Why do Venus and Uranus spin in the wrong direction?
Astronomers have a lot of puzzles to solve in space, and one in our own Solar System has stumped them for decades: why do Venus and Uranus spin in the opposite direction of the other planets that circle the Sun?
Venus spins from east to west on its axis, while Uranus is so far off center that it’s almost spinning on its side. Scientists don’t know why every other planet, including our own, spins from west to east, but ours does too.
The planets should all be spinning in the same direction. Our Solar System was made from a cloud of gas that collapsed and rotated, and it is thought that most planets, like Earth, still spin in the same direction as that cloud of gas.
Venus and Uranus are the only ones that don’t follow this rule. They have what is called “retrograde rotation,” which means that they spin in the opposite direction of the Sun. But how can that be?
One of the oldest theories is that Venus and Uranus used to spin in the same direction as Earth and the other planets, but they were hit by big things (maybe other planets) at some point, which made them spin in the opposite direction.
In the past few years, astronomers have looked for other reasons by studying Venus and Uranus on their own.
Simulations done in 2011 suggested that Uranus’s 98-degree spin was caused by a number of smaller collisions rather than one big one. This could also explain why the planet’s moons all spin in the same direction, which wouldn’t be the case if there had been just one big hit.
Astronomers came up with another idea in 2009: Uranus once had a big moon, and the pull of its gravity caused the planet to fall over on its side. At some point, another planet could have knocked the moon out of orbit, kind of like a game of cosmic pinball.
Scientists think that Venus, our closest neighbor, used to spin counterclockwise, then slowed down until it was almost still, and then started to spin clockwise like it does now.
This could explain why the planet rotates so slowly now. It takes Venus 243 Earth days to turn all the way around, but it only takes 225 Earth days to go around the Sun. So if you lived on Venus, you’d have longer days than years (and the Sun would rise in the west).
What makes a planet do that? Astronomers think that the strong gravitational pull of the Sun on Venus’s thick atmosphere, the tidal pulls of other planets, and the tidal pulls of the atmosphere could have all worked together to make the planet spin backwards.
This idea of tidal torques, in which the dense atmosphere on the warm, Sun-drenched side of a planet is pulled away from the cold side, is one of the most well-established reasons for Venus’ slow rotation, along with a collision between two planets.
At the moment, though, no one knows for sure what makes Venus and Uranus stand out from the other planets in our Solar System.
The next time we’ll get a good look at Venus will be when the BepiColombo probe, which is headed for Mercury and will launch in 2018, does a flyby.
That mission might give us new information that will help us figure out what’s going on, so we’ll be paying close attention.
