planets

deuce916: Do all planets orbit something, like earth around its sun, or do some just "hang" in space?

Geoff: I don't think anything in space just hangs there. Everything orbits something. It is quite possible that there are planets which have been thrown out of solar systems by either collisions or slingshot out of the stars gravitational field, probably done during the formation of the solar system.

However, these planets will be hurtling through interstellar space, and yet still part of the swirling galaxy, they would not be able to achieve escape velocity of the galaxy's gravitational well.

This means that they still orbit the galactic hub (just like all of the stars).

deuce916: Thank you Thor

illwerral: Not to rain on the parade, but part of the definition of a planet according to the IAU involves orbiting the Sun. Now of course that's a little odd since that would preclude ever finding planets outside our solar system, however orbiting a star would be reasonable to me.

And then, of course, on an interesting physics side-note.... the earth doesn't orbit the sun, nor does any other planet. They orbit a common center of mass, which in our solar system appears to be (but is only very close to) the sun.

In fact, the motion of our planets is not even circular, only very close to circular. It's more accurate to say that the orbits are elliptical with the sun at one focus of the ellipse, although this statement is still an approximation, although a much more accurate statement than the former.

Planetary motion is simple if you look at it from a simple perspective, however there is much to consider in the detailed study that physicists and astronomers engage in!

By the way... nothing 'hangs' in space! There is no fixed reference point in outer space, which means there is no notion of absolute motion or absolute stillness. Everything is moving with respect to everything else. And of course, there's too much in the way of gravitational fields in the universe to allow an object to simply not move. It will always have some force exerted on it by other celestial bodies hence it will always be accelerating a little bit at least.

Kalla: There are confirmed cases of rogue planets that float free through space, having been ejected from their former solar system by a supernovae or other such cosmic disturbance, and as Thor said, nothing just hangs in space. Everything follows a gravitational pull, be it that of another star, a blackhole, quasar, whatever you like, and even if something that does not appear to revolve around a central star (such as a galaxy), it is still being pulled to and against other galaxies (which can be seen in such instances as galactic collissions ie NGC 2207 and IC 2163 or even the Milky Way and Andromeda galaxies that are set to collide in several billions of years)

peak3433538: If everything orbits something, but what about meteor? So many meteors is floating in the solar system, and they not orbit some planets; they have themselves orbit that is not steady.

Geoff: They still orbit the sun, just in a more erratic manner. Their orbits are affected by each other, the planets and occasional collisions between them.

It's not that they are just sitting their waiting for something to collide with them, it is simply that their orbit intersects with the orbit of another body.

peak3433538: SO happy for your reply!

peak3433538: Why do the meterors orbit in a erratic manner and some planet such as the earth is regular?

Geoff: Mainly because of the Earth's mass. All of the major planets are much larger than meteors or comets.

If two meteors collide then they are sent off at odd angles because they are almost the same size.

All of the planets have settled into orbits that do not come close to each other, if they did then they would have collided billions of years ago. So the only things that hit a planet are far smaller than it is. This means that planets don't get knocked into odd orbits.

peak3433538: thank s, good Joe! Did you ever read the books of how the solar system to form? Could you recommend some books to me?

peak3433538: thank s, good Joe! Did you ever read the books of how the solar system to form? Could you recommend some books to me?

slasian: And what about the two forces? The one that pushes and the one that pulls centrifugal and centripetal forces, you forget them or what? Doesn’t they have any roll in planetary movement in addition to mass?

CheekyRob: Centrifugal and centripetal aren't forces. Centripetal acceleration is where an object moves round a curve and the object experiences a force due to it changing its direction. An example is standing on a spinning roundabout. As the roundabout spins and you hang onto it, your body wants to continue in a straight line (a tangent) but the roundabout is pulling you round with it, continually changing your direction. This makes you experience a force directed outwards. The force is due to the roundabout pulling you round, it is not a 'centripetal force'. Orbiting bodies don't experience this force because they are not hanging onto anything. They are in free fall through a vaccuum.

slasian: Really?