During the formation of the Solar System, there were probably many more (planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the solar nebula.The Terrestrial planets all have roughly the same structure—a central metallic core, mostly iron, with a surrounding silicate mantle.
The term inner planet should not be confused with inferior planet, which refers to those planets that are closer to the Sun than Earth is (i.e. This theory holds that 4.6 billion years ago the Solar System formed from the gravitational collapse of a giant molecular cloud.
This initial cloud was likely several light-years across and probably birthed several stars.
The first solid particles were microscopic in size.
These particles orbited the Sun in nearly circular orbits right next to each other, as the gas from which they condensed.
The geology of solar terrestrial planets mainly deals with the geological aspects of the four terrestrial planets of the Solar System, Mercury, Venus, Earth, and Mars, and one terrestrial dwarf planet, Ceres.
Only one terrestrial planet, Earth, is known to have an active hydrosphere.
Terrestrial planets are substantially different from the giant planets, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.
Terrestrial planets have a compact, rocky surfaces, and Venus, Earth, and Mars each also have an atmosphere. Terrestrial planets have numerous similarities to plutoids (objects like Pluto), which also have a solid surface, but are primarily composed of icy materials.
Gradually the gentle collisions allowed the flakes to stick together and make larger particles which, in turn, attracted more solid particles towards them. The objects formed by accretion are called planetesimals—they act as seeds for planet formation. They coalesced into larger objects, forming clumps of up to a few kilometers across in a few million years, a small time with comparison to the age of the Solar System.
After the planetesimals grew bigger in sizes, collisions became highly destructive, making further growth more difficult.
Only the biggest planetesimals survived the fragmentation process and continued to slowly grow into protoplanets by accretion of planetesimals of similar composition.