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What Makes Up Our Solar System?
Most people are aware that our solar system is made up of the Sun, eight planets
and one dwarf planet, but many aren’t aware that there is so much more to the
solar system. Our solar system consists of the sun, the terrestrial inner planets,
the asteroid belt, the gas giant outer planets (Jovian planets), the Kuiper belt, the
scattered disc, the heliopause, and the Oort cloud.
Composition of the Solar System
The Sun
The sun as we saw in the first unit is our solar system’s star. It belongs to the
main sequence of stars and fuses hydrogen into helium. Refer to our first unit for
any other characteristics.
Planets formed from the left over bits of matter. While the sun grew, material was
pulled in. Once it created light, the waves of heat pushed things away. The sun
could not push away the heavy rocks and metal, but most gases got pushed
further away. The gases floated like clouds, while gradually forming the Gas
Giant planets.
The Terrestrial Inner Planets
Mercury, Venus, Earth, and Mars all make up the inner planets. They are called
the terrestrial planets because they are composed primarily of rock and metal.
They also have relatively high densities, slow rotation, solid surfaces, no rings
and few satellites. The planets, Venus, Earth, and Mars have significant
atmospheres while Mercury has almost none.
The Asteroid Belt
The asteroid belt is found between Mars and
Jupiter. It is believed that the asteroids in the
asteroid belt are primoridal objects left over from the
formation of the solar system. While some have
suggested that they are the remains of a
protoplanet that was destroyed in a massive
collision long ago, the prevailing view is that
asteroids are leftover rocky matter that never
successfully formed into a planet. The "Main
Asteroid Belt" that is centered around 2.7 times the
Earth-Sun distance (astronomical unit or AU) from the Sun. It has been
estimated that the total mass of the Main Asteroid Belt may total less than
1/1000th of the mass of the Earth.
The Gas Giant Outer Planets (Jovian Planets)
Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like)
planets, because they are all gigantic compared to Earth, and they have a
gaseous composition like Jupiter. The gas planets are composed primarily of
hydrogen and helium and generally have low densities, rapid rotation, deep
atmospheres, rings and lots of satellites. It is thought that these planets may
have a small solid core as large as three to 20 Earth masses at their center.
The Kuiper Belt
The Kuiper Belt is a disk-shaped region past the orbit of Neptune extending
roughly 30 to 50 AU from the Sun and contains many small icy bodies. It is now
considered to be the source of the short-period comets.
Several Kuiper Belt objects have been discovered recently. They appear to be
small icy bodies similar to Pluto and Triton (but mostly smaller). There are more
than 800 known trans-Neptunian objects.
The Scattered Disc
The scattered disc (or scattered disk) is a distant region of the Solar System that
is sparsely populated by icy minor planets. While the nearest distance to the Sun
approached by scattered objects is about 30–35 AU, their orbits can extend well
beyond 100 AU. This makes scattered objects "among the most distant and cold
objects in the Solar System".
The Heliopause
The heliopause is the
boundary between the Sun's
solar wind and the
interstellar medium (All the
gas and dust found between
stars). The solar wind blows
a "bubble" known as the
heliosphere into the
interstellar medium. The
outer border of this "bubble"
is where the solar wind's
strength is no longer great
enough to push back the
interstellar medium. This is known as the heliopause, and is often considered to
be the outer border of the solar system. The zone between the termination shock
and the heliopause is known as the heliosheath. The heliosheath is the place
where the solar wind slows down and begins to interact with the interstellar
medium. The heliosheath has a few parts: the termination shock (the innermost
part of the boundary), the heliopause (the outermost part of the boundary) and
the part in between the inner and outer boundary.
The Oort Cloud
The Oort cloud is an immense spherical cloud surrounding the planetary system
and extending approximately 3 light years, about 30 trillion kilometers from the
Within the cloud, comets are typically tens of millions of kilometers apart. They
are weakly bound to the sun, and passing stars and other forces can readily
change their orbits, sending them into the inner solar system or out to interstellar
space. This is especially true of comets on the outer edges of the Oort cloud. The
structure of the cloud is believed to consist of a relatively dense core that lies
near the ecliptic plane and gradually replenishes the outer boundaries, creating a
steady state. One sixth of an estimated six trillion icy objects or comets are in the
outer region with the remainder in the relatively dense core.
The Oort cloud is the
source of long-period
comets and possibly
intermediate comets
that were pulled into
shorter period orbits
by the planets, such
as Halley and SwiftTuttle.
Long period comets
can appear at any
time and come from
any direction. Bright comets can usually be seen every 5-10 years. Two recent
Oort cloud comets were Hyakutake and Hale-Bopp. Hyakutake was average in
size, but came to 0.10 AU (15,000,000 km) from Earth, which made it appear
especially spectacular. Hale-Bopp, on the other hand, was an unusually large
and dynamic comet, ten times that of Halley at comparable distances from the
sun, making it appear quite bright, even though it did not approach closer than
1.32 AU (197,000,000 km) to the Earth.
Curiously, it seems that the Oort Cloud objects were formed closer to the Sun
than the Kuiper Belt objects. Small objects formed near the giant planets would
have been ejected from the solar system by gravitational encounters. Those that
didn't escape entirely formed the distant Oort Cloud. Small objects formed farther
out had no such interactions and remained as the Kuiper Belt objects.