Higher albedos and size distribution of large transneptunian objects

Transneptunian objects (TNOs) orbit beyond Neptune and do offer important clues about the formation of our solar system. Although observations have been increasing the number of discovered TNOs and improving their orbital elements, very little is known about elementary physical properties such as sizes, albedos and compositions. Due to TNOs large distances (>40 AU) and observational limitations, reliable physical information can be obtained only from brighter objects (supposedly larger bodies). According to size and albedo measurements available, it is evident the traditionally assumed albedo p = 0.04 cannot hold for all TNOs, especially those with approximately absolute magnitudes H ⩽ 5.5 . That is, the largest TNOs possess higher albedos (generally >0.04) that strongly appear to increase as a function of size. Using a compilation of published data, we derived empirical relations which can provide estimations of diameters and albedos as a function of absolute magnitude. Calculations result in more accurate size/albedo estimations for TNOs with H ⩽ 5.5 than just assuming p = 0.04 . Nevertheless, considering low statistics, the value p = 0.04 sounds still convenient for H > 5.5 non-binary TNOs as a group. We also discuss about physical processes (e.g., collisions, intrinsic activity and the presence of tenuous atmospheres) responsible for the increase of albedo among large bodies. Currently, all big TNOs (>700 km) would be capable to sustain thin atmospheres or icy frosts composed of CH4, CO or N2 even for body bulk densities as low as 0.5 g cm−3. A size-dependent albedo has important consequences for the TNOs size distribution, cumulative luminosity function and total mass estimations. According to our analysis, the latter can be reduced up to 50% if higher albedos are common among large bodies. , by analyzing orbital properties of classical TNOs ( 42 AU < a < 48 AU ), we confirm that cold and hot classical TNOs have different concentration of large bodies. For both populations, distinct absolute magnitude distributions are maximized for an inclination threshold equal to 4.5° at >99.63% confidence level. Furthermore, more massive classical bodies are anomalously present at a < 43.5 AU , a result statistically significant and apparently not caused by observational biases. This feature would provide a new constraint for transneptunian belt formation models.