Can planets exist without a star?

[yasic]

I should also note that it is, for all practical purposes, impossible to have a near perfectly circular orbit for the reason that a single push from a meteor or other such force which speeds the orbit up by any real amount, would push the planet out of its orbit and eventually out of the solar system as a whole...

And this is not even taking into account the problem of having our orbit in part decided by the other planets as well as the sun.

 

[Markus6]

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.

They're ellipses thanks to Johannes Kepler. Their paths fit the equations for elliptical orbits with the sun at one focus.

 

[Tinker Grey]

The equation for an ellipse is (in canonical form): x[sup]2[/sup]/a[sup]2[/sup] + y[sup]2[/sup]/b[sup]2[/sup] = 1.

The equation for a circle is: x[sup]2[/sup]/r[sup]2[/sup] + y[sup]2[/sup]/r[sup]2[/sup] = 1.

It is easy (relatively) to see that circle is an ellipse where a = b.

 

[Isambard]

There are creatures on Earth that survive without having any contact with the Sun. As long as there is some chemical reaction taking place to provide energy, I imagine life can exist.

 

[keith99]

I should also note that it is, for all practical purposes, impossible to have a near perfectly circular orbit for the reason that a single push from a meteor or other such force which speeds the orbit up by any real amount, would push the planet out of its orbit and eventually out of the solar system as a whole...

And this is not even taking into account the problem of having our orbit in part decided by the other planets as well as the sun.

Fail.

Orbits are stable. That is the beauty of an inverse square law, like gravitational attrection.

 

[Chesterton]

Fail.

Orbits are stable. That is the beauty of an inverse square law, like gravitational attrection.

Yeah, orbits are stable by definition. If unstable, then not in orbit.

A law only says that something is doing what it's doing, right?

 

[Penumbra]

Fail.

Orbits are stable. That is the beauty of an inverse square law, like gravitational attrection.

Yeah, orbits are stable by definition. If unstable, then not in orbit.

A law only says that something is doing what it's doing, right?

How is a stable orbit defined?

In practice, aren't all orbits unstable, because no orbit can possibly be permanent? Even in the depths of space, some gaseous molecules exist, and create minuscule amounts of friction that can slow down moving bodies over longs periods of time. Close to a planet (such as the case with artificial satellites such as the Hubble telescope), this is enough to pull an orbiting body down to earth within a few years.

-Lyn

 

[yasic]

Fail.

Orbits are stable. That is the beauty of an inverse square law, like gravitational attrection.

Orbits are stable by definition in a closed system (aka assuming no outside influence such as dust).

Non-circular orbits are practically stable.

A circular orbit is technically in a state of 'unstable equilibrium' in a closed system.

However, the universe does not have ideal closed systems, so a circular orbit cannot exist long at all.

 

[Wiccan_Child]

Fail.

Orbits are stable. That is the beauty of an inverse square law, like gravitational attrection.

Yeah, orbits are stable by definition. If unstable, then not in orbit.

A law only says that something is doing what it's doing, right?

I don't think orbits are stable by definition. You can have an unstable orbits.

How is a stable orbit defined?

In practice, aren't all orbits unstable, because no orbit can possibly be permanent? Even in the depths of space, some gaseous molecules exist, and create minuscule amounts of friction that can slow down moving bodies over longs periods of time. Close to a planet (such as the case with artificial satellites such as the Hubble telescope), this is enough to pull an orbiting body down to earth within a few years.

-Lyn

Yes and no. A stable orbit is one in which any deviation from the norm dies off. An unstable orbit is one in which any deviation escalates until it can hardly be said to be orbiting at all.

Consider a simple pendulum. It has two equilibrium points (equivalent to our orbits): pointing straight up and pointing straight down. Any deviation from those points will cause motion, but one point is stable, and the other is unstable. If you deviate it from the 'straight down' point, it will swing back and forth until it once again rests at that same point. If you deviate it from the 'straight up' point, it will swing down and back and forth until it settles down at the 'straight down' point.

That's stability.

Orbits are stable by definition in a closed system (aka assuming no outside influence such as dust).

Non-circular orbits are practically stable.

A circular orbit is technically in a state of 'unstable equilibrium' in a closed system.

However, the universe does not have ideal closed systems, so a circular orbit cannot exist long at all.

Hence why all orbits are elliptical .

 

[[serious]]

circular and elliptical orbits are equally stable. After all, a circular orbit is nothing more than an elliptical orbit in which the two foci have a separation of 0. A slight push will change the distance between foci, but no more than a slight push would change any set of foci.