The coolest 'stars' are free-floating planets

TL;DR

This paper presents evidence that free-floating planets, like OTS44, form through star-like processes, exhibit accretion, and share disk properties with stars, extending star formation theories to planetary masses.

Contribution

It demonstrates that objects down to a few Jupiter masses form via star-like mechanisms, with active accretion and disk properties similar to stars, expanding understanding of planetary formation.

Findings

OTS44 shows active accretion with broad Paschen beta emission.

Disk mass of OTS44 is about 10 Earth masses.

The disk-to-central-mass ratio is approximately 1% across a wide mass range.

Abstract

We show that the coolest known object that is probably formed in a star-like mode is a free-floating planet. We discovered recently that the free-floating planetary mass object OTS,44 (M9.5, ~12 Jupiter masses, age ~2 Myr) has significant accretion and a substantial disk. This demonstrates that the processes that characterize the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses. We detected in VLT/SINFONI spectra that OTS44 has strong, broad, and variable Paschen beta emission. This is the first evidence for active accretion of a free-floating planet. The object allows us to study accretion and disk physics at the extreme and can be seen as free-floating analog of accreting planets that orbit stars. Our analysis of OTS44 shows that the mass-accretion rate decreases continuously from stars of several solar masses down to free-floating planets. We determined, furthermore, the disk mass (10 Earth masses) and further disk properties of OTS44 through modeling its SED including Herschel far-IR data. We find that objects between 14 and 0.01 solar masses have the same ratio of the disk-to-central-mass of about 1%. Our results suggest that OTS44 is formed like a star and that the increasing number of young free-floating planets and ultra-cool T and Y field dwarfs are the low-mass extension of the stellar population.