Two Peculiar Low-Density Sub-Saturn-Mass Planets

Bakos et al. (2016) present the discovery of two transiting low-density sub-Saturn-mass planets identified as HAT-P-47b and HAT-P-48b. Both planets orbit relatively bright F-type stars and have masses half-way between those of Neptune and Saturn. HAT-P-47b and HAT-P-48b were discovered through a combination of transit and radial velocity observations. Both HAT-P-47b and HAT-P-48b are very low density planets, and they are currently the two lowest mass planets known to have radii exceeding that of Jupiter's.

Figure 1: Artist’s impression of an exoplanet.

HAT-P-47b has 0.206 ± 0.039 times the mass and 1.313 ± 0.045 times the radius of Jupiter, giving it only ~11 percent the density of water. Its orbital period around its host star is 4.732 days and its estimated equilibrium temperature is 1605 ± 22 K. The host star of HAT-P-47b has 1.387 ± 0.038 times the mass and 1.515 ± 0.040 times the radius of the Sun. It also has 4.15 ± 0.27 times the Sun's luminosity and its surface temperature is 6703 ± 50 K. HAT-P-47b and its host star are both located ~900 light years away.

HAT-P-48b has 0.168 ± 0.024 times the mass and 1.131 ± 0.054 times the radius of Jupiter, giving it only ~14 percent the density of water. Its orbital period around its host star is 4.409 days and its estimated equilibrium temperature is 1361 ± 25 K. The host star of HAT-P-48b has 1.099 ± 0.041 times the mass and 1.223 ± 0.046 times the radius of the Sun. It also has 1.67 ± 0.14 times the Sun's luminosity and its surface temperature is 5946 ± 50 K. HAT-P-48b and its host star are both located ~1000 light years away.

Figure 2: Transit light curves indicating the presence of HAT-P-47b and HAT-P-48b. Bakos et al. (2016)

Figure 3: Mass-radius diagram of sub-Saturn-mass transiting planets (i.e. planets with less than 0.3 times the mass of Jupiter). HAT-P-47b and HAT-P-48b are highlighted by large boxes. Solar System planets are indicated by blue triangles. Both HAT-P-47b and HAT-P-48b stand out by their very low densities. Bakos et al. (2016)

Figure 4: Planetary mean density versus mass for transiting planets that have masses known to less than 20 percent uncertainty. HAT-P-47b and HAT-P-48b are highlighted by large boxes. The size of the points scales with planetary radius, while the colour indicates equilibrium temperature. HAT-P-47b and HAT-P-48b fall in a relatively unpopulated region of the parameter space; they are the lowest density sub-Saturn-mass objects. Solar System planets are indicated by blue triangles. Bakos et al. (2016)

Reference:
Bakos et al. (2016), "HAT-P-47b AND HAT-P-48b: Two Low Density Sub-Saturn-Mass Transiting Planets on the Edge of the Period--Mass Desert", arXiv:1606.04556 [astro-ph.EP]