Trans-Neptunian object
They are thought to be composed of mixtures of rock, amorphous carbon and volatile ices such as water and methane, coated with tholins and other organic compounds.
After Pluto's discovery, American astronomer Clyde Tombaugh continued searching for some years for similar objects but found none.
For a long time, no one searched for other TNOs as it was generally believed that Pluto, which up to August 2006 was classified as a planet, was the only major object beyond Neptune.
Eris, the most massive TNO, was discovered in 2005, revisiting a long-running dispute within the scientific community over the classification of large TNOs, and whether objects like Pluto can be considered planets.
[nb 1] The diagram to the right illustrates the distribution of known trans-Neptunian objects (up to 70 au) in relation to the orbits of the planets and the centaurs for reference.
The Edgeworth–Kuiper belt contains objects with an average distance to the Sun of 30 to about 55 au, usually having close-to-circular orbits with a small inclination from the ecliptic.
It is estimated that there are between 240,000 and 830,000 scattering objects bigger than r-band absolute magnitude 12, corresponding to diameters greater than about 18 km.
However, the interpretations are typically ambiguous as the spectra can fit more than one model of the surface composition and depend on the unknown particle size.
More significantly, the optical surfaces of small bodies are subject to modification by intense radiation, solar wind and micrometeorites.
Consequently, the thin optical surface layer could be quite different from the regolith underneath, and not representative of the bulk composition of the body.
Small TNOs are thought to be low-density mixtures of rock and ice with some organic (carbon-containing) surface material such as tholins, detected in their spectra.
[17] Colour indices are simple measures of the differences in the apparent magnitude of an object seen through blue (B), visible (V), i.e. green-yellow, and red (R) filters.
[18] For reference, two moons, Triton and Phoebe, the centaur Pholus and the planet Mars are plotted (yellow labels, size not to scale).
This distinction leads to suggestion that the surface of the largest bodies is covered with ices, hiding the redder, darker areas underneath.
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.
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).
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).
[36] The existence of planets beyond Neptune, ranging from less than an Earth mass (Sub-Earth) up to a brown dwarf has been often postulated[37][38] for different theoretical reasons to explain several observed or speculated features of the Kuiper belt and the Oort cloud.
