The Diversity of Cold Worlds: Age and Characterization of the Exoplanet COCONUTS-2b

TL;DR

This study uses JWST data to characterize COCONUTS-2b, a cold brown dwarf, determining its age, mass, and atmospheric properties, and comparing it with similar objects to understand cold giant exoplanet atmospheres.

Contribution

First detailed atmospheric and physical characterization of COCONUTS-2b using JWST spectra, establishing its planetary mass and potential formation mechanism.

Findings

COCONUTS-2b is a planetary mass object (~7.5 MJup) with a well-constrained age (~414 Myr).

Spectral analysis reveals differences in vertical mixing compared to similar brown dwarfs.

Membership in the Ursa Major group supports its age and origin.

Abstract

Studying cold brown dwarfs is key to understanding the diverse characteristics of cold giant exoplanets atmospheres. COCONUTS-2, is a wide binary system composed of a T9 brown dwarf and an M3 star, which presents a unique opportunity to characterize a cold benchmark brown dwarf. As part of a JWST program to study the range of physical and atmospheric properties of the coldest brown dwarfs, we obtained NIRSpec G395H spectra (R~2700, 2.87-5.13 um) and MIRI F1000W, F1280W, and F1800W photometry for COCONUTS-2b. In this work, we find a 99% probability of the system belonging to the Corona of Ursa Major (414+/-23 Myr) using BANYAN Sigma and its full kinematics. We also re-estimate the astrometry of COCONUTS-2b using the MIRI data. We support the membership with a comparison of rotation period, metallicity and C/O ratio of the group with those of the COCONUTS-2 system. We also calculate its bolometric luminosity, which combined with our age estimation, allows us to derive its mass, effective temperature, surface gravity, and radius with high precision. As a result of our analysis, we support the conclusion that COCONUTS-2b is a planetary mass object (7.5+/-0.4 MJup) which was likely formed via the same mechanism as stars. In addition we compare the JWST spectrum to another object in the sample, J082507.35+280548.5 (0825+2805), a Y0.5 brown dwarf, which is a candidate member of the same moving group, but has a lower mass (3.7+/-0.2 MJup). We identify absorption feature differences which could indicate that 0825+2805 has stronger vertical mixing.