Figure 1: Artist’s impression of an exoplanet obscuring its host star. Image credit: Pauline Moss.
The large radius of WASP-121b indicates that it is significantly inflated. WASP-121b orbits so close to its host star, it is just ~1.15 times the minimum distance from its host star where it will start to become tidally disrupted. Due to the proximity to its host star, WASP-121b receives an extreme amount of irradiation which heats the planet to an estimated temperature of roughly 2,360 K. WASP-121b joins a handful of intensely irradiated planets with super-inflated radii.
By measuring a phenomenon known as the Rossiter-McLaughlin (RM) effect, WASP-121b is found to be in a polar orbit around its host star. The orbit of WASP-121b passes almost directly over the poles of its host star. Being so close to its host star, WASP-121b is expected to be significantly deformed due to the intense tidal force it is subjected to. WASP-121b is predicted to be deformed into the shape of a triaxial ellipsoid, with the longest axis pointed towards its host star. The longest axis is estimated to be twice Jupiter’s diameter, while the planet’s shortest axis (i.e. the planet’s polar axis) is estimated to be 1.79 times Jupiter’s diameter.
Figure 2: Light curve showing the transit of WASP-121b in front of its host star. The transit depth indicates that the planet is 1.87 times the radius of Jupiter. Delrez et al. (2015).
Figure 3: Radial velocity curve for the host star of WASP-121b. The amplitude of the radial velocity curve indicates that the planet is 1.18 times the mass of Jupiter. Delrez et al. (2015).
Figure 4: Observed Rossiter-McLaughlin (RM) effect as WASP-121b transits its host star. Delrez et al. (2015).
Delrez et al. (2015), “WASP-121 b: a hot Jupiter in a polar orbit and close to tidal disruption”, arXiv:1506.02471 [astro-ph.EP]