Our colleague J.L. Ortiz, from IAA, has been leading a recent and very interesting work on one of the five official dwarf planets: Eris. This dwarf planet, orbited by its satellite Dysnomia, seems to have comparatively large albedo and density. Its presumably homogeneous surface prevented to determine its spin period for long until very recently, when it was found out that the main body might be tidally locked to the 15.8 days orbit of its known satellite.
Artist’s rendering of Eris and its moon Dysnomia. NASA/ESA/STScI. From https://science.nasa.gov/dwarf-planets/eris/
Ortiz’s idea was to analyze the more than 400 photometric data of Eris from Gaia DR3, taking advantage of both its extraordinarily high accuracy, and long baseline. Periodograms of this data set soon revealed that Eris system was characterized by two frequencies. One was located at the known long period of 15.8~days. Nevertheless, periodograms showed that the most significant peak was incommensurably located at 18.9~hours. This highest peak is puzzling. Ortiz et al. suggest that Eris system might hide a yet unknown satellite that could account for many of the distinctive characteristics of Eris, including its high albedo, density, and the non-Keplerian orbit of Dysnomia.
If you are interested in this story, you can find more in our recently published paper
Featured image: Excerpt from Figure 1 in Ortiz et al., 2025.