Institute of Astronomy

Plane of the solar system

Published on 15/01/2014 
Question: 

This has somewhat been playing on my mind for a little while now, We've all seen the posters in school telling us the order of the eight planets and they're all neatly put in a straight line and it came to me, that seriously cannot be how the planets orbit the sun in a straight line some must be off in a tilt. So I went and tried to do some research and most sources do put all the planets in a somewhat near line not really varying from a straight rotation around the sun... So I was wondering is that image correct do all the planets tend to rotate around the sun on an even plane if so then our solar system must be extremely flat with huge vast spaces closely above and below planetary rotations that are never occupied.

The solar system is indeed very flat, the orbits of all of the planets are within a few degrees of the same plane.  This plane is also very close to the plane of the Sun's equator.  The flatness of the solar system is one of the pieces of evidence that suggests that the planets formed within a disc around the young Sun.

The space above and below the plane in which the planets lie is not entirely unoccupied, as many asteroids and comets have much higher inclinations.  The main part of the asteroid belt for example reaches up to about 20 degrees above and below the plane of the planets and some of the outlying groups can reach 30 degrees.  Long period comets, like Hale-Bopp, come into the inner solar system pretty much evenly from all directions, so we believe that the Oort cloud, where they originate, is roughly spherical.  All of these small bodies probably didn't form with such inclined orbits though, they were scattered by the planets (particularly Jupiter) to reach their current orbits.

Not all planetary systems show the same amount of flatness though. Some of the new planetary systems that we have been finding around other stars are somewhat different.  Some of them are actually even more flat than the solar system, but then there are others that where the opposite is true and there are huge differences between the planet orbit and the stellar equator.  We suspect that the cases where the orbits are not aligned are systems that have had much more violent histories than the solar system with close encounters between some of the planets.

Page last updated: 15 January 2014 at 12:18