Search for giant planets in M67 III: excess of hot Jupiters in dense open clusters

TL;DR

This study reports a higher-than-expected occurrence of hot Jupiters in the dense open cluster M67, suggesting stellar encounters and planet-planet scattering as key formation mechanisms, contrasting with field star populations.

Contribution

It provides the first long-term RV survey of M67, revealing a significant excess of hot Jupiters and challenging existing theories on planet formation in open clusters.

Findings

Discovery of a new hot Jupiter around YBP401.

High hot Jupiter occurrence rate (~5.6%) in M67.

Excess hot Jupiters not explained by metallicity or stellar mass.

Abstract

Since 2008 we used high-precision radial velocity (RV) measurements obtained with different telescopes to detect signatures of massive planets around main-sequence and evolved stars of the open cluster (OC) M67. We aimed to perform a long-term study on giant planet formation in open clusters and determine how this formation depends on stellar mass and chemical composition. A new hot Jupiter (HJ) around the main-sequence star YBP401 is reported in this work. An update of the RV measurements for the two HJ host-stars YBP1194 and YBP1514 is also discussed. Our sample of 66 main-sequence and turnoff stars includes 3 HJs, which indicates a high rate of HJs in this cluster (~5.6% for single stars and ~4.5% for the full sample ). This rate is much higher than what has been discovered in the field, either with RV surveys or by transits. High metallicity is not a cause for the excess of HJs in M67, nor can the excess be attributed to high stellar masses. When combining this rate with the non-zero eccentricity of the orbits, our results are qualitatively consistent with a HJ formation scenario dominated by strong encounters with other stars or binary companions and subsequent planet-planet scattering, as predicted by N-body simulations.