Sun-Like Star with Two Low Density Sub-Saturns

Using data collected by the repurposed Kepler Space Telescope, now known as K2, Petigura et al. (2015) present the discovery of two low density sub-Saturn planets orbiting a Sun-like star identified as EPIC-203771098. The planets are designated with the suffixes “b” and “c”. Planet “b” is the inner planet and planet “c” is the other planet. EPIC-203771098 was observed by K2 from 23 August to 14 November 2014. The planets were detected by K2 as they periodically transit EPIC-203771098. The transit depths from the transit light curves indicate that planet “b” has 5.68 ± 0.56 times the radius of Earth and planet “c” has 7.82 ± 0.72 times the radius of Earth.

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]