This conversation is closed.

## If the sun's gravity is strong enough to pull a comet from the kuiper belt why does it not pull the planets into it?

I was told that the sun's gravitational pull is so strong that it pulls comets in from the kuiper belt, but why does it not do the same to the planets? Since a comet is so small in size ,mass and density and so far out how is this possible? An old teacher of mine said it has to do with the planets speed and its own gravity, but doesn't a comet move at a faster speed than the planets? Does the fact that comets do not have a gravitational field play into this? Or am I being mislead? This may be a basic question but I have not found an answer that explains this. This question and a few others is making me think I need to take astrophysics in college . Thank you.

• #### Roy Bourque

• 0
Nov 18 2012: Alexander,

A comet's speed in the Kuiper belt is not faster than a planet. The further an object is from the sun, the slower it moves in its orbit.

If something redirects the comet's direction toward the sun, the sun's gravity will take it over. As it gets closer to the sun, its speed increases due to the force of gravity. As it departs away from the sun, its speed decreases the further it gets.

Comets do have a gravitational field. Gravity is a function of mass. Everything has gravity including you, otherwise, you would not be held down to the earth. the earth's gravity interacts with your gravity to create an attraction.

Try an experiment; take a small ball at the end of an elastic and spin it around your head. The elastic is pulling the ball in, but the speed of the ball is pulling it away. Gravity is like an elastic, only it gets weaker as distance increases. But the principle is still the same. The speed of the planets keeps them from falling into the sun. If they were to slow down, the sun's gravity would pull them closer to the sun. It doesn't take much to change the direction or speed of a comet. It would take an enormous force to change the speed or direction of a planet, which is why comets get pulled in but planets don't.

Simple arithmetic; The earth is 93 million miles from the sun. It takes 365.25 days to go around the sun. Circumference is 3.1416 times diameter. How fast is the earth moving?
The moon is 240,000 miles from the earth (rough estimate). It takes 29.5 days to go around the earth. How fast is the moon moving?
When you calculate it out, it becomes surprising at just how fast celestial objects move through space.
• #### Kitty Hawk

• 0
Nov 15 2012: The key to any orbit is velocity: it you can move something fast enough in the correct direction, you can get an orbit (think Tether-ball with a really elastic rope). Gravity accelerates the earth toward the sun at about 6mm/s/s, but the earth is moving about 30 km/s in a different direction (almost 90 degrees from that acceleration) and is 1.47E11 meters away. With just that velocity, after a second passes the earth would move 30km at that angle, which means its distance from the sun is now... 1.47E11m PLUS 3mm. (remember that for the sides of a right triangle, [ A^2 + B^2 = C^2 ])

Now for the acceleration. You know [ d = 0.5 * a * t^2 ], so the acceleration for that second moved us... 3mm closer to the sun. We are once again 1.47E11m away. There are other things going on, but keep in mind that calculus was invented specifically to solve this problem!
• #### Alexander Witt

• 0
Nov 15 2012: Thank you, Thought it was something like that, but dwell too long on one issue and you start to over-think things and then start pondering other things and trying to make it fit into the original question.
Like I said before my science teacher doesn't know much about space and what makes it tick, He does know a great deal about geology.
What erks me is that I believe if your going to become a H.S. science teacher you at least be well versed in what you plan on teaching.
• #### Kitty Hawk

• 0
Nov 15 2012: I know what you mean. If you ask me, the phrase "that's just the way it is" defeats the whole purpose of teaching something! It might seem counter-intuitive, but ask you mathematics teacher if your science teacher can't explain something. Very often, it's because the situation has something special about it like this.
• #### Alexander Witt

• 0
Nov 16 2012: Thanks for helping my son figure this out, I on the other hand am not a smart man when it comes to these things. The way his teacher brushed him off was very disappointing. But it sheds a bright light onto why America has fallen behind in the science department. When the teachers don't care its the students and the future that end up suffering.
Again I appreciate you helping him, he was lit up when I came home. Proud to have gotten the answer to his question and even more proud to understand it too.
makes me very happy and proud to see him grow up to want to learn about this stuff and to want to excel at it. He is smarter than I and that's exactly why i push him to study so hard, to not struggle through life as I have. - Dan

P.s. expect more questions from him, as he would like to get a job in one of the astro science fields
• #### Alexander Witt

• 0
Nov 15 2012: I understand that the kuiper belt has its own orbital structure and gravitational forces keep it there. I also understand that debris in that belt sometimes collide and objects are pushed out form there, causing it to "fall".
I guess the question should have been what keeps us from being pulled into the sun?
Is it centrifugal force? Or am I just over-thinking everything?

Thank you John Smith and Lejan, like I said before it may be a simple question, but my science teacher cant explain it at all, he just says " that's just the way it is" then moves onto something completely unrelated
• #### John Smith

• 0
Nov 15 2012: "If the sun's gravity is strong enough to pull a comet from the kuiper belt why does it not pull the planets into it?"

If you could stop a planets movement around the Sun (its orbit) it would fall in. Whether an object falling towards the Sun ends up in an orbit or crashing into the Sun depends on how that object approached the Sun in the first place: if it moved slowly, or just on a straight collision course with the Sun it will crash into the Sun, if it approached in such a way that it will "overshoot" the Sun, the Sun will still try to pull the object in and the object will enter an elliptical or circular orbit around the Sun that's stable because the centrifugal force of the orbit negates the gravitational force. In essence the object will be in eternal freefall (this is why astronauts are weightless when in orbit around the Earth).
• #### Lejan .

• 0
Nov 15 2012: If it was, by now there was no kuiper belt anymore.

Also any comet, any mass, has it very own gravitational field, as gravitation is intrinsic to any matter.

http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation

The reason why every now and then an object leaves the kuiper belt is, because our whole solar system and the kuiper belt iteslf is not in equilibrium condition. Objects still collide with one another which causes some to change course.

According to wikipedia, Kenneth Edgeworth hypothesis in 1943 states like this:
"... the outer region of the solar system, beyond the orbits of the planets, is occupied by a very large number of comparatively small bodies" and that, from time to time, one of their number "wanders from its own sphere and appears as an occasional visitor to the inner solar system''

Once that happens, and an object wanders on elliptical orbit across the solar system, any time it gets near its original 'startpoint' within the kuiper belt, it does influence remaining objects slightly by its gravitational field, and over time, some other objects may get 'off cours' as well. If all was static, this could not happen.