Floating binary planets from ejections during close stellar encounters

TL;DR

This paper proposes that free-floating binary giant planets (JuMBOs) can form from ejected double giant planets during close stellar encounters, with simulations showing their typical orbital characteristics and environmental dependence.

Contribution

It introduces a new formation mechanism for JuMBOs via stellar encounters, supported by direct simulations and environmental rate estimates.

Findings

JuMBOs often have semi-major axes about three times their original planetary separation.

Ejected JuMBOs exhibit high eccentricities with a superthermal distribution.

Formation rates of JuMBOs depend strongly on stellar cluster density.

Abstract

The discovery of planetary systems beyond our solar system has challenged established theories of planetary formation. Planetary orbits display a variety of unexpected architectures, and free-floating planets appear ubiquitous. The recent detection of candidate Jupiter Mass Binary Objects (JuMBOs) by the James Webb Space Telescope (JWST) has added another puzzling layer. Here, through direct few-body simulations, we demonstrate that JuMBOs could arise from the ejection of double giant planets following a close encounter with a passing star, if the two planets are nearly aligned at the closest approach. These ejected JuMBOs typically possess an average semi-major axis approximately three times the orbital separation within their original planetary system and a high eccentricity, characterized by a superthermal distribution that sets them apart from those formed primordially. We estimate the JuMBO formation rate per planetary system in typical and densely populated clusters, revealing a significant environmental dependence. In dense clusters, this formation rate can reach a few percent for wide planetary systems. Comparative analysis of JuMBO rates and properties with current and forthcoming JWST observations across various environments promises insights into the conditions under which these giant planets formed in protoplanetary disks, thereby imposing constraints on theories of giant planet formation.