Y Dwarfs: The Challenge of Discovering the Coldest Substellar Population in the Solar Neighborhood

Stars form in the Galaxy with a wide range in mass. If the mass is below 7% of the Sun's, then the object does not become hot enough for stable hydrogen burning. These substellar objects are called brown dwarfs. Maps of the sky at infrared wavelengths have found large numbers of brown dwarfs. However only 24 objects have been found (as of April 2017) that are cold enough to be classified as "Y dwarfs": these have atmospheres that are cooler than 500 K (or 200 C, 400 F) and have masses only 5 - 20 times that of Jupiter. The coolest Y dwarf currently known, discovered in 2014, has a temperature around freezing, has a mass of about 5 Jupiter masses, and is only 2 pc away from the Sun. These small and cold objects are faint and difficult to find. This chapter describes the discovery and characterization of the Y dwarfs. Finding more of these very cold planet-like brown dwarfs will require an as-yet unplanned space mission mapping large areas of sky at wavelengths around 5 microns.