# A Hot Saturn Near (but unassociated with) the Open Cluster NGC 1817

## TL;DR

This paper reports the discovery and validation of a hot Saturn-sized exoplanet around a star initially thought to be part of the NGC 1817 open cluster, but Gaia data shows it is unassociated, highlighting the importance of precise measurements.

## Contribution

The study demonstrates the validation of a new exoplanet using seeing-limited photometry, adaptive optics, and Gaia data, and clarifies the star's non-membership in the cluster.

## Key findings

- Discovered a hot Saturn-sized planet around EPIC 246865365.
- Validated the planet with a false positive probability of 0.01%.
- Confirmed the host star is not associated with NGC 1817 using Gaia data.

## Abstract

We report on the discovery of a hot Saturn-sized planet (9.916 +/- 0.985 R_earth) around a late F star, EPIC 246865365, observed in Campaign 13 of the K2 mission. We began studying this planet candidate because prior to the release of Gaia DR2, the host star was thought to have been a member (> 90% membership probability) of the approximately 1 Gyr open cluster NGC 1817 based on its kinematics and photometric distance. We identify the host star (among three stars within the K2 photometric aperture) using seeing-limited photometry and rule out false positive scenarios using adaptive optics imaging and radial velocity observations. We statistically validate EPIC 246865365b by calculating a false positive probability rate of 0.01%. However, we also show using new kinematic measurements provided by Gaia DR2 and our measured radial velocity of the system that EPIC 246865365 is unassociated with the cluster NGC 1817. Therefore, the long-running search for a giant transiting planet in an open cluster remains fruitless. Finally, we note that our use of seeing-limited photometry is a good demonstration of similar techniques that are already being used to follow up TESS planet candidates, especially in crowded regions.