Letter to the Editor

Tidally locked rotation of the dwarf planet (136199) Eris discovered via long-term ground-based and space photometry^⋆

¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary
² CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
e-mail: szakats.robert@csfk.org
³ ELTE Eötvös Loránd University, Institute of Physics, Budapest, Hungary
⁴ Eötvös Loránd University, Pázmány Péter sétány 1/A, 1171 Budapest, Hungary
⁵ Max-Planck-Institut für extraterrestrische Physik, P.O. Box 1312 85748 Garching, Germany
⁶ Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ Department of Astronomy, Institute of Geography and Earth Sciences, Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁸ Centre for Astrophysics and Space Science, Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁹ ELKH-SZTE Stellar Astrophysics Research Group, Szegedi út, Kt. 766, 6500 Baja, Hungary
¹⁰ Wigner Research Centre for Physics, PO Box 49 Budapest 1525, Hungary

Received: 13 October 2022
Accepted: 14 November 2022

Abstract

The rotational states of the members in the dwarf planet-satellite systems in the trans-Neptunian region are determined by formation conditions and the tidal interaction between the components. These rotational characteristics serve as prime tracers of their evolution. A number of authors have claimed a very broad range of values for the rotation period for the dwarf planet Eris, ranging from a few hours to a rotation that is (nearly) synchronous with the orbital period (15.8 d) of its satellite, Dysnomia. In this Letter, we present new light curve data for Eris, taken with ∼1–2 m-class ground based telescopes and with the TESS and Gaia space telescopes. The TESS data did not provide a well-defined light curve period, but it could be used to constrain light curve variations to a maximum possible light curve amplitude of Δm ≤ 0.03 mag (1-σ) for P ≤ 24 h periods. Both the combined ground-based data and Gaia measurements unambiguously point to a light curve period equal to the orbital period of Dysnomia, P = 15.8 d, with a light curve amplitude of Δm ≈ 0.03 mag, indicating that the rotation of Eris is tidally locked. Assuming that Dysnomia has a collisional origin, calculations with a simple tidal evolution model show that Dysnomia must be relatively massive (mass ratio of q = 0.01–0.03) and large (radius of R_s ≥ 300 km) to have the potential to slow Eris down to a synchronised rotation. These simulations also indicate that (assuming tidal parameters usually considered for trans-Neptunian objects) the density of Dysnomia should be 1.8–2.4 g cm⁻³. This is an exceptionally high value among similarly sized trans-Neptunian objects, setting important constraints on their formation conditions.