Evidence of a Sub-Saturn around EPIC~211945201

TL;DR

This paper reports the discovery and confirmation of a sub-Saturn exoplanet around EPIC 211945201, using combined photometric and spectroscopic data, providing valuable data on intermediate-mass exoplanets.

Contribution

It presents the first conclusive spectroscopic confirmation of a sub-Saturn exoplanet with precise mass and radius measurements, expanding the catalog of well-characterized intermediate-mass planets.

Findings

Confirmed a sub-Saturn planet with radius 6.12 R⊕ and mass ~27 M⊕.

Provided a density estimate of 0.65 g/cm³, indicating a Saturn-like composition.

Placed an upper mass limit of 42 M⊕ at 3-sigma confidence.

Abstract

We report here strong evidence for a sub-Saturn around EPIC~211945201 and confirm its planetary nature. EPIC~211945201b was found to be a planetary candidate from {\it K2} photometry in Campaigns 5 \& 16, transiting a bright star ($V_{\rm mag}=10.15$, G0 spectral type) in a 19.492 day orbit. However, the photometric data combined with false positive probability calculations using VESPA was not sufficient to confirm the planetary scenario. Here we present high-resolution spectroscopic follow-up of the target using the PARAS spectrograph (19 radial velocity observations) over a time-baseline of 420 days. We conclusively rule out the possibility of an eclipsing binary system and confirm the 2-$\sigma$ detection of a sub-Saturn planet. The confirmed planet has a radius of 6.12$\pm0.1$$~R_{\oplus}$, and a mass of $27_{-12.6}^{+14}$~$M_{\oplus}$. We also place an upper limit on the mass (within the 3-$\sigma$ confidence interval) at 42~$M_{\oplus}$ above the nominal value. This results in the Saturn-like density of $0.65_{-0.30}^{+0.34}$ g~cm$^{-3}$. Based on the mass and radius, we provide a preliminary model-dependent estimate that the heavy element content is 60-70 \% of the total mass. This detection is important as it adds to a sparse catalog of confirmed exoplanets with masses between 10-70 $M_{\oplus}$ and radii between 4-8 $R_{\oplus}$, whose masses and radii are measured to a precision of 50\% or better (only 23 including this work).