Why do all of the planets in the solar system rotate around the sun on the same plane?

xfacilitatorx

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). http://www.nineplanets.org/overview.html http://www.nineplanets.org/overview.html Apr 11 06, 7:40 PM

davejacobs

I couldn't find the answer to the question in the referenced web site. The questioner obviously knew that the planets orbit in roughly the same plane, but his/her interesting question is WHY. Apr 12 06, 8:38 AM

woodardr

I don't have a reference for this - it comes from memories of a Scientific American article I had to write a report on many many years ago. One of the major theories for how the solar system formed was that a disc of space dust and rocks began orbiting around the sun at the same time as the sun was forming (think of an *enormous* version of Saturn's rings). Over time, the dust and rocks joined together to form the planets in a process called accretion. Since the disc of space junk would've been in a single plane, the accreted planets would also form in the same plane. Two oddities about this theory: 1) there is leftover space junk still floating in the plane. A band of asteroids (space rocks and junk) still fill the gap between Mars and Jupiter. It has been thought that Jupiter's size creates enough gravitational havoc in this area (since the sun isn't the only major gravitational player in the area) to keep a 10th planet from accreting in this zone. The remaining space junk that hasn't accreted is the source of most of the doomsday theories of asteroids crashing into Earth and killing everything. 2) Pluto doesn't orbit in the same plane - it's close, but nowhere near as consistently in the same plane as the others. This is one of the basic arguements a number of astronomers have for claiming Pluto may not count as an official planet. Apr 12 06, 9:51 AM

davejacobs

OK, the planets came from a primordial disc of matter. But why a DISC, why not just a shapeless lump? Apr 13 06, 2:53 AM

darkpresence

Because of angular momentum. http://en.wikipedia.org/wiki/Introduction_to_angular_momentum The raw matter of the solar system may have a cloud to begin with but as the sun began to collapse and form, it would have dragged the orbiting matter into a disc. As the link may (or may not) make clear, the drag is greater from the centre of the spinning mass (in this case, the sun). "Stars and planets are formed gently at first.The gas and dust cloud slowly gets smaller and denser, and in the diffuse, cold and extremely low pressure solar nebula all matter must have existed as gas or as tiny grains of dust and ice. Imagine an enormous cloud of snowflakes and ice-coated dust. The density and pressure gradually increases at the centre of the solar nebula, as gravity tugs ever more tightly. As the concentrations increase, atoms collide frequently with each other, generating heat. If this process had continued in isolation the whole gas cloud would have fallen into one object and there would have been just a sun with no planets. But because the solar nebula must have been rotating, its angular momentum sculpted the nebula into a flattened disc, warm in the centre, cold at the edges. When they formed, all the planets would have been in the same flat plane, at least in the beginning." --From "The Moon: a biography" by David Whitehouse. Mar 11 10, 2:56 PM