The planets can be classified into two major groups: (1) the terrestrial
planets of Mercury, Venus, Mars, and Earth and (2) the giant planets of
Jupiter, Saturn, Uranus, and Neptune.Comets are porous aggregates of water ice, frozen methane, frozen
ammonia, dry ice, and dust. The solar system is surrounded by the Kuiper
Belt and the Oort cloud of these objects. Something nudges one of the icy
bodies and it falls into a long elliptical orbit around the Sun. As it approaches
the Sun, increased radiation evaporates ices and pushes ions
and dust into a long visible tail. Asteroids are rocky or metallic bodies that
are mostly located in a belt between Mars and Jupiter. The remnants of
comets, fragments of asteroids, and dust are called meteoroids. A meteoroid
that falls through Earth's atmosphere and melts to a visible trail of
light and smoke is called a meteor. A meteoroid that survives the trip
through the atmosphere to strike the surface of Earth is called a meteorite.
Most meteors are fragments and pieces of dust from comets. Most meteorites
are fragments that resulted from collisions between asteroids.
The protoplanet nebular model is the most widely accepted theory of
the origin of the solar system, and this theory can be considered as a series
of events, or stages. Stage A is the creation of all the elements heavier
than hydrogen in previously existing stars. Stage B is the formation of a nebula from the raw materials created in stage A. The nebula contracts
from gravitational attraction, forming the protosun in the center with a
fat, bulging accretion disk around it. The Sun will form from the protosun,
and the planets will form in the accretion disk. Stage C begins as the
protosun becomes established as a star. The icy remains of the original
nebula are the birthplace of comets. Asteroids are other remains that did
undergo some melting.
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