Icy World in the Shape of an Elongated Triaxial Ellipsoid

Trans-Neptunian Objects (TNOs) are far-flung objects that orbit the Sun beyond the orbit of Neptune. E. Fernández-Valenzuela et al. (2015) present observations of 2008 OG19, revealing it to be a highly elongated TNO. 2008 OG19 orbits the Sun in an eccentric orbit; 38.6 AU from the Sun at its closest and 93.8 AU from the Sun at its furthest. As 2008 OG19 rotates, its brightness changes with time, creating an almost symmetric double-peaked light curve with two minima and two maxima.

The light curve’s peak to valley amplitude is 0.437 ± 0.011 mag (i.e. a brightness variation factor of roughly 1.5). The periodicity of the light curve indicates that 2008 OG19 has a rotational period of 8.727 ± 0.003 hours. Such a brightness variation is consistant with 2008 OG19 having the shape of a highly elongated triaxial ellipsoid. As different cross-sections of the triaxial ellipsoid rotate in and out of view, the observed brightness of 2008 OG19 changes, creating a double-peaked light curve each rotation. 2008 OG19 is estimated to have an equivalent diameter of approximately 620 km, making it similar in size to Varuna, another highly elongated TNO.

The presence of light and dark features on the surface of 2008 OG19 cannot be the cause of the brightness variation because the magnitude of the brightness variation is simply too large. Furthermore, the symmetrical double-peaked light curve is consistent with a rotating triaxial ellipsoid because it is unlikely for an object to have surface features that are symmetrical on both hemispheres.

Reference:
E. Fernández-Valenzuela et al. (2015), “2008 OG19: A highly elongated Trans-Neptunian Object”, arXiv:1511.06584 [astro-ph.EP]