This may be a silly question............

[Bluey]

But why do all the planets orbit around the sun in the same direction, I expect the answer was taught in nursery school, but I must have missed that day:icon_eek:

[Telrad]

No question is a silly question..More correctly, the turning of the Sun and the orbits of the planets are all related to the same other thing- angular momentum. The solar system formed from material that had some angular momentum, it's hard to avoid (ever tried throwing a ball with no spin on it at all?). When gravity pulls it all together, the tiny angular momentum begins to make its presence felt. One way to think of this is, as a ball of gas contracts gravitationally, the characteristic speeds increase like the inverse of the square root of the characteristic size of the ball of gas. But an object in angular momentum (as a contracting body will if its self-gravity is the only force around) has its characteristic rotation speed is linearly proportional to the inverse of the characteristic size. Thus the tendency to be rotating all the same way grows much faster than the tendency for the particles to move faster as the whole business contracts. At some point, the characteristic rotation speed equals the characteristic speeds of the particles, and the whole business must then be rotating around the same way. But to be orbiting and all going around the same way, they have to all be in a plane. So contracting balls of gas eventually reach a plane that is all going around the same way.

That would prevent further collapse, so you need other mechanisms that actually extract angular momentum so the collapse can continue (this is an ongoing area of research, not yet known). Magnetic fields are often invoked. But the bottom line is, you see the origin of the fact that the planets go around the same way, and are in the same plane- they carry the "rotation" of the whole solar system (more so than does the Sun).

[Alan_B]

Yep thats exactly what I was taught in nursery school

[Bluey]

Wow, perhaps I don't feel such a numpty not knowing that now! Thanks for such a prompt reply, much appreciated.

It amazes me, the more we learn about the universe, it all seems to be one great big balancing act that all come crashing down one day!

[Telrad]

Wow, perhaps I don't feel such a numpty not knowing that now! Thanks for such a prompt reply, much appreciated.

It amazes me, the more we learn about the universe, it all seems to be one great big balancing act that all come crashing down one day!

You're welcome Bluey

[Bluey]

Yep thats exactly what I was taught in nursery school

You obviously went to a different nursery school to me

[LukeSkywatcher]

All the planets orbit the sun in the same direction,however not all of the planets rotate on their axis in the same direction. One of the planets rotates in the opposite direction to the rest. Cant for the life of me remember which one. I want to say Venus but i cant be sure.

[661-pete]

All the planets orbit the sun in the same direction,however not all of the planets rotate on their axis in the same direction. One of the planets rotates in the opposite direction to the rest. Cant for the life of me remember which one. I want to say Venus but i cant be sure.

Strictly speaking, there are two planets which rotate retrograde. Venus has a rotation period of 243 days in a retrograde direction. But Uranus, with a rotation period of 17 hours and an axial tilt of 98° - just over a right angle, is also technically rotating retrograde - though it's not usually described as such.

When we come to satellites of planets, retrograde orbits are more commonplace. Several of Jupiter's minor moons are retrograde - and the largest moon with such a behaviour is Triton of Neptune.

Apart from the above, why the uniform direction of rotation? Well I suppose, to put it simply, the entire Solar System is simply conserving the angular momentum of the gas and dust cloud out of which it was formed. If any of the planets had not originated there but been captured by the Sun's gravity later in its life, presumably they would have an equal chance of orbiting forwards or backwards (as do many comets). But retrograde orbits would be inherently more unstable because of perturbations from the other planets.

[LukeSkywatcher]

Apart from the above, why the uniform direction of rotation? Well I suppose, to put it simply, the entire Solar System is simply conserving the angular momentum of the gas and dust cloud out of which it was formed.

Very true. Think of it like this:

If you stir a cup of coffee in a certain direction.......when you think the coffee has stopped "rotating" add a single drop of milk into it and it will rotate in the same direction as you stirred the coffee orginally. Obviosly if you leave it too long the coffee will have settled and the milk drop will just diffuse into the coffee.