One can never get enough of oddballs. A new exoplanet named WASP-17 b was found orbiting the wrong way relative to the direction of it's star's rotation. It is on a retrograde orbit.
Typically a star's spin, as well as the orbital motion of all it's planets go in the same direction of the swirls of the primeval gas and clouds from which they formed. This is the case with our own solar system--but not so with WASP-17, which is the first star system known with a planet orbiting in an odd way.
The story says that a violent collision with another massive rock sent the WASP-17's planet flying off in the other direction.
But how did the Astronomers find out about the direction of WASP-17b's orbit in the first place? A transit light curve alone does not tell whether an exoplanet passes across it's star from left to right or vise-versa.
It turns out that the WASP Team needed help from the planet hunters at the Geneva Observatory, who specialize in Radial Velocity (RV) measurements.
Here's the low-down on how Radial Velocity was used for WASP-17b which was primarilly a transiting exoplanet, "astronomers can identify the direction of a planet's orbit because of slight discrepancies in the radial velocity data when a planet transits a star. Because a star is rotating, one side of it is moving towards (or away) from Earth faster than the other side. During a transit, the planet covers first one side of the star and then the other, causing a slight but measurable shift in the radial velocity readings. If during the transit the star first appears to be moving relatively slowly towards the Earth, but then faster as the transit progresses, then the planet is orbiting in the same direction as the star's rotation. But if the reverse is the case – as it is for WASP-17 – then the planet is in a retrograde orbit."
That description is actually the The Rossiter–McLaughlin effect, and it was also mentioned to me by the twitter hive mind. I think The Rossiter–McLaughlin effect is the key part in understanding how to find out an exoplanet's direction of orbit. It's a phenomenon that weaves together the elements of the Doppler Shift, the star's wobble and Photometry.
It seemed counter-intuitive for me at first, knowing that the Transit Method used to find WASP-17 b is primarily used for detecting exoplanets from star systems whose orbital plane is edge-on with our view. While Radial Velocity (RV) I thought was used only for non-transiting exoplanets--whose orbital plane is not edge-on with our line of sight.
But now i realized that the planet-hunting methods can be used in tandem as "Planet-hunting Mash-ups" to make novel discoveries, as what happened with WASP-17b.
It's truly great to know that now we can find out the direction of an exoplanet's orbit, no matter how odd it is.