OK science types, what's the answer to this?

Question

Planets orbit the stars, they're attracted due to gravitation. Why don't they get closer and closer till they collide? If the attraction's not strong enough for this then why don't planets escape their orbit?

Think I'm gonna have to put this to the vote, all the answers fall into either a planet's orbit is constant, or it's gradually getting pulled towards the star

Answer 1

it has to do with orbital mechanics....gravity can not only have forces and pull and push but also can be resposible for creating a stable environment...orbits are achieved by this principle and is only disturbed by the influence of outside objects or changes in the characteristics of the principal players for the stability

Answer 2

A planet's orbit is constant, if the planets were gradually being pulled toward the sun there would be a very visible change in weather and temperature, and I guess Mercury could have already burned up in the intense heat. And the planets don't collide into the sun or escape from their orbit, they stay in orbit because the sun's gravity put together with the planets' inertia is just enough to keep them in orbit.

Answer 3

the spinning centrifugal force is keeping the planets apart while gravity is attracting them. These forces don't always balance and so planets do not always have spherical orbits, instead the could be elliptical etc. Don't forget though that planets do not just orbit around a star... the stars themselves are moving in orbit within the galaxies (in this case the milky way) and these galaxies are also orbiting each other.

Generally it is acknowledged that the galaxies are moving away from each other (ever since the big bang) but the rate is slowing down. It is unknown if they will ever stop moving away from each other (ie if the force of gravity is strong enough to stop the acceleration away). As such there appear to be 3 possibilities.

1. The universe continues to expand (gravity not strong enough).
2. It stays roughly where it is in relation to everything else.
3. It begins to contract again (gravity strong) ultimately leading to the collapse of everything again into a single point and maybe causing another big bang.

Answer 4

When the sun was forming it created a kind of Whirlpool that caused about 1% of the matter in the system to stay in orbit. By getting just the right speed to stay in a stable orbit.

Since space is almost a vacuum then there isn't anything that will change an object speed, so orbits stay stable.

More then that, changes to an object speed are quite small compared to it's orbit speed. So even if there is change in doesn't affect much the orbit itself.

There a lot of forces that do affect the speed. For example the solar wind push us away from the sun and in the same time slow as when we travel into it... But when getting to the total it's extremely small compare to the orbit speed.

Answer 5

Actually, the planet's orbits (and the various satellite's orbits around their planets) are NOT constant, they're gradually getting larger. For example, the moon moves about 3.8 cm per year away from the earth.

This is due to tidal bulges (ironically caused by the gravitational force of the moon itself). These tidal bulges on the Earth in turn exert a gravitational force on the moon. Since the Earth rotates faster (once every 24 hours) than the moon orbits (once every 27.3 days), the bulge tries to "speed up" the moon. Conversely, the moon "pulls back", slowing the Earth's orbit.

Tidal friction, caused by the movement of the tidal bulge around the Earth, takes energy out of the Earth and puts it into the Moon's orbit, making the orbit larger.

This does indeed slow the Earth's rotation, by about 2 milliseconds every hundred years.

Answer 6

Thats the trick of how an orbit works...

If you held a planet still and let go, it would fall right into the star.
Thing is, they arent still, the orbit they are in is such that they are moving sidways (respect to a line straight to the star) so, they miss.
Fair enough for a first shot, but how come they do it again and again?
Well lets say that a particular planet isnt moving very fast, just enough to miss the star, it will fall closer and closer, getting faster and faster, so that it ends up real close but moving fast, very fast. Thats the trick right there, it orbits either close and fast or far and slow.

Technical term for this is conservation of energy, a planets energy is either potential energy (high and slow) or kinetic energy (low and fast).

Most orbits are not pure circles, instead they are elipses with the star inside the elipse and near one end

Answer 7

Look guy, the planets are orbiting the stars at tremendous speeds and due to these speeds the planets are in turn trying to fling themselves into interstellar space, but due also to the insane gravity of it's parent star the planet will not leave because to gravity holds it there, and the opposite also is true, the star wants to draw in the planets, but the flinging motion is holding it out, basically the planet wants to go , the star wants to pull it , so they have a nice little game of tug and war for the rest of eternity. Got a record player? put a penny on it and spin it at 33 rpm, then 45, then 78, notice the penny beginning to fly away, the planets are the penny, but the sun is also an anchor, that will not let them fly away.

Answer 8

The attractive gravitational forces are balanced by the centrifugal force acting to pull the planet away from the star.
Try swinging a weight on a piece of string around your head to experience centrifugal force trying to pull the weight away while being balanced by the tension of the string.Let go and weight flies away.Slow down and weight gets closer to you.If the planetary forces are not balanced the planet will either accelerate into orbit closer to the star or slow out farther until forces are balanced again.
Equilibrium is thus re-established.

Answer 9

Sometimes things do fall into the sun.. Sometimes things do get flung out of the solar system. Earth and other planets in the solar system are in fairly stable orbits.

Things in motion will continue in a straight line unless something applies a force to them. The Earth is traveling at a particular speed relative to the surface of the sun. The force due to the gravity of the Earth-Sun system...the centripetal force, pulls the Earth towards the sun. Howerver, the Earth is traveling fast enough that this effect merely pulls it around the sun. The Earth is falling towards the sun but makes it over "the edge" before actually falling closer a considerable amount.

Answer 10

There have been a lot of answers and many of them are accurate and good technical explanations. I won't bore you by repeating them.

If you would like a more simplistic and practical explanation, try this....

Take a ball on the end of a rubber band (get one of those cheap little paddle ball games) hold the rubber band in your hand and start twirling the ball overhead.

The ball is a planet, the rubber band is gravity and you are the star.

As others have stated, momentum is in the planet and it prefers to follows Newton's law of motion and continue to fly forward in a straight line. Gravity is in the rubber band and tries to pull the ball inward. Notice that the rubber band stretches out a little bit? That is the balancing point between the linear momentum and the gravity.


The only real difference is that the star doesn't twirl the planet. In the example, you are having to do that because air resistance would otherwise slow the ball down. In space, you would not have to twirl the ball.

I hope that helps.

Answer 11

The force of gravity obeys the inverse square law, the further away you are the weaker its effect. The orbiting planet is attempting to fly off in a straight line owing to the centrifugal (outward) force of its velocity. Captured by the gravity of the star it takes up an orbit at a distance at which the inward attraction of the star's gravity is exactly equal to the outward centrifugal force of the planets momentum.