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

Abstract

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<SUB>⊕</SUB> and a radius of R<SUB>p</SUB> = 9.49 ± 0.16 R<SUB>⊕</SUB>. 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<SUB>⊕</SUB> and Kepler-103c has a mass of M_{p,c} = 58.5^{+11.2}_{-11.4} M<SUB>⊕</SUB>. These are 2.5σ and 5σ results, respectively. With radii of R_{p,b} = 3.49^{+0.06}_{-0.05} R<SUB>⊕</SUB> and R_{p,c} = 5.45^{+0.18}_{-0.17} R<SUB>⊕</SUB>, 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.