Gna? Or Something else in the Far Outer Solar System?

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

Authors:

Vlemmings et al

Abstract:

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 greater than 5.8σ in the image plane and greater than 14σ 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 ∼3.0 mJy) exhibiting a large proper motion (∼87 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 ∼220−880 km. Alternatively it is a much larger, planet-sized, object, gravitationally unbound, and located within ∼4000 AU, or beyond (out to ∼0.3~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.