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

TL;DR

This study combines optical and thermal data to determine that trans-Neptunian object (225088) 2007 OR10 is larger, darker, and rotates more slowly than previously thought, suggesting a MacLaurin ellipsoid shape and volatile retention.

Contribution

First comprehensive thermal and rotational analysis of (225088) 2007 OR10 using combined Herschel and K2 data, revealing new size, albedo, and rotation characteristics.

Findings

Diameter of approximately 1535 km, making it one of the largest TNOs.

Slow rotation period of about 44.81 hours.

Likely shape is a MacLaurin ellipsoid with surface inhomogeneities.

Abstract

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 p_V=0.089^{+0.031}_{-0.009}. The light curve analysis revealed a slow rotation rate of P_rot=44.81+/-0.37 h, superseded by a very few objects only.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 P_rot=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 -- CH_4, CO and N_2 -- on the surface.