Solar
System
Solar system objects
The wide variety of objects that exist in the solar system fall into
several categories. In recent years many of these categories have been
found to be less clear-cut than once thought. This encyclopedia employs
the following divisions:
The Sun is a spectral class G2 star that contains 99.86% of the system's
mass.
The planets of the solar system are those nine bodies traditionally labelled
as such: Mercury, Venus , Earth, Mars , Jupiter , Saturn , Uranus , Neptune
and Pluto .
Sizeable objects that orbit these planets are moons. For a complete listing,
see that article.
Artificial satellites orbiting the planets, mainly Earth, as well as probes
heading into deep space.
Dust and other small particles that orbit these planets form planetary
rings.
Space debris of artificial origin that can be found in orbit around Earth.
Planetesimals, from which the planets were originally formed, are sub-planetary
bodies that accreted during the first years of the solar system and that
no longer exist. The name is also sometimes used to refer to asteroids
and comets in general, or to asteroids below 10 km in diameter.
Asteroids are objects smaller than planets that lie roughly within the
orbit of Jupiter and are composed in significant part of non-volatile
minerals. They are subdivided into asteroid groups and families based
on their specific orbital characteristics.
Asteroid moons are asteroids that orbit larger asteroids. They are not
as clearly distinguished as planetary moons, sometimes being almost as
large as their partners.
Trojan asteroids are located in either of Jupiter's L4 or L5 points, though
the term is also sometimes used for asteroids in any other planetary Lagrange
point as well.
Meteoroids are asteroids that range in size from roughly boulder sized
to particles as small as dust.
Comets are composed largely of volatile ices and have highly eccentric
orbits, generally having a perihelion within the orbit of the inner planets
and an aphelion beyond Pluto. Short-period comets exist with apoapses
closer than this, however, and old comets that have had most of their
volatiles driven out by solar warming are often categorized as asteroids.
Some comets with hyperbolic orbits may also originate outside the solar
system.
Centaurs are icy comet-like bodies that have less-eccentric orbits so
that they remain in the region between Jupiter and Neptune.
Trans-Neptunian objects, which are icy bodies whose semi-major axes lie
beyond Neptune's. These are further subdivided:
Kuiper belt objects have orbits lying between 30 and 50 AU (astronomical
units, an AU is approximately equal to the mean distance between Earth
and Sun). This is thought to be the origin for short-period comets. Pluto
is sometimes classified as a Kuiper belt object in addition to being a
planet, and the Kuiper belt objects with Pluto-like orbits are called
Plutinos. The remaining Kuiper belt objects are classified as Cubewanos
in the main belt and scattered disk objects in the outer fringes.
Oort cloud objects, currently hypothetical, have orbits lying between
50,000 and 100,000 AU. This region is thought to be the origin of long-period
comets.
The newly discovered object 90377 Sedna, with a highly elliptical orbit
extending from about 76 to 850 AU, does not obviously fit in either category,
although its discoverers argue that it should be considered a part of
the Oort cloud.
Small quantities of dust are present in the interplanetary medium and
are responsible for the phenomenon of zodiacal light. Some of the dust
is likely interstellar dust from outside the solar system.
Jupiter constitutes most of the mass of the solar system outside the Sun:
0.1% of the mass of the solar system. In turn, Saturn constitutes most
of the remaining mass, then Uranus and Neptune, then Earth and Venus.
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