  In recent years brown dwarfs have been extended to a new Y-dwarf class with
effective temperatures colder than 500K and masses in the range 5-30 Jupiter
masses. They fill a crucial gap in observable atmospheric properties between
the much colder gas-giant planets of our own Solar System (at around 130K) and
both hotter T-type brown dwarfs and the hotter planets that can be imaged
orbiting young nearby stars (both with effective temperatures of in the range
1500-1000K). Distance measurements for these objects deliver absolute
magnitudes that make critical tests of our understanding of very cool
atmospheres. Here we report new distances for nine Y dwarfs and seven very-late
T dwarfs. These reveal that Y dwarfs do indeed represent a continuation of the
T dwarf sequence to both fainter luminosities and cooler temperatures. They
also show that the coolest objects display a large range in absolute magnitude
for a given photometric colour. The latest atmospheric models show good
agreement with the majority of these Y dwarf absolute magnitudes. This is also
the case for WISE0855-0714 the coldest and closest brown dwarf to the Sun,
which shows evidence for water ice clouds. However, there are also some
outstanding exceptions, which suggest either binarity or the presence of
condensate clouds. The former is readily testable with current adaptive optics
facilities. The latter would mean that the range of cloudiness in Y dwarfs is
substantial with most hosting almost no clouds -- while others have dense
clouds making them prime targets for future variability observations to study
cloud dynamics.
