‘Our cradle planet, the Earth, and the Mars, where we are staying, are two terrestrial planets within the Solar System. Since ancient time, people all over the Earth had make conjectures about the formation of astronomical objects, especially their home.’

‘I know that human’s technology has improved a lot since the New Stone Age; and now, you can even produce artificial human.’

‘Yes, and the theory about the formation of the Solar System is more advanced than the legends in the prehistoric periodic. Not only to explain phenomenon, but our theory can also predict before discovery.’

EVOLUTION OF THE UNIVERSE>FORMATION OF THE SOLAR SYSTEM>ZONE 3: COMETS

Where does the solar system end? The traditional answer has been at the orbit of Pluto (mean heliocentric distance: 39.4 AU). However, we now know the Sun’s family extends to much greater distances. It was long suspected that large planets failed to form beyond Neptune not because there were no icy planetesimals to accrete them from, but only because the planetesimals were dispersed too thinly. Some 50 years ago Kenneth Edgeworth and Gerard Kuiper independently predicted that such a belt of remnant building blocks still exists beyond Pluto. Recent astronomical observations, particularly by the HST, have confirmed the existence of these objects. Extrapolating from the sample observed, there must be more than 200 million objects, each a few kilometers in dimension, orbiting in the inner edge of what has come to be called the Kuiper belt. (Pluto itself, its satellite Charon, and the Neptunian moon Triton are probably the largest examples of Kuiper-belt objects.) The presence of all this solid matter extending the solar system’s diameter beyond Pluto is consistent with the large disks found in association with young solar-type stars.

Short-period comets are very likely Kuiper-belt planetesimals that have been perturbed into the inner solar system by mutual interactions or by (as yet undiscovered) Pluto-size objects. Long-period comets are thought to have had a different origin, from within Zone 2. As they neared their final sizes, the giant planets gravitationally “stirred” the icy planetesimals around them into eccentric orbits. (NASA refers to such an interaction with a planet as a “gravity assist”, and employs the technique to pump up the orbital energy of its spacecraft.) Most of the redirected planetesimals were ejected from the solar system altogether, doomed to wander “forever” in interstellar space. However, a great many did not quite reach the solar system’s escape velocity; these hangers-on, still feebly bound to the Sun at vast distances from it, make up the Oort cloud, whose representatives occasionally revisit the planetary system in the form of long-period comets.

So where does the solar system effectively end? Outside Pluto’s orbit, the Kuiper belt of icy planetesimals appears to extend to about 50 AU, then taper off. Much farther out lies the Oort cloud of comet nuclei, the last remote fringe of the solar system. The outer margin of the Oort cloud extends to roughly 1 light-year — and we should consider this the true limit of the Sun’s realm.

Nearly four centuries of telescopic observation, combined with five decades of space exploration, have taught us this essential truth about the solar system: while the Sun and its planetary system surely arose from one grand spiral of gas and dust in a flurry of collective activity, the results are hardly a homogeneous set of characterless orbs. Instead, this grand scheme of formation has yielded amazing diversity. It is humbling to realize that still other totally different kinds of planets, beyond our imagining, must be circling stars elsewhere in the galaxy.