Did a stellar fly-by shape the planetary system around Pr 0211 in the cluster M 44?

TL;DR

This study investigates whether a stellar fly-by could explain the eccentric planetary orbit in the Pr 0211 system within the M44 cluster, using simulations to assess the likelihood and history of such events.

Contribution

The paper demonstrates that close stellar fly-bys are a plausible explanation for the eccentric orbit in Pr 0211 and assesses their frequency in cluster environments through simulations.

Findings

Close fly-bys are relatively common in M44, especially within the first 3 Myr.

Approximately 6.5% of stars experience a fly-by capable of shaping planetary orbits in the first 2 Myr.

12-20% of stars could undergo such fly-bys over the cluster's lifetime.

Abstract

Out of the $\sim$ 3000 exoplanets detected so far, only fourteen planets are members of open clusters: among them an exoplanet system around Pr 0211 in the cluster M44 which consists of at least two planets with the outer planet moving on a highly eccentric orbit at 5.5 AU. One hypothesis is that a close fly-by of a neighbouring star was responsible for the eccentric orbit. We test this hypothesis. First we determine the type of fly-by that would lead to the observed parameters and then use this result to determine the history of such fly-bys in simulations of the early dynamics in an M44-like environment. We find that although very close fly-bys are required to obtain the observed properties of Pr 0211c, such fly-bys are relatively common due to the high stellar density and longevity of the cluster. Such close fly-bys are most frequent during the first 1-2 Myr after cluster formation, corresponding to a cluster age $\leq$ 3 Myr. During the first 2 to 3 Myr about 6.5% of stars actually experience a fly-by that would lead to such a small system-size as observed for Pr0211 or even smaller. It is unclear whether planets generally form on such short timescales. However, afterwards the close fly-by rate is still 0.2-0.5 Myr$^{-1}$, which means extrapolating this to the age of M44 12%-20% of stars would experience such close fly-bys over this timespan. Our simulations show that the fly-by scenario is a realistic option for the formation of eccentricity orbits of the planets in M44. The occurrence of such events is relatively high leading to the expectation that similar systems are likely common in open clusters in general.