# Why are the orbits of planets in the same plane?

Asked by krrazypassions (1337) June 24th, 2011

The former ninth planet Pluto orbited in a different plane- but the eight planets orbit in the same plane. Why?

Is it because the sun itself is orbiting around the center of Milky Way galaxy and the planets can orbit only in the plane perpendicular to the direction in which the sun is moving?

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Gravity. Think about objects passing each other and pulling on each other. Over gazillions of years, they are going to pull each other to the shortest distance between them that they can maintain, and that’s going to be in a plane.

So I just made that up. Let’s see is a real physicist can set us straight.

wundayatta (58484)

@wundayatta Sure they will arrange in a plane to achieve equilibrium… but what decides which particular plane they chose? is it the movement of sun in the galaxy?

@krrazypassions That sounds reasonable to me.

If we align with the plane of the galaxy, then we would be seeing the galaxy edge on. When you think of the way the Milky way looks, that would make sense.

wundayatta (58484)

@wundayatta hmmm.. that means the path of the earth in the galaxy is like a spring or DNA helix- spiralling along the central axis marked by the sun’s forward motion.. thus each moment, we are in unique space-time co-ordinates. (In other case, we might visualise the orbit to be elliptical that retraces its path every year- overlapping the same space-coordinates- but thats not so as the sun itself is moving)

Planetary systems coalesce out of the star’s protoplanetary disk of gas and dust.

The nebula, or cloud of gas, that collapses to form the star is churning with random motions, but these will tend to average out to a particular angular momentum, which will establish a rotational motion. Once that motion is established, then the cloud will flatten in the plane perpendicular to the axis of rotation.

That direction of rotation is essentially random, and has no relationship to the orientation of the galaxy. The plane of our solar system is inclined at about 63 degrees to the plane of the Milky Way.

thorninmud (20407)

@thorninmud cool.. so the plane of the planetary orbits is perpendicular to the axis of rotation of the sun.. ok :)

Not exactly. Better to say that both the direction of rotation of the sun and the plane of the solar system are determined by the average angular momentum of the nebula.

thorninmud (20407)

@thorninmud so the plane of the planetary orbits is perpendicular to the axis of rotation of the sun which is perpendicular to the axis of rotation of the nebula?

No, the axes are parallel, but are perpendicular to the orbital plane. The axis of rotation of the sun is roughly the same as the axis of rotation of the original nebula, as it began to collapse. If you were to find the average axis around which the entire solar system spins, it would be virtually identical to that original axis. The Sun’s axis would be close to that, but wanders slighty due to the influence of the more massive planets.

thorninmud (20407)

So the axis of sun and axis of the nebula are the same- and the plane of the solar system is perpendicular to these axes… and this axis of the sun has nothing to do with the orientation of the galaxy-
like the axis of Uranus has nothing to do with the orientation of the solar system- it is dipped by a large angle so that it rather rolls like a ball along the elliptical orbit instead of spinning like a top going along the orbit as earth does.

so, the plane of the Moon’s orbit must be perpendicular to the axis of the Earth.. while the axis of the moon has nothing to do with it.

The axis of the moon has been determined over time by the Earth’s gravitational pull. Even if it had some other axis originally (following whatever cataclysm broke it away from Earth), it was just a matter of time until tidal locking brought it into line with Earth’s axis.

thorninmud (20407)

firstly, I must know,how planet rotate,clockwise or unticlockwise and some moons never rotate at all.I think rotation is creating gravity,and polarity, negative or positive.everything is in equilibrium suspended in space.As I know we are tilted 33 deg. from the Sun.I believed which ever pole of the earth is stronger is more likely to be attracted to the sun.

hgsophia (3)