Comets and early Solar System Comets are often said to contain primordial material of Solar System: it would be the material of the original protoplanetary disk that originated planets and all the other objects.
Documents that state this can be found here (page 1) and here (page 38), for example.
But what are the reasons of this belief? Why are scientists almost sure that comets kept this material unchanged for about 4.5 billion years?
 A: The basic observation is the ecliptic plane .

Observations show that the other planets, with the exception of Pluto, also orbit the sun in essentially the same plane. The ecliptic plane then contains most of the objects which are orbiting the sun. This suggests that the formation process of the solar system resulted in a disk of material out of which formed the sun and the plane

The second observation is that presently the earth is moving through a relative vacuum. Asteroidsand comets appear in regular trajectories, i.e. are trapped by the sun.
It is all ofcourse a matter of modeling and accepting the model if it agrees with the observations.

Astronomers estimate that at the time the Solar system formed, its proto-planetary disk contained the equivalent of about twenty Jupiter-masses of gas and dust. This so-called "minimum mass solar nebula (MMSN)" is derived from the current masses of the rocky planets and calculations of how they formed; a minimum mass is used in case the planet formation mechanism is somehow less efficient than expected. (Some earlier estimates had MMSN values up to about 100 Jupiter-masses.) As a nebula ages and its planets develop, its disk mass naturally decreases; current models estimate that a planetary system can form in under five million years.
Comets are small, irregularly shaped bodies in the solar system composed mainly of ice and dust that typically measure a few kilometers across. They travel around the sun in very elliptical orbits that bring them very close to the Sun, and then send them out past Neptune. There are two categories of comet, based on the amount of time they take to orbit the Sun. Short-period comets take less than 200 years, and long-period comets take over 200 years, with some taking 100,000 to 1 million years to orbit the Sun. The short-period comets are found near the ecliptic, which means they are orbiting the Sun in same plane as the planets. The short-period comets are thought to originate in the Kuiper Belt, an area outside Neptune's orbit (from about 30 to 50 AU) that has many icy comet-like objects. The long-period comets tend to have orbits that are randomly oriented, and not necessarily anywhere near the ecliptic.

The model works , and hence it is reasonable to suppose that the far flung pieces from the proto planetary cloud that keep to the elliptic will contain matter similar to the one that condensed to the planets themselves.
