Protoplanets are moon-sized planet embryos within protoplanetary discs. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide. According to planet formation theory, protoplanets perturb each other's orbits slightly and thus collide in giant impacts to gradually form the real planets. A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ... This article or section does not cite any references or sources. ... The eight planets and three dwarf planets of the Solar System. ...
Early protoplanets had more radioactive elements, the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center; while lighter elements rose to the surface; such a process is known as planetary differentiation. Composition of some meteorites show that differentiation took place in some asteroids. Radioactive decay is the set of various processes by which unstable atomic nuclei (nuclides) emit subatomic particles. ... Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ... The eight planets and three dwarf planets of the Solar System. ... The periodic table of the chemical elements A chemical element, or element for short, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ... In cosmogony, planetary differentiation is a process by which the denser portions of a planet will sink to the center; while less dense materials rise to the surface. ... Willamette Meteorite A meteorite is a natural object originating in outer space that survives an impact with the Earths surface without being destroyed. ...
The giant impact theory proposes that Earth's Moon formed from a colossal impact of a hypothetical protoplanet, named Theia, with Earth early in the solar system's history. The Big Splash The giant impact theory (or Big Splash or Big Whack; cf. ... Apparent magnitude: up to -12. ... Theia (THAY-uh) is the hypothetical planet that, according to the giant impact theory of the Moons formation, collided with Earth over four billion years ago. ...
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