The orbit of WASP-33b is precessing

Hot Jupiter planets are in tight orbits around their host star, and since that star will not be perfectly spherical, small gravitational perturbations should cause the orbit to precess. A team led by Marshall Johnson has now shown that this is indeed happening in WASP-33.

WASP-33 is a very hot, rapidly rotating A-type star. This means the planet is only detected by the “shadow” that it causes in the profiles of the spectral lines of the star during transit.

Since the star is rotating the spectral lines will be broadened by the Doppler effect, with photons from the approaching limb being blue-shifted and photons from the receding limb being red-shifted. As the planet transits the star, it blocks the light from one small region of the star’s surface. This removes the photons that are Doppler shifted with the velocity of that part of the star’s surface.

The trace of the planet across the star’s surface during transit can therefore be seen as a stripe moving in velocity across the profile of the star’s spectral lines. This is seen in these false-colour images of the spectral line of WASP-33, taking during two transits, six years apart:

WASP-33 line profiles

WASP-33 line profiles

The white diagonal stripe is the path of the planet, blocking out the photons below it. The stripe is clearly in a different place in the two observations. This means that the path of the orbit has changed. Johnson et al give the following schematic of how they think the orbit of the planet has changed between the two observations.

WASP-33 precession

This observation validates the theory that the orbit should be precessing, and is only the second detection of nodal precession in an exoplanet orbiting a single star, after the example of Kepler-13 Ab.