M. Gillon et al. (2014) report the discovery of WASP-103 b, an ultra-short-period planet at the edge of tidal disruption. WASP-103 b orbits an F-type star at a distance of just ~2 stellar radii from the star's surface, taking a mere 22.2 hours to complete an orbit. The WASP transit survey is sensitive to detecting ultra-short-period giant planets when these planets happen to cross in front of their host stars. WASP-103 b has 1.49 times the mass and 1.53 times the diameter of Jupiter. This newfound planet joins a small group of gas giants that are known to be at the verge of being tidally disrupted by their host stars. The group include planets such as WASP-12 b and WASP-19 b.
Artist’s impression of a gas giant. Credit: Daniel Mallia.
WASP-103 b is significantly inflated and has a bulk density that is only 55 percent the density of water. The low density of WASP-103 b is not just because of the intense irradiation it receives due to its extreme closeness to its host star. Tidal heating is also expected to contribute significantly to the planet's "bloatedness" since the planet's orbit is only 15 to 20 percent away the Roche Limit. Any closer, the planet is expected to be tidally destructed by the gravity of its host star.
Ultra-short-period gas giants that are right at the edge of being tidally disrupted might experience mass loss and significant tidal distortion. One such planet, WASP-12 b, is known to be surrounded by planetary material that has escaped it. In the case of WASP-103 b, the extreme irradiation it receives, the planet's inflated size and the brightness of its host star makes it favourable for atmospheric characterisation with existing ground-based and space-based telescopes. Observing signs of mass loss and tidal distortion for such extreme planets can shed light on the final stages in the lives of hot-Jupiters.
M. Gillon et al. (2014), "WASP-103b: a new planet at the edge of tidal disruption", arXiv:1401.2784 [astro-ph.EP]