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 Sun. 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 higher-inclination 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.