B−V 0.70, V−R 0.39, e.g. Orcus) to RR (very red, B−V 1.08, V−R 0.71, e.g. Sedna) with BR and IR as intermediate classes. BR (intermediate blue-red) and IR (moderately red) differ mostly in the infrared bands I, J and H. Typical models of the surface include water ice, amorphous carbon, silicates and organic macromolecules, named tholins, created by intense radiation. Four major tholins are used to fit the reddening slope: Titan tholin, believed to be produced from a mixture of 90% N2 (nitrogen) and 10% (methane) Triton tholin, as above but with very low (0.1%) methane content (ethane) Ice tholin I, believed to be produced from a mixture of 86% and 14% C2H6 (ethane) (methanol) Ice tholin II, 80% H2O, 16% CH3OH (methanol) and 3% As an illustration of the two extreme classes BB and RR, the following compositions have been suggested for Sedna (RR very red): 24% Triton tholin, 7% carbon, 10% N2, 26% methanol, and 33% methane for Orcus (BB, grey/blue): 85% amorphous carbon, +4% Titan tholin, and 11% H2O ice Size determination and distribution Characteristically, big (bright) objects are typically on inclined orbits, whereas the invariable plane regroups mostly small and dim objects. It is difficult to estimate the diameter of TNOs. For very large objects, with very well known orbital elements (like Pluto), diameters can be precisely measured by occultation of stars. For other large TNOs, diameters can be estimated by thermal measurements. The intensity of light illuminating the object is known (from its distance to the Sun), and one assumes that most of its surface is in thermal equilibrium (usually not a bad assumption for an airless body). For a known albedo, it is possible to estimate the surface temperature, and correspondingly the intensity of heat radiation. Further, if the size of the object is known, it is possible to predict both the amount of visible light and emitted heat radiation reaching Earth. A simplifying factor is that the Sun emits almost all of its energy in visible light and at nearby frequencies, while at the cold temperatures of TNOs, the heat radiation is emitted at completely different wavelengths (the far infrared). Thus there are two unknowns (albedo and size), which can be determined by two independent measurements (of the amount of reflected light and emitted infrared heat radiation). Unfortunately, TNOs are so far from the Sun that they are very cold, hence producing black-body