Our Solar System formed from a single, spinning gas cloud in a nebula not unlike the Orion Nebula we can see in the sky today. The whole cloud was spinning the same direction, which means that as it collapsed, almost everything in our Solar System that formed from it is spinning the same direction, namely, the same direction the Sun rotates. The few things that don’t were knocked into weird orbits by collisions or close gravitational encounters with other objects. For the next biggest thing in our Solar System–Jupiter–to orbit the opposite direction would almost certainly be impossible.
Well, nobody told that to WASP-17b. WASP-17b is already notable for being a “puffy planet” with one of the lowest densities of any known planet. It turns out that it’s also orbiting the opposite direction from the rotation of its star, or retrograde, in defiance of all common decency of angular momentum.
Wait a minute, you might say. How can we even measure that? Well, it turns out that it’s not that hard. We can tell which way a planet is orbiting from something called the Rossiter-McLaughlin Effect.
The Rossiter-McLaughlin Effect takes advantage of the Doppler shift caused by the star’s rotation. The left half of the star in the picture above is rotating toward us, so the light it emits is slightly blueshifted. The right half is rotating away, so its light is slightly redshifted.
A normal planet, which orbits in the same direction as the rotation, blocks part of the bluish half of the star first, then part of the reddish half. But WASP-17b orbits backwards, so it blocks the red light before the blue light. Soon after, we found another retrograde planet: HAT-P-7b.
So we can measure it, but we still have the problem that the biggest planet in a solar system can’t wind up orbiting backwards. There’s no gravitational anchor by which to move it. Exoplanets surprise us all the time, but as far as we know, that’s still true. So what happened?
Option 1 is that WASP-17b is not the largest planet in its solar system. Perhaps there is a more massive planet in a more distant orbit, not visible from Earth, whose gravity knocked WASP-17b into its weird backward motion. This is believed to happen frequently for hot jupiters, although most of them just wind up in tilted orbits and not backward one.
Option 2 is that the star itself simply flipped over. This could happen if the star’s magnetic field interacted in some strange way with the planet-forming disk early in its life. In this case, the planet is “right”, and the star is “wrong”. Magnetic fields are the go-to culprit in astronomy when something weird happens, to the point where it can sometimes become a joke. However, we are learning that they really are very important, and we are only beginning to understand all the ways they can affect stars and galaxies.