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The unprecedented sensitivity of the Atacama Large millimeter/submillimeterarray (ALMA) is providing many new discoveries. Several of these areserendipitous to the original goal of the observations. We report the discoveryof previously unknown continuum sources, or a single fast moving new source, inour 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 $25$ arcsecond region around the asymptotic giant branch star W Aql in the continuumaround 345 GHz. At a third epoch, covering $50x50$ arcseconds, the source(s)were not seen. We have investigated if the detections could be spurious, ifthey could constitute a population of variable background sources, or if theobservations revealed a fast moving single object. Based on our analysis, weconclude that a single object (with a flux of $\sim3.0$ mJy) exhibiting a largeproper motion ($\sim87$ arcsec/yr) is the most likely explanation. Until thenature of the source becomes clear, we have named it Gna. Unless there are yetunknown, but significant, issues with ALMA observations, we have detected apreviously unknown objects in our solar system. Based on proper motion analysiswe 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 amuch larger, planet-sized, object, gravitationally unbound, and located within$\sim4000$ AU, or beyond (out to $\sim0.3$~pc) if it is strongly variable. Ourobservations highlight the power of ALMA in detecting possible solar systemobjects, but also show how multiple epoch observations are crucial to identifywhat are otherwise probably assumed to be extra-galactic sources.

The serendipitous discovery of a possible new solar system object with ALMA