Using HARPS-N to characterise 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 exoplanet. We use {\it 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 3.5-$\ddot{I}ƒ$ mass estimate of $M_p=109_{-32}^{+30}$ M${}_{\oplus }$ and a radius of $R_p=9.49\pm 0.16$ R${}_{\oplus }$. This means that PH-2b has a Saturn-like bulk density and is the only planet of this type with an orbital period $P>200$ days that orbits a single star. We find that Kepler-103b has a mass of $M_{\text{p,b}}=11.7_{-4.72}^{+4.31}$ M${}_{\oplus }$ and Kepler-103c has a mass of $M_{\text{p,c}}=58.5_{-11.4}^{+11.2}$ M${}_{\oplus }$. These are 2.5$\ddot{I}ƒ$ and 5$\ddot{I}ƒ$ results, respectively. With radii of $R_{\text{p,b}}=3.49_{-0.05}^{+0.06}$ R${}_{\oplus }$, and $R_{\text{p,c}}=5.45_{-0.17}^{+0.18}$ R${}_{\oplus }$, these results suggest that Kepler-103b has a Neptune-like density, while Kepler-103c is one of the highest density planets with a period $P>100$ days. 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.

Accepted for publication in MNRAS