Large size and slow rotation of the trans-Neptunian object (225088) 2007 OR10 discovered from Herschel and K2 observations

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We present the first comprehensive thermal and rotational analysis of the second most distant trans-Neptunian object (225088) 2007 OR10. We combined optical light curves provided by the Kepler space telescope -- K2 extended mission and thermal infrared data provided by the Herschel Space Observatory. We found that (225088) 2007 OR10 is likely to be larger and darker than derived by earlier studies: we obtained a diameter of d=1535⁺⁷⁵₋₂₂₅ km which places (225088) 2007 OR10 in the biggest top three trans-Neptunian objects. The corresponding visual geometric albedo is pᵥ=0.089⁺⁰.⁰³¹₋₀.₀₀₉. The light curve analysis revealed a slow rotation rate of Pᵣot=44.81+/-0.37 h, superseded by a very few objects only. The most likely light-curve solution is double-peaked with a slight asymmetry, however, we cannot safely rule out the possibility of having a rotation period of Pᵣot=22.40+/-0.18 h which corresponds to a single-peaked solution. Due to the size and slow rotation, the shape of the object should be a MacLaurin ellipsoid, so the light variation should be caused by surface inhomogeneities. Its newly derived larger diameter also implies larger surface gravity and a more likely retention of volatiles -- CH₄, CO and N₂ -- on the surface.