The Gliese 86 Binary System: A Warm Jupiter Formed in a Disk Truncated at $\approx$2 AU

TL;DR

This study combines observational data and simulations to show that the giant planet in the Gliese 86 system formed within a truncated protoplanetary disk around 2 AU, despite close stellar companions.

Contribution

It provides the first detailed analysis of how a giant planet can form in a severely truncated disk caused by a close binary companion.

Findings

Giant planet formation occurred in a disk truncated at approximately 2 AU.

The disk had enough mass to form the planet's core and envelope under high accretion rates.

The system's orbital history suggests a close stellar approach of about 9 AU during its main sequence phase.

Abstract

Gliese 86 is a nearby K dwarf hosting a giant planet on a $\approx$16-day orbit and an outer white dwarf companion on a $\approx$century-long orbit. In this study we combine radial velocity data (including new measurements spanning more than a decade) with high angular resolution imaging and absolute astrometry from Hipparcos and Gaia to measure the current orbits and masses of both companions. We then simulate the evolution of the Gl 86 system to constrain its primordial orbit when both stars were on the main sequence; the closest approach between the two stars was then about $9\,$AU. Such a close separation limited the size of the protoplanetary disk of Gl 86 A and dynamically hindered the formation of the giant planet around it. Our measurements of Gl 86 B and Gl 86 Ab's orbits reveal Gl 86 as a system in which giant planet formation took place in a disk truncated at $\approx$2$\,$AU. Such a disk would be just big enough to harbor the dust mass and total mass needed to assemble Gl 86 Ab's core and envelope, assuming a high disk accretion rate and a low viscosity. Inefficient accretion of the disk onto Gl 86 Ab, however, would require a disk massive enough to approach the Toomre stability limit at its outer truncation radius. The orbital architecture of the Gl 86 system shows that giant planets can form even in severely truncated disks and provides an important benchmark for planet formation theory.