Item

Abstract

The thermal emission of Trans-Neptunian objects and Centaurs has been observed at mid- and far-infrared wavelengths—with the biggest contributions coming from the Spitzer and Herschel space observatories—and the brightest ones also at submillimeter and millimeter wavelengths. These measurements allowed to determine the sizes and albedos for almost 180 objects, and densities for about 25 multiple systems. The derived very low thermal inertias show evidence for a decrease at large heliocentric distances and for high-albedo objects, which indicate porous and low-conductivity surfaces. The radio emissivity was found to be low (ϵ_r = 0.70 ± 0.13) with possible spectral variations in a few cases. The general increase of density with object size points to different formation locations or times. The mean albedos increase from about 5%–6% (Centaurs, scattered-disk objects) to 15% for the detached objects, with distinct cumulative albedo distributions for hot and cold classicals. The color-albedo separation in our sample is evidence for a compositional discontinuity in the young solar system. The median albedo of the sample (excluding dwarf planets and the Haumea family) is 0.08, the albedo of Haumea family members is close to 0.5, best explained by the presence of water ice. The existing thermal measurements remain a treasure trove at times where the far-infrared regime is observationally not accessible.