Kepler-78b is a transiting planet that is 1.2 times the radius of Earth and orbits a young , active K dwarf every 8 hours .
The mass of Kepler-78b has been independently reported by two teams based on radial velocity measurements using the HIRES and HARPS-N spectrographs .
Due to the active nature of the host star , a stellar activity model is required to distinguish and isolate the planetary signal in radial velocity data .
Whereas previous studies tested parametric stellar activity models , we modeled this system using nonparametric Gaussian process ( GP ) regression .
We produced a GP regression of relevant Kepler photometry .
We then use the posterior parameter distribution for our photometric fit as a prior for our simultaneous GP + Keplerian orbit models of the radial velocity datasets .
We tested three simple kernel functions for our GP regressions .
Based on a Bayesian likelihood analysis , we selected a quasi-periodic kernel model with GP hyperparameters coupled between the two RV datasets , giving a Doppler amplitude of 1.86 \pm 0.25 m s ^ { -1 } and supporting our belief that the correlated noise we are modeling is astrophysical .
The corresponding mass of 1.87 ^ { +0.27 } _ { -0.26 } M _ { \oplus } is consistent with that measured in previous studies , and more robust due to our nonparametric signal estimation .
Based on our mass and the radius measurement from transit photometry , Kepler-78b has a bulk density of 6.0 ^ { +1.9 } _ { -1.4 } g cm ^ { -3 } .
We estimate that Kepler-78b is 32 \pm 26 % iron using a two-component rock-iron model .
This is consistent with an Earth-like composition , with uncertainty spanning Moon-like to Mercury-like compositions .