Nib asked
Why do planets orbit in a flat plane?
Gravity alone would allow orbits tilted at any angle. The flatness is a fossil of the Sun's birth: a spinning cloud collided with itself until its up-and-down motion cancelled, leaving a thin disk, and planets grew inside it.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1any tilt allowed
Newton's gravity pulls every planet toward the Sun, and an orbit can sit at any angle around it. If you threw planets in at random, you would expect orbits like the tilted rings of an atom model, not one neat sheet. So the flatness of the real solar system needs its own explanation.
Gravity allows orbits at any tilt, so it cannot be what makes the real ones flat.
a common mix-upFlat orbits are not a built-in consequence of gravity. They are leftovers from how the solar system formed.
2but they're flat
The tilt of each planet's orbit, measured against the plane of Earth's orbit, is 7.0 degrees for Mercury, 3.4 for Venus, 1.9 for Mars, 1.3 for Jupiter, 2.5 for Saturn, 0.8 for Uranus and 1.8 for Neptune. Compared with the full 90 degrees that was possible, that is almost perfectly flat.
All eight planets orbit within about 7 degrees of one plane.
Which planet's orbit is tilted most against Earth's?
Show the answer
Mercury, at about 7 degrees.
3the cloud
The Sun and planets formed from a cloud of gas and dust, about 4.6 billion years ago. At first the cloud's bits were moving in every direction, with no flat sheet anywhere.
The solar system began as a roughly round cloud with motion in every direction.
4collapse and spin
As the cloud shrinks under its own gravity it spins faster, because spin, or angular momentum, cannot disappear. Gas clumps moving up meet clumps moving down, collide, and swap their up-and-down motion into heat. Motion around the spin axis is shared by everything, so it survives. The cloud flattens along the axis and keeps circling.
Collisions cancel motion along the spin axis; the spin itself cannot be cancelled, so the cloud flattens.
Why does the spin survive when the up-down motion does not?
Show the answer
Everything circles the same way, so collisions do not cancel it. Angular momentum is conserved.
5the disk
The cloud ends up as a thin, rotating disk of gas and dust with the baby Sun at its heart. Dust sticks into pebbles, pebbles into planets, all inside the disk, so every planet inherits the disk's plane and its direction of spin. This is why all eight planets go around the same way, too.
Planets form inside the flat disk, so they inherit its plane and its direction.
6the whole idea
The flat plane of the planets is a fossil of the Sun's birth: a spinning cloud collapsed, collisions cancelled its up-and-down motion but not its spin, and planets formed inside the resulting disk. Gravity alone would allow any tilt. The long-period comets that come from the distant Oort cloud arrive from every direction, which shows that flatness is not a law, only a history.
Flat orbits are a fossil of a spinning disk, not a law of gravity.
The planets' flat sheet is a fossil: the shape of the cloud that spun them into being.