The number of recorded stellar occultations by TNOs has grown considerably in recent years. At this moment, several objects have already been measured more than once with occultations. This allows us to better explore the tridimensional shape of TNOs. Combined with photometry data, this procedure is a powerful tool to explore the fraction of contact binaries in the transneptunian region. This ratio is poorly constrained and it is of great relevance to test evolutionary models of the Solar System.
(19521) Chaos is a large classical TNO observed five times through occultations between November 2020 and September 2023. The first three occultations were low in SNR or had few chords. However, they still provided us with 3 astrometric points with milliarcsecond accuracy, which lead to a considerable refinement of the ephemerides. As a consequence, two further occultations could be observed in better conditions. The results were unforeseen. A two-lobed shape is recongnizable in both occultations. The relative orientation of the lobes between events indicates that Chaos is a compact/contact binary in a pole-on geometry (rotation axis close to the line of sight). We used a Bayesian approach to fit shape models to all 5 occultations and further explore this hypothesis.
If confirmed, the finding would very significant considering that the only confirmed contact binary TNO is Arrokoth, which is ~20 times smaller than Chaos.