WISEA J083011.95+283716.0: A Missing Link Planetary-Mass Object
  We present the discovery of WISEA J083011.95+283716.0, the first Y dwarf
candidate identified through the Backyard Worlds: Planet 9 citizen science
project. We identified this object as a red, fast-moving source with a faint
$W2$ detection in multi-epoch \textit{AllWISE} and unWISE images. We have
characterized this object with Spitzer Space Telescope and \textit{Hubble Space
Telescope} follow-up imaging. With mid-infrared detections in
\textit{Spitzer}'s \emph{ch1} and \emph{ch2} bands and flux upper limits in
Hubble Space Telescope $F105W$ and $F125W$ filters, we find that this object is
both very faint and has extremely red colors ($ch1-ch2 = 3.25\pm0.23$ mag,
$F125W-ch2 \geq 9.36$ mag), consistent with a T$_{eff}\sim300$ K source, as
estimated from the known Y dwarf population. A preliminary parallax provides a
distance of $11.1^{+2.0}_{-1.5}$ pc, leading to a slightly warmer temperature
of $\sim350$ K. The extreme faintness and red Hubble Space Telescope and
Spitzer Space Telescope colors of this object suggest it may be a link between
the broader Y dwarf population and the coldest known brown dwarf WISE
J0855$-$0714, and highlight our limited knowledge of the true spread of Y dwarf
colors. We also present four additional Backyard Worlds: Planet 9 late-T brown
dwarf discoveries within 30 pc.
