Kepler-68: Three Planets, One with a Density between that of Earth and Ice Giants

NASA's Kepler Mission has revealed two transiting planets orbiting Kepler-68. Follow-up Doppler measurements have established the mass of the innermost planet and revealed a third Jovian-mass planet orbiting beyond the two transiting planets. Kepler-68b, in a 5.4 day orbit, has Mp_{\rm P}=8.3^{+2.2}_{-2.4}$ M ⊕, Rp_{\rm P}=2.31^{+0.06}_{-0.09}$ R ⊕, and $\rho _{\rm P}=3.32^{+0.86}_{-0.98}$ g cm-3, giving Kepler-68b a density intermediate between that of the ice giants and Earth. Kepler-68c is Earth-sized, with a radius $R_{\rm P}=0.953^{+0.037}_{-0.042}$ R ⊕ and transits on a 9.6 day orbit; validation of Kepler-68c posed unique challenges. Kepler-68d has an orbital period of 580 ± 15 days and a minimum mass of M Psin i = 0.947 ± 0.035MJ . Power spectra of the Kepler photometry at one minute cadence exhibit a rich and strong set of asteroseismic pulsation modes enabling detailed analysis of the stellar interior. Spectroscopy of the star coupled with asteroseismic modeling of the multiple pulsation modes yield precise measurements of stellar properties, notably T eff = 5793 ± 74 K, M sstarf = 1.079 ± 0.051 M ☉, R sstarf = 1.243 ± 0.019 R ☉, and ρsstarf = 0.7903 ± 0.0054 g cm-3, all measured with fractional uncertainties of only a few percent. Models of Kepler-68b suggest that it is likely composed of rock and water, or has a H and He envelope to yield its density ~3 g cm-3.