Figure 1: Artist’s impression of a gaseous exoplanet.
Figure 2: Transit light curves indicating the presence of planet “b” (red) and planet “c” (blue). Petigura et al. (2015)
Subsequent radial velocity measurements of EPIC-203771098 with the High Resolution Echelle Spectrometer (HIRES) on the Keck Telescope in Hawaii allow the masses of the two planets to be determined. Planet “b” has 21.0 ± 5.4 times the mass of Earth and planet “c” has 27.0 ± 6.9 times the mass of Earth. With the sizes and masses of both planets known, the densities of planets “b” and “c” are 0.63 ± 0.25 g/cm³ and 0.31 ± 0.12 g/cm³, respectively.
The low densities of both planets indicate that they have thick hydrogen-helium envelopes. Furthermore, interior models suggest both planets have fairly massive cores. The estimated core masses for planets “b” and “c” are 17.6 ± 4.3 (~75 percent of the planet’s total mass) and 16.1 ± 4.2 (~50 percent of the planet’s total mass) times the mass of Earth, respectively.
Both planets are relatively hot as they orbit quite close to EPIC-203771098. Planet “b” has an orbital period of 20.9 days and it receives 60 ± 14 times the intensity of insolation Earth gets from the Sun, resulting in an estimated equilibrium temperature of 767 ± 177 K. Planet “c” has an orbital period of 42.4 days and it receives 24 ± 5 times the intensity of insolation Earth gets from the Sun, resulting in an estimated equilibrium temperature of 606 ± 139 K.
Reference:
Petigura et al. (2015), “Two Transiting Low Density Sub-Saturns from K2”, arXiv:1511.04497 [astro-ph.EP]