COCONUTS: COol Companions ON Ultrawide orbiTS

Giant planets and brown dwarfs on very wide (> 500 au) orbits are valuable benchmarks to understand ultracool atmospheres, yet such objects are rare and their formation remains uncertain. Constructing a comprehensive catalog of ultracool companions will significantly benefit the characterization of planetary and substellar atmospheres. Also, studying the distributions of companions in separation and mass is essential to understand how they formed, since different scenarios predict different distributions. However, such companion distributions are not well-constrained due to the incompleteness and heterogeneity of the current census. To establish a complete sample, we are performing a dedicated large-scale search for wide-orbit giant planets and brown dwarf companions, targeting a volume-limited sample of 300,000 primary stars. We have identified candidates using multi-wavelength photometry and multi-epoch astrometry from Gaia DR2, Pan-STARRS1, UKIDSS, and WISE, and then assess our candidates' companionship and ultracool nature using ground-based astrometric, photometric, and spectroscopic follow-up. We will present our survey design and preliminary discoveries. Our work is the first volume-limited survey of wide-orbit substellar companions and we aim to build up a robust, well-defined sample of companion benchmarks to bring insight into wide companion formation and to improve our understanding of ultracool atmospheres and evolution.