The grazing transit of WASP-67b

The hot-Jupiter exoplanet WASP-67b is a curiosity, being the only known exoplanet with a grazing transit, such that not all of the planet transits the host-star’s disc. This means that the characteristic “second contact” and “third contact” points are missing from the transit lightcurves. These are the points where, usually, the whole planet is now in front of the star, and the transit is then flat-bottomed, apart from the relatively small effects of stellar “limb darkening”.

The four "contact" points of a planetary transit, illustrated for Mercury transiting our Sun.

The four “contact” points of a planetary transit, illustrated for Mercury transiting our Sun.

In WASP-67b’s grazing transit the planet is never completely in transit and thus the transit lightcurve has a continuously varying V shape. The grazing nature of WASP-67b was confirmed by a detailed study of new transit lightcurves by Mancini et al (2014), who used the GROND instrument on ESO’s 2.2-m telescope at the La Silla observatory.

The transit of WASP-67b from Mancini et al. (2014)

The transit of WASP-67b from Mancini et al. (2014)

The lack of second and third contact makes the system parameters hard to tie down, and thus obtaining a secure estimate of the planet’s radius and density requires Mancini et al’s high-quality lightcurves. WASP-67 is also notable for being in one of the target fields of the revamped Kepler spacecraft’s K2 mission, and thus we can expect ongoing detailed study of this system.

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