Once the planetesimals have reached the size of Ceres, with diameters of around 1000 km, gravitational effects begin to play an additional role in their growth. Small planetesimals collide by chance, when they happen to be in each other’s way, but larger planetesimals attract each other gravitationally and enforce collisions from a much wider volume around their flight path. In addition, heat created by impacts and by the decay of radioactive isotopes from the interstellar material melts the interior of some of the planetesimals and produces sedimentation of the heavy material, such as iron, into their cores. Eventually, planetesimals accumulate into planets. Simulations have shown that the development from planetesimals to planets occurs in time spans of several 10 million years. In a timedependent calculation by Wetherill, the motion of 500 planetesimals in their orbit around the Sun was modeled. These initially had masses between that of Ceres and half that of the Moon, and were distributed across a distance range between 0.4 and 2 AU. In the course of the calculation, the number of planetesimals decreases due to three types of catastrophic events. First, some of them collide to form larger bodies; second, some fall into the Sun; and third, some are thrown far out to the outer boundaries of the solar system or escape from it altogether. In the latter scenario, a close encounter between two planetesimals gives one of them sufficient energy to overcome solar gravity. In the two phases of the calculation, the eccentricity of the planetesimals is plotted against the semimajor axis of their elliptical orbit around the Sun.
Since the accretion disk lasted only for about 3–4 million years the formation of the terrestrial planets was mainly due to the development in the planetesimal disk. The accumulation of the Earth by planetesimals essentially ended about 30–40 million years after the start of the solar system 4.567 billion years ago. However, the subsequent phase of heavy bombardment lasted for at least 600 million years from the time where the planet embryo formed around 4.56 billion years ago until the end of the heavy bombardment phase at about 3.9 billion years ago. This time was characterized by the collisions of very large Ceres-, Moon- and even Mars-sized bodies in the first few tens of millions of years and later by many giant ocean-vaporizing impacts.
Discovery Adds Mystery to Earth's Genesis2
Earth and the other rocky planets aren't made out of the solar system's original starting material. the sun's basic building blocks differ significantly from those of Earth, the moon and other denizens of the inner solar system.
Oxygen has three stable isotopes: oxygen-16 (eight neutrons), oxygen-17 (nine neutrons) and oxygen-18 (ten neutrons). The sun has significantly more oxygen-16, relative to the other two isotopes, than Earth. Some process enriched the stuff that formed our planet — and the other rocky bodies in the inner solar system — with oxygen-17 and oxygen-18 by about 7 percent. Furthermore: solar wind has about 40 percent less nitrogen-15 (compared to nitrogen-14) than do samples taken from Earth's atmosphere.
1. Intelligent Life in the universe, page 51
Planetary system formation: exposing naturalistic storytelling