How do they find new planets?
A few days ago, the WASP project (Wide Angle Search for Planets) announced the discovery of their 17th planet called WASP-17b (the star is called WASP-17, WASP-17b is the first planet to be discovered around this star). WASP discovers new planets by using  the "transit method". With this method hundreds of images of the same patch of sky are collected over several nights and by advanced analysis methods slight changes in the brighness in any of the stars are found. The slight dimming of a star could be due to a large planet (Jupiter- or Saturn-sized) passing in front of the star. But qute often the dimming is caused by the transit of a background double star - or if the star is a binary star itself and the eclipse is only partial: a gracing eclipse. The dimming is typically of the order 1-2 percent.

How do they know it's a planet?
To confirm that such a transit is due to a planet it is necessary to make additional measurements to determine the mass of the transiting object. This is done by taking a series of spectrograms (by sending light from the telescope through a prism or a grating). In each spectrogram hundreds of spectral absorption lines are visible. Each line is caused by the absorption of light in the atmosphere of the star due to electronic transitions in the atoms that make up the hot plasma that comprise the atmosphere. Each atomic line has a certain wavelength corresponding to the transition energy. The exact same wavelengths can be measured accurately in laboratories on Earth. However, when studying stellar spectrograms the lines are shifted due to the movement of the star relative to the telescope situated on Earth. After correcting for Earth's rotation and the movement of Earth around the Sun, the movement of the distant star is measured; this is realtive to the barycenter of our solar system. In this way the velocity curve of the star is measured and it typically consists of many individual spectrograms collected on several nights.

The WASP-17b planet has an orbital period of about 3.7 days. By combining the light curve (from images, showing the dimming) with an accurate velocity curve (from spectrograms, showing the movement) scientists can determine the orbit of the planet and the radius and mass of the planet: in this case it is an eccentric orbit (it is not a circular orbit, but an ellipse), the radius is about twice that of Jupiter, and the mass is only half that of Jupiter. In other words it has a much lower density than Jupiter, probably be cause it is relatively hot since it is so close to its host star.

WASP-17b is in a retrograde orbit
The most exciting thing about WASP-17b is that the orbit around its host star is retrograde. That means it is travelling in the opposite direction as the rotation of the central star. This phenomenon is also known for several of the moons in our Solar system. It is thought that this is due to collisions or close encounters of massive objects (other moons or other planets). Therefore scientists have suggested that the retrograde orbit of WASP-17b is due to a relatively recent close encounter. This would also explain the eccentric orbit of the planet, since tidal forces will tend to circularize such orbits, especially when the planet is very close to the star.

How do they know it's a retrograde orbit?
This is done from a detailed analysis of the radial velocity curve. The change in velocity is due to the pull of the planet causing the much larger star to move - but only slightly. This explains the main part of the velocity change, but there is also a more subtle effect changing the velocity, the so-called Rossiter-McLaughlin effect. As the planet moves in front of the star, part of the light will not be visible. Let us imagine that this is the right part of the stellar disc and that 2 percent of the light is therefore not being measured. Let us say the star is rotating from left to right, meaning the left part is coming towards us and the right part is receding. The Doppler effect will cause the light on the left part to be blue shifted (the right part will be red shifted; the light near the centre of the disc will not be shifted). Since 2 percent of the red-shifted light is not being observed the measurements will indicate that the star is coming towards us (relative to the situation outside the eclipse). This Dopper-shift will change during the eclipse - from a blue shift to a red shift. This is exactly the opposite of what is observed in all other known exo-planets and this is one of the things that makes WASP-17b special.

Who are "they" ?
The paper presenting the results can be found here: Anderson et al. 2009.

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