons, but they are caused by the tilt of our axis, not our distance from the Sun). However, even though our distance only changes by 3.4% from the closest to the furthest distance (1.7% each way from the middle, doubled), that is still a variation of 3 million miles. But suppose we had a variation in distance a thousand times smaller than that -- only 3 thousand miles. That is smaller than the size of the Earth, so some part of the Earth would always be at the same distance from the Sun, and it would be fair to say that the orbit was, for all practical purposes, a perfectly circular orbit. It's just that out of all the orbits we could have between the one we have, and the one just mentioned, there would be nearly a thousand with a larger variation in distance, and only two with an equal or smaller variation in distance. So it is much more likely that we will have a slightly non-circular orbit than a perfectly circular one.

All the above assumes that whatever orbit we have stays the same forever. But as I mentioned, more nearly circular orbits are favored in systems with large planets. That's because the planets very slightly affect each other as they go around their star. Particularly when one planet is passing between the star and another planet, there are small tugs which add to or subtract from the star's effect on the planet. Those tugs gradually change the orbit, causing it to sometimes get a little bigger and sometimes a little smaller, and to sometimes get a little more nearly circular and sometimes a little less nearly circular. So even if a planet had a perfectly circular orbit now, it would not be likely to have one a few orbits from now, when it has had a chance to lap (or be lapped by) other planets, and affected by their gravity.

I might note that when people ask me about this topic, sometimes they are looking for some deep philosophical explanation, rather than a practical one, which is what I have tried to present here. If you were looking for a different sort of explanation, please feel free to send a follow-up question, with a more detailed explanation of what kind of answer you need, and whether you want a non-mathematical answer (as I tried to make this one), or a more formal, mathematically rigorous answer.