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

TL;DR

This paper reports the serendipitous discovery of a potentially new solar system object with high proper motion, detected by ALMA, suggesting it could be a previously unknown planet-sized body within our solar system or beyond.

Contribution

The study demonstrates ALMA's capability to detect unknown solar system objects through multi-epoch observations, revealing a candidate object with high proper motion and unknown nature.

Findings

Detected a fast-moving object named Gna with high significance.

Estimated Gna's size to be 220-880 km if bound to the Sun.

Proposed Gna could be a bound or unbound object at varying distances.

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 $>5.8\sigma$ 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.