WASP-103b is a Hot-Jupiter Stretched by Tidal Forces

WASP-103b is a transiting hot-Jupiter in an ultra-short period orbit around a F8V star with 1.4 times the diameter and 1.2 times the mass of the Sun. Transit and radial velocity measurements show that WASP-103b has 1.6 times the diameter and 1.5 times the mass of Jupiter. Because WASP-103b orbits so close to its host star, it is expected to raise significant tides on its host star and experience tidally-induced orbital decay. Over a time interval of 10 years, the orbital period of WASP-103b could decrease by ~100 seconds.

Tidally-induced orbital decay may be detectable by precisely measuring when WASP-103b transits its host star to look for any slight deviations in periodicity. At present, this is not detectable as it requires many years of observations with high quality transit timing data. Nevertheless, more precise transit observations of WASP-103b have improved the time of mid-transit to an accuracy of 4.8 seconds. For comparison, the time of mid-transit was only accurate to 67.4 seconds at the time of the planet’s discovery. A more accurate time of mid-transit would help in future searches for tidally-induced orbital decay.

The extreme closeness of WASP-103b to its host star causes it to be tidally stretched into an ellipsoid with its longest axis oriented towards its host star. Assuming Rᴊᴜᴘ denotes the equatorial radius of Jupiter (i.e. 71,492 km); the dimensions of WASP-103b are 1.721 ± 0.075 Rᴊᴜᴘ at the substellar point, 1.710 ± 0.072 Rᴊᴜᴘ at the antistellar point, 1.537 ± 0.043 Rᴊᴜᴘ at its poles, and 1.571 ± 0.047 Rᴊᴜᴘ at its sides. WASP-103b is significantly distorted from a spherical shape, with its longest axis ~10 percent longer than its shortest axis.

Reference:
Southworth et al. (2014), “High-precision photometry by telescope defocussing. VII. The ultra-short period planet WASP-103”, arXiv:1411.2767 [astro-ph.EP]