| The Kuiper belt and Oort cloud are populations of icy planetesimals located beyond the orbit of Neptune. |
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| Planet cores and terrestrial rocky planets would accrete from the planetesimals. |
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| Because they were formed in two very different areas, the planetesimals in the two belts have different compositions. |
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| Although planets, moons and planetesimals may seem to be very different astronomically, their common origin shows that this is not so. |
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| When the planets formed 4.6 billion years ago, they formed from an agglomeration of many planetesimals, or small solid celestial bodies. |
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| Like the hills and valleys on a contour map the gravity wells of suns, planets, moons, and planetesimals showed up. |
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| The effect of the gas is to damp the orbital perturbations induced by the companion star but also to align neighbouring planetesimals orbits. |
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| The situation becomes more favourable to planetary accretion when taking into account gas drag effect on planetesimals. |
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| The Kuiper belt is a hypothetical massive flattened disc of billions of icy planetesimals supposedly left over from the formation of the solar system. |
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| First is the classical scenario in which the giant planets begin roughly in their final locations and the disk of planetesimals and embryos in the terrestrial planet region extends all the way into the outer asteroid belt region. |
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| As planetesimals grow still larger, their gravitational attraction increases, allowing them to become even more effective at accreting nearby planetesimals. |
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| Figure 2 Effect of the companion star perturbations on a population of test planetesimals. |
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| A giant planet was discovered there, close to the primary star, but the perturbations from the secondary star should have prevented the accretion of planetesimals. |
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| To study the dynamical conditions allowing accretion in such a system, the crucial parameter is thus the impact velocity among planetesimals during the accretion phase. |
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| The nebular theory indicates that particles within the flattened disk then collided and stuck together to form asteroid-sized objects called planetesimals. |
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| One of these planetesimals developed into Earth. |
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| Here, however, the accretion of icy planetesimals produced objects with masses 10 times that of Earth, sufficient to cause the gravitational collapse of the surrounding gas and dust in the solar nebula. |
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| During the process of planet formation, submicronic aggregates of grains grow and form the planetesimals which in turn by collisions will form planets. |
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| Another consequence of gaseous friction is the progressive orbital drift towards the central star it induces on planetesimals, with in particular the progressive emptying of the outer regions of the system. |
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| Some of these planetesimals combined to become the nine large planets. |
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| Other planetesimals formed moons, asteroids, and comets. |
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