  The unprecedented sensitivity of the Atacama Large millimeter/submillimeter
array (ALMA) is providing many new discoveries. Several of these are
serendipitous to the original goal of the observations. We report the discovery
of previously unknown continuum sources, or a single fast moving new source, in
our ALMA observations. Here we aim to determine the nature of the detections.
The detections, at $>5.8\sigma$ in the image plane and $>14\sigma$ in the
$(u,v)-$plane, were made in two epochs of ALMA observations of a $25$ arc
second region around the asymptotic giant branch star W Aql in the continuum
around 345 GHz. At a third epoch, covering $50x50$ arcseconds, the source(s)
were not seen. We have investigated if the detections could be spurious, if
they could constitute a population of variable background sources, or if the
observations revealed a fast moving single object. Based on our analysis, we
conclude that a single object (with a flux of $\sim3.0$ mJy) exhibiting a large
proper motion ($\sim87$ arcsec/yr) is the most likely explanation. Until the
nature of the source becomes clear, we have named it Gna. Unless there are yet
unknown, but significant, issues with ALMA observations, we have detected a
previously unknown objects in our solar system. Based on proper motion analysis
we find that, if it is gravitationally bound, Gna is currently located at
$12-25$ AU distance and has a size of $\sim220-880$ km. Alternatively it is a
much larger, planet-sized, object, gravitationally unbound, and located within
$\sim4000$ AU, or beyond (out to $\sim0.3$~pc) if it is strongly variable. Our
observations highlight the power of ALMA in detecting possible solar system
objects, but also show how multiple epoch observations are crucial to identify
what are otherwise probably assumed to be extra-galactic sources.
