Trans-Neptunian objects
A trans-Neptunian object (TNO) is any object in the Solar System that orbits the Sun at a greater distance on average than Neptune.
Small TNOs are thought to be low density mixtures of rock and ice possibly containing some organic surface material. Some may have compositions similar to that of comets. Some larger TNOs have higher densities which suggests a much higher rock content.
 |
| Caption: Graph showing orbital characteristics and size of KBOs The horizontal axis is distance from the Sun (the line shows minimum and maximum distance), and the vertical axis shows the inclination to the orbit of the Solar System (from Wikipedia). |
Astronomers currently know of three divisions in this region of the outer
Solar System:
• the Kuiper belt,
• the Scattered disk, and
• the Oort cloud.
Kuiper Belt
The Kuiper Belt (sometimes called the Edgeworth-Kuiper Belt) is a relatively roughly doughnut-shaped region of the Solar System extending from about 30 AU to 44 AU from the Sun. Kuiper Belt Objects (KBOs) mainly have near circular orbits or slightly elliptical orbits. They are thought to have largely formed in their present position, although a significant fraction may have originated in the vicinity of Jupiter, and been ejected by it to the far regions of the Solar System.
Modern computer simulations show the Kuiper belt to have been strongly influenced by the primordial evolution of the planets Jupiter and Neptune. During the early period of the Solar System, Neptune's orbit is thought to have migrated outwards from the Sun due to interactions with minor bodies. In the process, Neptune swept up, or gravitationally ejected all the primordial bodies closer to the Sun than about 30 AU. This explains why there appears to be a well-defined inner edge of the Belt, apart from those which fortuitously were in a 2:3 orbital "resonance" with Neptune (for every 3 solar orbits of Neptune these bodies orbit twice - this provides these objects with a sort of gravitational safety net - Pluto was the first of these objects to be discovered).
Scattered Disk
The properties of the Scattered Disk are still being determined as more objects of this class are discovered. Currently, it is believed to have formed when KBOs were "scattered" (or deflected) by gravitational interactions with the outer planets, primarily Neptune. This explains the observations that these objects have highly-eccentric (non-circular) and orbits highly-inclined to the plane of the Solar System.
Oort Cloud
The Oort Cloud has not been observed but is hypothesised to exist in order to account for 1/ the observed distribution of comet orbits, and 2/ the replenishment of new comets into the inner Solar System. The Oort Cloud is proposed to contain millions of comet nuclei; remnants of the original nebula that collapsed to form the Sun and planets five billion years ago. Oort cloud objects probably formed much closer to the Sun but gravitational interactions with the primordial gas giant planets such as Jupiter ejected them into a roughly spherical distribution far from the Sun and out of the plane of the Solar System. The Cloud is most densely populated around 50,000 to 100,000 AU (Sun-Earth distance) from the Sun. This is approximately 2000 times the distance from the Sun to Pluto or roughly one light year, almost a quarter of the distance from the Sun to Proxima Centauri, the star nearest the Sun.
|
| Caption: Diagram of the outer Solar System (from Wikipedia). |
Size determination
It is difficult to estimate the diameter of TNOs because they are quite distant and therefore faint. For very large objects, with very well known orbital elements (namely, Pluto and Charon), diameters can be precisely measured from the time duration of their occultation of distant stars, or by direct measurement using the Hubble Space Telescope.
 |
| Caption: Diagram showing relative sizes of Solar System objects, in particular TNOs (from Wikipedia). |
Solar System object's diameters can also be estimated by the amount of sunlight they reflect. Such estimates are critically dependent on the estimate of their albedo (reflectivity). The known range of TNO albedo can vary from 0.05 to 0.50 leading to a factor of about 3 uncertainty in diameter. Some large TNOs can have their diameters further constrained by observations of their thermal emissions in the infrared part of the electromagnetic spectrum.
