Abstract

Spectroscopy of exoplanets is essential to study their atmospheres but such analysis faces two challenges: (1) emission spectra of these objects suffer from small samples, low signal-to-noise, and/or low spectral resolution, and (2) the models needed to interpret these data inadequately capture the complex atmospheric processes. Non-irradiated gas-giant planets and brown dwarfs (as wide-orbit companions to stars or free-floating objects) are amenable to high-quality spectroscopy and thereby constitute excellent laboratories to study self-luminous exoplanet atmospheres. To expand the census of such imaged-planet analogs, I am carrying out the COol Companions ON Ultrawide orbiTS (COCONUTS) program, a volume-limited search for wide-orbit (>500 au) planetary-mass and substellar companions. Mining Pan-STARRS1 and AllWISE, COCONUTS has more than doubled the current census of wide-orbit brown dwarf companions to date. I will discuss the survey design and discoveries, as well as plans of extending our yields to planetary masses using the latest/upcoming sky surveys (e.g., UKIDSS, LSST). Combining spectroscopy of wide-orbit companions with those of free-floating objects, I am also conducting a data-driven examination of ultracool model atmospheres. I have performed a uniform forward-modeling analysis for 55 late-T brown dwarfs (~600-1200 K) and analyzed their spectral-fitting residuals to systematically test the cloudless model atmospheres. I will discuss extending such analysis to wider temperature and wavelength ranges to validate models over larger parameter space. Finally, the growing census of wide-orbit companions and the empirically-validated model atmospheres will lead towards robust characterization of imaged exoplanets.