largest known solid objects in the Solar System that is less dense than water. Why this should be is not well understood, because objects of its size in the Kuiper belt often contain a fair amount of rock and are hence pretty dense. To have a similar composition to others large KBOs, it would have to be exceptionally porous, which was believed to be unlikely given the compactability of water ice; this low density thus astonished astronomers. Studies by Grundy et al. suggest that at the low temperatures that prevail beyond Neptune, ice is brittle and can support significant porosity in objects significantly larger than , particularly if rock is present; the low density could thus be a consequence of this object failing to warm sufficiently during its formation to significantly deform the ice and fill these pore spaces. Satellite The discovery of a minor-planet moon was reported in IAUC 8812 on 22 February 2007. The satellite was detected using the Hubble Space Telescope in August 2005. The satellite was found at 0.16 arcsec from the primary with an apparent magnitude difference of 2.5. It orbits the primary in days, at a distance of , yielding a system mass of . The eccentricity of the orbit is . This moon is estimated to be in diameter. Assuming the same albedo as the primary, it would have a diameter of 190 km, assuming an albedo of 0.05 (typical of other cold, classical KBOs of similar size) a diameter of 260 km. References External links MPEC 2002-V08 Astronomers surprised by large space rock less dense than water, Ron Cowen, Nature, 13 November 2013 Scientist finds medium sized Kuiper belt object less dense than water, Bob Yirka, Phys.org, 14 November 2013 Classical Kuiper belt objects Discoveries by the Spacewatch project Possible dwarf planets Binary trans-Neptunian objects 20021030