Low-Density Saturn-Mass Planets

Hellier et al. (2016) present the discovery of yet another low-density Saturn-mass planet identified as WASP-131b. The planet orbits a G0V star with an orbital period of 5.3 days. Its proximity to its host star causes it to be heated to an estimated equilibrium temperature of 1460 ± 30 K. Transit observations, together with follow-up radial velocity measurements, indicate that WASP-131b has 1.22 times the radius and 0.27 times the mass of Jupiter. This gives the planet a low density of only 15 ± 2 percent the density of Jupiter. Other known low-density Saturn-mass planets include WASP-21b, WASP-39b and Kepler-427b. All these planets appear to orbit low metallicity host stars (i.e. host stars with low abundance of elements heavier than hydrogen and helium).

Another planet whose discovery was reported in the same paper is WASP-130b. The planet orbits a G6V star with an orbital period of 11.6 days. Unlike WASP-131b, WASP-130b is a compact Jupiter-mass planet. Transit and radial velocity observations show that WASP-130b has 0.89 ± 0.03 times the radius and 1.23±0.04 times the mass of Jupiter. WASP-130b is similar to, but somewhat less compact than HATS-17b, a Jupiter-mass planet with 0.78 times the radius and 1.34 times the mass of Jupiter. The compactness of both planets suggests that they possess massive metallic cores. Correspondingly, both planets also orbit high metallicity host stars (i.e. host stars with high abundance of elements heavier than hydrogen and helium). WASP-130b is termed a warm-Jupiter as its estimated equilibrium temperature of 833 ± 18 K is not hot enough for it to be classified as a hot-Jupiter.

Reference:
Hellier et al. (2016), "WASP-South transiting exoplanets: WASP-130b, WASP-131b, WASP-132b, WASP-139b, WASP-140b, WASP-141b & WASP-142b", arXiv:1604.04195 [astro-ph.EP]