WASP is the Wide Area Search for Planets housed in the U.K. which uses a camera to monitor starlight and watches for the fading caused by planets passing between the star and earth. WASP has discovered 17 planets so far and it’s most recent discovery is cleverly named WASP-17 and so far is the most unique planet yet found.

One thing that is intriguing about WASP-17 is that the planet is twice the size of Jupiter but 70 times less dense than the Earth. This makes the planet about as dense as packing peanuts and also the largest planet yet found. Astronomers think this bloated-ness is due to the planets orbit which is highly elliptical which causes seasonal tides. These tides heat up and cool the whole planet causing it to expand.

But, what makes this find even more fascinating is the fact that the planet is orbiting the star backwards to the star’s rotation. Up to this point all of the planets astronomers have observed orbit their star in the same direction as the star’s rotation. WASP-17’s retrograde orbit could have been caused by a near miss of another planet early during the formation. This near collision would slingshot the planet into revolving backwards.

Scientists used a method called transit photometry for this discovery. Scientists can determine a star’s rotation through measuring the speed of the star - as one half rotates towards us it appears to be moving slower than the other half that rotates away from us. Similarly, scientists can determine the direction of a planets revolution through watching how the star fades and brightens.