Here we present an extremely cold, planetary-mass brown dwarf which bridges the temperature gap between the warmer Y dwarf population and the coldest brown dwarf ever discovered. W0830 was identified through the Backyard Worlds: Planet 9 citizen science collaboration, which brings together over 150,000 people around the world in identifying cold, fast-moving sources through coadded WISE images. We have characterized this object with Hubble and Spitzer Space Telescope follow-up photometry. The available evidence points to a ~Y1 source at Teff ~ 350 K with a planetary mass of 4-13Mjup, as extrapolated from the known Y dwarf population. This object joins a small, yet growing sample of “missing link” objects connecting brown dwarfs to giant planets in terms of temperature.

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