As we know, the Solar System contains over 178 objects that revolve around the central star or Sun. We can see some of these objects from Earth with the unaided eye or an Earth-based telescope. Still, the majority have only been detected by developing instruments such as the Hubble Space Telescope or unmanned probes such as Voyager. These operate outside Earth’s atmosphere and collect more information about the composition and behavior of objects in the Solar System, which has enabled researchers to hypothesize about their origins.
It is generally thought that a cloud of interstellar gas and dust is known as a ‘nebula’ disturbed by some major event in space – possibly a supernova – about 5 billion years ago and collapsed under its own gravity. This cloud rotated rapidly as it got smaller and denser and heated up to several thousand degrees, causing some of its elements to vaporize into a gas. The cloud center became so hot that it eventually exploded into a star, with the cooler gases flowing around it. In time, the gases condensed into dust, metals, and various kinds of ice in the cold outer reaches of space. These solid particles collided with each other to form larger objects or asteroids as they continued to spin around the central star.
As these asteroids increased in size, their gravity pulled in all the material in their immediate surroundings, and the largest of these became planets. The very different composition of the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Neptune, and Uranus) leads astronomers to believe that their distances from the Sun caused them to develop at different rates and in different ways. According to the most widely-held opinions, the planets closest to the Sun where all the ice particles were vaporized because of the incredible heat (the Sun is 15.6 million degrees Kelvin at the core), were formed mostly of rock, silicates, and metals with high melting points as these particles collided and were pulled together by gravity. These inner planets have thin atmospheres or none at all, and few, if any satellites, which would show that most of the material was either pulled into their own gravity or burnt away in the heat of the Sun. The inner and outer planets are separated by an asteroid belt comprising a material that was probably prevented from coalescing into a world by interfering with Jupiter’s immense gravity. Beyond this area, as more dust and ice particles escaped destruction by the Sun, four larger planets formed over a longer period of time in a far colder environment as the material was thrown out from the center by the spinning star’s centrifugal force. About a million years after the original nebula’s cooling, the Sun emitted a stream of charge, protons, and electrons known as the solar wind, which blew the remaining gases outwards, to be sucked in by the outer planets which became gas giants. These planets attracted many objects into their vast gravity fields, some of which are big enough to be termed ‘satellites’ and countless smaller fragments that form rings around the planets.
The discovery of more objects in the Solar System in recent times has led to the need for further classification. Far beyond the outer planets lies Pluto, which was originally considered the ninth planet but has since been found to be a binary system of two dwarf planets, the other being Charon. Pluto’s origins may be in the recently discovered Kuiper Belt, the source of many of the comets that travel through the Solar System; this theory is based on Pluto’s rock/ice composition to that of a comet. At one time, also thought to be a moon of Neptune, Pluto/Charon was reclassified in 2006 as one of three dwarf planets discovered so far: Eris and Ceres.
All scientific knowledge is only as good as the ability of scientists to collect and understand the evidence, and as new advances are made in astronomy, the present theories as to the extent of the Solar System may be disproved, as in the case of Pluto. At this time, the above account represents the consensus of current opinions on the matter.
Rosetta Probe's (Image credit: ESA) An ESA mission was launched in 2004 to visit the comet “67P/Churyumov-Gerasimenko” to discover the history of comets and whether they played any part in bringing water