An HST search for planets in the lower Main Sequence of the globular cluster NGC 6397

TL;DR

This study used HST to search for transiting planets in the globular cluster NGC 6397, analyzing over 5,000 light curves but finding no planetary candidates, and identified 12 new variable stars.

Contribution

First HST-based deep transit search in NGC 6397, providing constraints on planet occurrence in dense cluster environments and reporting new variable stars.

Findings

No high-significance planetary candidates detected

Very low photometric jitter observed in early-M cluster members

12 new variable stars identified

Abstract

Searches for planetary transits carried out in open and globular clusters have yielded to date only a handful of weak, unconfirmed candidates. These results have been interpreted either as being insignificant, or as evidence that the cluster chemical or dynamical environment inhibits the planetary formation or survival. Most campaigns were limited by small sample statistics or systematics from ground-based photometry. In this work we performed a search for transiting planets and variables in a deep stellar field of NGC 6397 imaged by HST-ACS for 126 orbits. We analyzed 5,078 light curves, including a pure sample of 2,215 cluster-member M0-M9 dwarfs. The light curves have been corrected for systematic trends and inspected with several tools. No high-significance planetary candidate is detected. We compared this null detection with the most recent results from Kepler, showing that no conclusive evidence of lower planet incidence can be drawn. However, a very small photometric jitter is measured for early-M cluster members (<~2 mmag on 98% of them), which may be worth targeting in the near future with more optimized campaigns. Twelve variable stars are reported for the first time.