Why Ecliptic Plane?
Date: Fall 2012
Why are the planets in our solar system in a 2D plane with the sun? Why do we not find planets "below" the sun?
The ecliptic is the background (the constellations) of the Sun as the result of the Earth's orbit.
It is thought that the Solar System was formed from a protoplanetary disc, so the planets are usually close a 23.4 degree deviance from the celestial pole.
Please note the planets are not in a 2D plane. Mars and Venus bob in and out of the ecliptic plane. Mars also appears to reverse from a planar observance (it actually does not reverse). Mercury, Jupiter, Neptune, Uranus and Saturn are out of the plane.
The lack of coplanar relationship is why we use the word conjunction instead of alignment in reference to the planets. The references give very good visual relationships in many different frames of reference.
Keep looking to the sky! Peter E. Hughes, Ph.D. Milford, NH
We sometimes find objects with orbits that are inclined with respect to the ecliptic plane (Pluto is an example), but in many cases we see planets orbiting in a 2-D, or ecliptic, plane. This is due to the way a solar system forms, and we see examples all over the observable sky. This is due primarily to gravity and conservation of angular momentum.
Typically, a star forms from a large cloud of gas that comes together under the influence of gravity. This cloud will usually have some nonzero angular momentum, and because of conservation of angular momentum, as the mass is drawn toward the center the cloud spins faster - just like a spinning figure skater pulling her arms in towards her body. As the cloud spins faster, the matter near the axis of rotation is drawn inward by gravity while rotation keeps the matter near the equator from doing so as rapidly. The resulting shape is a disk, much like you see when someone spins pizza dough in the air.
As the gas at the center of the disk becomes sufficiently dense due to gravity, the gas ignites and a star is formed. The material that's left in the disk can coalesce into various form, including planets. And since the planets formed from material in this disk, their orbits stay within the disk.
Click here to return to the Astronomy Archives
Update: December 2011