Spectroscopy of Free-floating Planetary-mass Objects and Their Disks with JWST

Free-floating planetary-mass objects (FFPMOs) are known to harbor disks at young ages. Here, we present 1-13 μm spectra for eight young FFPMOs with masses of 5-10 MJup (at ages of 1-5 Myr), using the NIRSpec and MIRI instruments on the James Webb Space Telescope. We derive fundamental properties of these targets, and find spectral types of M9.5 to L4, with effective temperatures of 1600-1900 K. The photospheric spectra of our targets show a clear diversity at similar temperatures, especially in the 3-5 μm range, unaccounted for by existing atmospheric models. We find a silicate absorption feature in the photosphere of one of our targets, the first such detection in very young FFPMOs, indicating silicate clouds in their cool atmospheres. Six of our objects show mid-infrared excess emission above the photosphere, as well as silicate emission features, demonstrating the presence of disks. The shape and strength of the latter features constitute strong evidence of grain growth and crystallization, similar to what is seen in more massive brown dwarfs and stars. We also detect emission lines from hydrocarbon molecules in the disks of several targets. These are the lowest-mass isolated objects found so far with silicate and hydrocarbon emission features arising in their disks. The presence of disks and their characteristics point to the potential for the formation of rocky companions around free-floating planetary-mass objects.