Editing Trans-Neptunian object (section)

===Size determination and distribution ===
[[File:Selected Planemos.svg|thumb|Size comparison between the [[Moon]], Neptune's moon Triton, Pluto, several large TNOs, and the dwarf planet Ceres. Their respective shapes are not represented.]]

Characteristically, big (bright) objects are typically on inclined orbits, whereas the [[invariable plane]] regroups mostly small and dim objects.<ref name="Rabinowitz 2005"/>

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 [[infrared|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). TNOs are so far from the Sun that they are very cold, hence producing black-body radiation around 60 [[micrometre]]s in [[wavelength]]. This wavelength of light is impossible to observe from the Earth's surface, but can be observed from space using, e.g. the [[Spitzer Space Telescope]]. For ground-based observations, astronomers observe the tail of the black-body radiation in the far infrared. This far infrared radiation is so dim that the thermal method is only applicable to the largest KBOs. For the majority of (small) objects, the diameter is estimated by assuming an albedo. However, the albedos found range from 0.50 down to 0.05, resulting in a size range of 1,200–3,700&nbsp;km for an object of [[Absolute magnitude#Solar System bodies (H)|magnitude]] of 1.0.<ref>{{cite web|url=http://www.minorplanetcenter.org/iau/lists/Sizes.html |title=Conversion of Absolute Magnitude to Diameter |publisher=Minorplanetcenter.org |access-date=2013-10-07}}</ref>