Before we can expand our thinking out to “all planets”, the easiest wayto start looking at planets and how they rotate is to look at our ownsolar system, which we can investigate in far more detail (and far moreeasily) than anywhere else in the Universe. What we see in our own solarsystem is that all of the major planets are rotating around their own internal axis.
We’re wellacquainted with the rotation of the Earth, even if we haven’t thoughtabout it in this way — the Earth’s rotation is what creates our days. Asa result, we all know how long it takes for the Earth to rotateonce — 24 hours. But 24 hours isn’t the rule within our solar system; infact, of all the other planets, only Mars rotates at a similar speed.Mars completes one rotation in 24 hours and 40 minutes; nearly identicalto our home planet.
Venus,our planetary sister gone horribly awry, rotates much slower than theearth — one rotation takes 243 days and 26 minutes; this makes it theslowest rotator in our solar system. Mercury comes in second slowest,and rotates once in 58 days, 15 hours, and 30 minutes. The gas giantsall seem to have similar rotation speeds, all of which are faster thanany of the inner, rocky worlds. Jupiter rotates once every 9 hours and55 minutes. Saturn recently had its rotation speed re-measuredto be 10 hours and 36 minutes. Uranus rotates once every 17 hours and14 minutes, whereas Neptune rotates once every 16 hours and 6 minutes.
Mosteverything in our solar system rotates in the same direction — the samedirection as the Earth. If we had a bird’s eye view of our solarsystem, where we’d flown into space “up” via the North Pole, and lookedback down, most of the planets would be rotating counterclockwise — orfrom the West towards the East.
You can usually remember which way the earth is rotating by thinking about thetime zones; the further east you go, the earlier it is — they’re pushedtowards daylight sooner than the west.
Sowhy does almost everything rotate in the same way? A hint of the answerlies in the Sun. The sun also rotates, and it rotates in the samedirection as the majority of the planets — counterclockwise from ourbird’s eye view. Since the Sun is also following our consistent rotationpattern, we’re going to have to make our way back to the formation ofthe solar system in order to make sense of this.
Our sun formed out of a cloud of gas and dust — enormous molecular clouds arethe only places where, under the persistent pull of gravity over time,material gets dense enough to begin to collapse and form a star (orten). However, during the collapse of the cloud of dust and gas, anyinitial motion in the cloud becomes quite important.
It’svery unusual for objects in the universe to be completely still inrelation to each other (in fact, they would have to be at absolute zeroto prevent any motion), so there’s a little bit of stirring about withinthe cloud that is always present. As the cloud collapses, the averagedirection of motion of the cloud is kept — and as gravity pullseverything closer to the center, the conservation of angular momentumcauses that average motion to speed up. You can do this experiment athome if you have a chair that spins. If you start yourself turning alittle bit with your arms and legs extended, and then pull your legs andarms in quickly, you’ll find yourself spinning much more rapidly thanyou were to start out. Pulling your arms in plays the same role thatgravity is playing in the collapse of the early solar system.
Because therotation speed is so much faster than it was initially, the gas formsinto a flat disk that’s all rotating in the same direction. If you’veever played with a ball of bread dough or silly putty, and spun it onyour finger, you’ll find it flattens out into a thin disk fairlyquickly. The star at the center is therefore being formed in anenvironment where everything is already rotating — as the star collapseseven more, its rotation will increase even more. All the proto-planetsare also forming out of gas which is rotating, so it makes sense thatthey too will absorb the rotation of the disk they formed in. So inprinciple, we expect most planets to form with a rotation that’sconsistent with the star at the center of their solar system.
However,“most everything” isn’t everything — we’ve got two notable exceptionswithin our solar system to the counterclockwise rotation rule: Venus andUranus. Uranus rotates 90 degrees off from everything else. If youconsider the plane of all the planets’ orbits around the sun as a flatsurface, most planets spin as though they were a coin spun on its edge,flicked counterclockwise. Uranus, on the other hand, spins like a beadrolled along the ground, instead of spinning vertically. Venus is evenodder- it spins clockwise. As far as we can tell, this means Venus issomehow upside down.
Whathappened to these planets? The picture I painted above indicates thatthey should have formed with a rotation aligned with every other planetin our solar system. In fact, they probably did form with a rotationthat matched. However, the very early solar system was a violent place,filled with many more proto-planets which were inclined to collide withother objects. Some of these collisions were likely with the earlyversions of the planets which exist today. A particularly bad collisionor series of collisions could cause such an energetic punch to the otherplanet that the planet was effectively tipped over onto its side (inthe case of Uranus) or upside down completely (in the case of Venus.)
Toexpand to the Universe outside of our own solar system; physics is thesame everywhere in the Universe, so solar systems should form in thesame way everywhere. However, we wouldn’t expect every planet to rotatein the same direction as its parent star, since — like our own solarsystem — the planets may have had a very collision-intensive early life.