A Volume-Limited Survey and Spectroscopic Characterization of Planetary-Mass and Substellar Benchmarks

Giant planets and brown dwarfs that are wide-orbit (> 500 au) companions to stars are excellent benchmarks to study self-luminous ultracool atmospheres, yet such objects are rare and the current companion census is incomplete. We are conducting the COol Companions ON Ultrawide orbiTS (COCONUTS) program, a volume-limited survey for wide-orbit companions targeting 300,000 primary stars within 100 pc. We have identified candidates using astrometry and photometry from wide-field sky surveys (Pan-STARRS1 and AllWISE) and then confirmed new companions using astrometric/photometric/spectroscopic follow-up. Thus far, COCONUTS discoveries have more than doubled the current census of wide-orbit (> 500 au) substellar companions and represent an order-of-magnitude larger yield than any previous search.

Characterization of these wide-orbit companions, as well as free-floating planets and brown dwarfs, is commonly performed by comparing their spectra with model atmospheres, but these models rely on unavoidably simplified assumptions that inadequately capture the complex atmospheric processes. To test state-of-the-art models, we have constructed a novel Bayesian forward-modeling framework and applied it to low-resolution near-infrared spectra of 55 cloudless (late-T type) exoplanets and brown dwarfs. Our data-driven analysis has found systematic offsets between data and models and for the first time studied such data-model inconsistencies as a function of wavelengths and physical parameters to identify specific shortcomings of model predictions, constituting the first systematic examination of cloudless model atmospheres.

Combining our empirically-validated model atmospheres with the observed spectra for all COCONUTS discoveries and previously known companions, we are producing a robust catalog of ultracool benchmarks with uniform and well-characterized properties. These objects will enhance our knowledge of the atmospheres on imaged exoplanets and brown dwarfs and also help understand these objects' formation and evolution based on their demographics.