Can planets exist without a star?

[shinbits]

Could it be possible that a planet with an atmosphere and satelites exists, which doesn't orbit around a star or is near one?

Some other questions: how does the sun or a planet's magnetic field result in satalites orbiting around it, rather than being pulled into it?

And why do planets rotate?

 

[Chesterton]

Could it be possible that a planet with an atmosphere and satelites exists, which doesn't orbit around a star or is near one?

I've heard that it's possible, but whether it'd be called a planet or not is questionable. Some define "planet" as a body orbiting a star, in which case the body wouldn't be a planet if it formed in space and never orbited a star. But if it did orbit a star once and got ejected from it's orbit, I don't know if they'd call that a planet or not.

Here's an article about what might be a "planet with planets": http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html

 

[Nathan45]

chesterton is basically correct.

Brown dwarf - Wikipedia, the free encyclopedia

 

[Nathan45]

Some other questions: how does the sun or a planet's magnetic field result in satalites orbiting around it, rather than being pulled into it?

the reason they orbit has absolutely nothing to do with magnetism. It's just momentum. There is no (significant) friction in space, so they never lose momentum.

the reason planets and moons orbit is because they are constantly in motion, but the direction is constantly altered by gravity. However, their motion is fast enough and there is no friction, so the gravity never pulls them in it just keeps them in orbit...

i.e. the gravity will cause them to curve, altering the direction of their motion... kindof like a baseball when you throw it it curves downward...

but the thing about moons/planets is that but by the time they've curved a noticeable amount from gravity, they've already overshot the object they're orbiting... and then gravity is comming from a different direction, so they're pulled in that direction, causing them to curve again, overshoot again...

i'm not sure if i'm explaining this very well, but basically the gravity keeps them circling but because they have momentum and no friction it never pulls them in. I think to explain it better than this i'd have to break out some really complicated math lol.

 

[Nathan45]

And why do planets rotate?

also momentum,

the planets are spherical and there's no friction preventing their rotation from continuing for very long periods of time. (although they will stop rotating eventually due to tidal locking--google that-- but that'll take billions of years).

the planets and the sun all formed together 4.6 billion years ago. Some of the planets rotate because the original dust cloud that formed them was rotating and they never stopped... others rotate because they were hit by a large object at an oblique angle which caused them to spin in the direction they were hit and they never really stopped spinning.

 

[shinbits]

Thank you, Chesterton, you were seriously helpful.

Nathan, your explanation is rather quite skillful. I clearly understood what you were saying. You have my gratitude.

 

[Chesterton]

But the real reason the Earth rotates is just God being thoughtful, you know, turning off the light while we sleep.

 

[LifeToTheFullest!]

But the real reason the Earth rotates is just God being thoughtful, you know, turning off the light while we sleep.

Tell that to those living at the poles.

 

[AV1611VET]

Another thing that keeps planets orbiting the sun (and not falling into it) is called Conservation of Angular Momentum.

Angular Momentum is the product of: Mass x Velocity x Radius of the Orbit.

Since the product stays the same number, the factors change to compensate; and this is why a planet speeds up when it is closest to the sun, and slows down when it is farthest away --- (since the orbits are elliptical).

For example:

20 = 4 x 5 x 1

• 4 = velocity of the planet
• 5 = radius of the planet's orbit
• 1 = mass of the planet

As the planet approaches the sun, the mass never changes, but the radius shortens.

Since the product (20) will never change, and the radius just shortened, the velocity must change to compensate.

Thus: 20 = 8 x 2.5 x 1.

In other words, the planet speeds up.

 

[shinbits]

^ Nathan said (if I understood him correctly) was that a planet has it's moment curved. Wouldn't that mean that the planets caught by the star's gravity simply continue to orbit the speed of whatever caused them to move through space at that speed in the first place?

In other words, Let's say a planet was inititally traveling through space at a "slow" rate, then got caught by the gravity of a star; isn't it entirely possible that a planet further away may have initially been moving at a much quicker speed, and therefore orbits the star at a faster rate than the planet with a shorter orbit?

 

[Nathan45]

Thus: 20 = 8 x 2.5 x 1.

In other words, the planet speeds up.

well all of the planets* have a nearly perfectly circular orbit so this speeding up and slowing down never actually takes place for planets like it does for comets, etc, because the distance never changes.

*Sorry, Pluto doesn't count as a planet anymore.

 

[shinbits]

Another thing that keeps planets orbiting the sun (and not falling into it) is called Conservation of Angular Momentum.

Angular Momentum is the product of: Mass x Velocity x Radius of the Orbit.

Since the product stays the same number, the factors change to compensate; and this is why a planet speeds up when it is closest to the sun, and slows down when it is farthest away --- (since the orbits are elliptical).

For example:

20 = 4 x 5 x 1

• 4 = velocity of the planet
• 5 = radius of the planet's orbit
• 1 = mass of the planet

As the planet approaches the sun, the mass never changes, but the radius shortens.

Since the product (20) will never change, and the radius just shortened, the velocity must change to compensate.

Thus: 20 = 8 x 2.5 x 1.

In other words, the planet speeds up.

^ Nathan said (if I understood him correctly) was that a planet has it's moment curved. Wouldn't that mean that the planets caught by the star's gravity simply continue to orbit the speed of whatever caused them to move through space at that speed in the first place?

In other words, Let's say a planet was inititally traveling through space at a "slow" rate, then got caught by the gravity of a star; isn't it entirely possible that a planet further away may have initially been moving at a much quicker speed, and therefore orbits the star at a faster rate than the planet with a shorter orbit?

 

[Nathan45]

^ Nathan said (if I understood him correctly) was that a planet has it's moment curved. Wouldn't that mean that the planets caught by the star's gravity simply continue to orbit the speed of whatever caused them to move through space at that speed in the first place?

Not really. If you throw a ball downwards it will speed up as it falls due to gravity. Similarly, if you throw a ball upwards it will slow down and eventually fall back.

The same principle applies to non-circular orbits, they'll speed up as they get closer to the star, slow down as they get farther away.

although this is moot for planets because the planet's orbits are all circular... the distance is always the same so the speed is always the same. also the planets formed together with the sun, they weren't captured--we know this because it is extremely unlikely for a captured object to have a perfectly circular orbit.

In other words, Let's say a planet was inititally traveling through space at a "slow" rate, then got caught by the gravity of a star; isn't it entirely possible that a planet further away may have initially been moving at a much quicker speed, and therefore orbits the star at a faster rate than the planet with a shorter orbit?

the force of gravity decreases with distance... if something is moving very fast through space and its "farther away" it may not be captured by the star's gravity at all....

if it is captured it will certainly fall into a much "higher" (longer) orbit.

conversely, an object that starts out slower moving, will speed up considerably as it falls closer to the star due to the star's gravity. and it will fall into a much lower orbit.

 

[Nathan45]

to illustrate:

imagine dropping a baseball (with a little sideways push) from a helicopter 100 feet above a manhole. Even though you dropped the ball from directly above the manhole it will probably not hit the manhole because you gave it a little push (the original momentum before gravity). It will fall faster and faster as it gets closer to the ground but because of its starting momentum it will actually miss the manhole to the side in one direction or another.

elypitical orbits are sortof like that, except instead of hitting the ground near the manhole there's just a bunch of empty space and so it will circle around for another pass. But on the 2nd pass it will still have the same momentum it started with (momentum is conserved when there is no friction) so it will miss again in the exact same way, then on to the 3rd pass, ad infinitum.

 

[pgp_protector]

Sounds like you're asking about Rogue planet - Wikipedia, the free encyclopedia

 

[AV1611VET]

well all of the planets* have a nearly perfectly circular orbit so this speeding up and slowing down never actually takes place for planets like it does for comets, etc, because the distance never changes.

*Sorry, Pluto doesn't count as a planet anymore.

Ya --- I did go a little overboard ---

 

[shinbits]

although this is moot for planets because the planet's orbits are all circular... the distance is always the same so the speed is always the same. also the planets formed together with the sun, they weren't captured--we know this because it is extremely unlikely for a captured object to have a perfectly circular orbit.

I didn't know that.

I read every word of both your posts, very informative.

 

[Freodin]

although this is moot for planets because the planet's orbits are all circular... the distance is always the same so the speed is always the same.

Which is only approximately correct.

In reality, all planet´s orbits are eliptical.

 

[JustMeSee]

Nice thread. It is good to have knowledgeable around here to answer questions.

 

[yasic]

Which is only approximately correct.

In reality, all planet´s orbits are eliptical.

For earth, the furthest point from the sun is about 4% further than the closest point from the sun.

If you want to call that, in practical terms, an ellipse or a circle is really unclear, and would depend on the context your using it in.