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

TL;DR

This paper confirms the planetary nature and measures the masses of long-period exoplanets PH-2b and Kepler-103c using Kepler light curves and HARPS-N radial velocities, expanding the sample of well-characterized long-period planets.

Contribution

It provides the first mass estimates for the Kepler-103 system's planets and confirms PH-2b as a long-period, Saturn-like planet, enhancing understanding of long-period exoplanets.

Findings

PH-2b has a mass of 109^{+30}_{-32} M_⊕ and a radius of 9.49±0.16 R_⊕.

Kepler-103b has a mass of 11.7^{+4.31}_{-4.72} M_⊕ and a radius of 3.49^{+0.06}_{-0.05} R_⊕.

Kepler-103c has a mass of 58.5^{+11.2}_{-11.4} M_⊕ and a radius of 5.45^{+0.18}_{-0.17} R_⊕.

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-$\sigma$ mass estimate of $M_p = 109^{+30}_{-32}$ M$_\oplus$ and a radius of $R_p = 9.49\pm0.16$ 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 $M_{\text{p,b}} = 11.7^{+4.31}_{-4.72}$ M$_{\oplus}$ and Kepler-103c has a mass of $M_{\text{p,c}} = 58.5^{+11.2}_{-11.4}$ M$_{\oplus}$. These are 2.5$\sigma$ and 5$\sigma$ results, respectively. With radii of $R_{\text{p,b}} = 3.49^{+0.06}_{-0.05}$ R$_\oplus$, and $R_{\text{p,c}} = 5.45^{+0.18}_{-0.17}$ 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.