Using HARPS-N to characterize the long-period planets in the PH-2 and Kepler-103 systems

We present confirmation of the planetary nature of PH-2b, as well as the first mass estimates for the two planets in the Kepler-103 system. PH-2b and Kepler-103c are both long-period and transiting, a sparsely populated category of exoplanets. We use Kepler light-curve data to estimate a radius, and then use HARPS-N radial velocities to determine the semi-amplitude of the stellar reflex motion and, hence, the planet mass. For PH-2b we recover a 3.5σ mass estimate of M_p=109⁺³⁰₋₃₂ M⊕ and a radius of R_p = 9.49 ± 0.16 R⊕. This means that PH-2b has a Saturn-like bulk density and is the only planet of this type with an orbital period P > 200 d that orbits a single star. We find that Kepler-103b has a mass of M_p,b=11.7^+4.31_−4.72 M⊕ and Kepler-103c has a mass of M_p,c=58.5^+11.2_−11.4 M⊕. These are 2.5σ and 5σ results, respectively. With radii of R_p,b=3.49^+0.06_−0.05 R⊕ and R_p,c=5.45^+0.18_−0.17 R⊕, these results suggest that Kepler-103b has a Neptune-like density, while Kepler-103c is one of the highest density planets with a period P > 100 d. By providing high-precision estimates for the masses of the long-period, intermediate-mass planets PH-2b and Kepler-103c, we increase the sample of long-period planets with known masses and radii, which will improve our understanding of the mass–radius relation across the full range of exoplanet masses and radii.