Tanisha Kinnard, of Balclutha School, asks :-
Keith Ross of Taumarunui asked:-
Why are the planets flipped over a bit? Why is the Earth's axis of rotation slightly off vertical compared with the Earth-Sun plane?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
The sun and planets formed from a large cloud of dust and gas. At first the dust gathered into tiny planets or planetesimals. Over time the planetesimals gathered together and formed the bigger planets.
The closer a planetesimal was to the sun the faster it orbited. Planetismals circling closer to the sun than a growing planet were most likely to hit the sunny side of the planet. They were also travelling faster than the planet so gave the sunny side a push in the forward direction. Planetesimals from further out were going slower, mostly hit the dark side, and pushed in the backwards direction. Thus the planet picked up spin as it grew.
If all the planetesimals were orbiting in the same plane, like marbles rolling around on a table, then would all hit the planet near the equator. The planet's spin axis would then be vertical to its orbit, like Jupiter's is now. But they weren't that well organized. They hit the planet from different directions. A glancing impact near the pole of the planet would tilt the spin axis. So the tilts that most planets now have depends on how they were hit while they were growing.
Uranus must have had a really big off-centre impact for its axis is tilted sideways. Earth did too: we think the debris from that impact made the moon.
This explanation doesn't apply to the inner planets. For Mercury and Venus the strong pull of the sun's gravity has straightened their spins to vertical. Fortunately for Earth, the moon's gravity keeps our tilt nearly constant.