The K2 and TESS Synergy III: search and rescue of the lost ephemeris for K2's first planet

TL;DR

This paper successfully recovers and refines the transit ephemeris for the first K2 exoplanet, K2-2 b, using combined space and ground observations, ensuring precise future observations and characterization.

Contribution

It introduces a joint fitting method combining multiple datasets and a new stellar activity correction technique to accurately update the exoplanet's ephemeris.

Findings

Recovered the transit ephemeris with a 40-sigma discrepancy from initial data.

Achieved a transit timing uncertainty of less than 13 minutes through 2030.

Demonstrated the importance of systematic ephemeris updates for exoplanet characterization.

Abstract

K2-2 b/HIP 116454 b, the first exoplanet discovery by K2 during its Two-Wheeled Concept Engineering Test, is a sub-Neptune (2.5 $\pm$ 0.1 $R_\oplus$, 9.7 $\pm$ 1.2 $M_\oplus$) orbiting a relatively bright (KS = 8.03) K-dwarf on a 9.1 day period. Unfortunately, due to a spurious follow-up transit detection and ephemeris degradation, the transit ephemeris for this planet was lost. In this work, we recover and refine the transit ephemeris for K2-2 b, showing a $\sim40{\sigma}$ discrepancy from the discovery results. To accurately measure the transit ephemeris and update the parameters of the system, we jointly fit space-based photometric observations from NASA's K2, TESS, and Spitzer missions with new photometric observations from the ground, as well as radial velocities from HARPS-N that are corrected for stellar activity using a new modeling technique. Ephemerides becoming lost or significantly degraded, as is the case for most transiting planets, highlights the importance of systematically updating transit ephemerides with upcoming large efforts expected to characterize hundreds of exoplanet atmospheres. K2-2 b sits at the high-mass peak of the known radius valley for sub-Neptunes, and is now well-suited for transmission spectroscopy with current and future facilities. Our updated transit ephemeris will ensure no more than a 13-minute uncertainty through 2030.