WHY IS THE SOLAR SYSTEM FLAT, OR HOW TO MAKE A COSMIC PIZZA.
All planets in the Solar System travel in orbits that lie almost in the same plane. The inclination of planets’ orbits to the the ecliptic and to the plane of the Sun’s equator doesn’t exceed 3.4 degree, except for Mercury whose orbit is inclined 7 degrees with respect to the ecliptic.
Our understanding of how the Solar System formed provides an explanation of this remarkable alignment. We know today that planets formed within a flat protoplanetary disk rotating around the protostar that would later become Sun. Rocks, grains of dust, molecules of gas would crash into each other and adhere to larger chunks thanks to their gravity. Sooner or later, the biggest body would sweep its orbital neighbourhood clean of most remaining debris and become the orbit’s sole tenant.
But why did the Solar system start with a disk? Wasn’t it an irregular cloud of interstellar dust and gas to begin with? Why did it collapse into an orderly, flat disk?
You may have seen an explanation of this problem using a comparison with a ball of pizza dough. When the dough is rotated, the centrifugal force makes it stretch outwards till it becomes a flat pizza. This isn’t, however, a good analogy. The Solar System didn’t originate from a dense central clump of matter spun by a cosmic chef.
To paint a better picture of the formation of the Solar System, let’s imagine that we’re making the same pizza in the International Space Station, so that Earth’s gravity doesn’t bother us. We have all the time in the world so we’ll just shake a bag of flour in the middle of the station’s largest module, spray in the air some liquid eggs and olive oil and wait for the pizza to form itself.
The crumbs of flour are gravitationally attracted to one another, and the droplets of eggs and olive oil make them stick after they collide. By shaking the bag and spraying the liquid eggs, we’ve introduced some random motion into our cloud. The crumbs initially move in a very chaotic pattern, but most of them orbit the cloud’s centre of gravity (except for those that stick to the station’s walls - they are lost to us). The crumbs keep interacting with and crashing into one another. Their kinetic and gravitational energy is gradually transformed into heat and the whole cloud starts shrinking under its own gravity.
What determines the cloud’s shape now is its angular momentum. The angular momentum of the cloud is the sum of the angular momenta of all crumbs with respect to the cloud’s centre of mass. Even though we didn’t spin our flour on purpose to begin with, the pure random distribution of velocities results in some residual, non-zero angular momentum when we add them all together. The direction and value of this angular momentum describes the wholesale rotation of the cloud, even though it may be imperceptible to begin with. When the cloud shrinks, the principle of conservation of angular momentum causes it to rotate faster and faster.
Conservation of angular momentum will also make it difficult for individual crumbs to fall directly into the centre of the floating pizza. Each crumb will end up crossing the cloud’s equatorial plane up and down, but most of them won’t get any closer to its centre. As crumbs stick to one another, their up and down velocities average out and become nearly zero. Their angular momenta however remain conserved, so they keep orbiting the cloud’s axis. The crumbs slowly fall into the vicinity of the equatorial plane and our pizza cloud starts forming. The equatorial regions become denser and thinner and if we’re patient enough we’ll end up with a floating, rotating disk of pizza dough.
We’ll finish our experiment here, since we don’t want our pizza to break apart into tiny dough planets. This shows that culinary analogies will get you only so far. Even in space.
For a more scholarly description of the formation of a protoplanetary disk, click here or here.
The Universe wrote about the formation of the Solar System in these recent articles [redirects to Facebook]: 1, 2, 3, 4
Image: Artist’s impression of a protoplanetary disk.
This post originally appeared at The Universe on Facebook.