How can a planet be orbiting the wrong way?

[gnarledreaper]

My understanding of gravity is limited and this sounds bizarre, i read an article about it in newscientist yesterday and found a bbc link for proof, but you all may know about this http://news.bbc.co.uk/1/hi/sci/tech/8197683.stm

 

Does gravity have a field that rotates as the star turns, is that what drags a planet into a stable orbit and keeps it there?, how can an orbit be kept stable in reverse to the spin direction of the star?

[Mr Skeptic]

No*, for the most part it's that when a solar system is formed from a spinning cloud of dust and gas, everything should be spinning the same way. To have anything significant spinning in the opposite direction would require either a lot of coincidences or something from outside the system.

 

Orbits are stable due to the effects of gravity and centrifugal force.

[Spyman]

The orbit for a planet is caused by a balance between the speed of the planet and the force of gravity from the star. The force of gravity is directed from the center of the planet to the center of the star and totally independent of the angle of rotation or spin of the star. Thus the planet could be said to constantly be falling towards the star but due to a sideway speed it also continues to miss it.

 

A star-planet system is thought to form from a swirling cloud an as such the stars and the planets should keep this initial rotation but otherwise there is no relation between the stars spin direction and the planets orbital direction.

 

If a planet gets very disturbed by something like a large collision or a close encounter with something massive enough it could change path or even get hurled out of the system. A rogue planet from another system could also come close enough to a star that it gets captured and then the infalling angle will determine its orbital path.

 

Understanding orbits

There are a few common ways of understanding orbits:

¤ As the object moves sideways, it falls toward the central body. However, it moves so quickly that the central body will curve away beneath it.

¤ A force, such as gravity, pulls the object into a curved path as it attempts to fly off in a straight line.

¤ As the object moves sideways (tangentially), it falls toward the central body. However, it has enough tangential velocity to miss the orbited object, and will continue falling indefinitely. This understanding is particularly useful for mathematical analysis, because the object's motion can be described as the sum of the three one-dimensional coordinates oscillating around a gravitational center.

http://en.wikipedia.org/wiki/Orbit

Edited December 17, 2010 by Spyman