Derby and District Astronomical Society

The Journal of the Derby and District Astronomical Society
January - April 2006

A History of the Solar System
By Dave Selfe

Our Solar System

What exactly is the 'Solar System'? Well our Solar System consists of the Sun and all of the objects that are gravitationally bound to it. These objects are the planets and their associated satellites, the asteroids, the Kuiper Belt Objects (KBOs) and the comets that orbit the sun in the far off Oort Cloud. In later articles of Aries, we will study these objects more closely and we shall discover what a wonderful and diverse solar system we reside in.

Our solar system is dominated by the Sun, which constitutes something around 99.8% of the total mass of the whole of the solar system. When we consider that amount – 99.8% of the total mass of the whole of the solar system – we may find difficulty in grasping just how huge an amount this actually is. Well if we realize that the planet Jupiter has more mass than all the other planets added together and if we also consider that the mass of the Sun is about 1,000 times the mass of Jupiter then maybe we get just the vaguest idea of just how massive our Sun is. And let us not forget that our Sun is just a very average star!

Our Solar System
A stylised view of our Solar System (not to scale). Image Credit: NASA/CXC/SAO

After the Sun, the planets make up the next major constituents of the Solar System and these are probably the most explored objects of all in the Solar System. Besides the numerous ground and space based telescopes that have examined them, there have been many spacecraft put in orbit around them and robotic missions dropped to the surfaces to investigate further. In recent years, even the comets and asteroids have had spacecraft sent to examine them.

The Solar Nebula

The current theory for the formation of the Solar System is that it began to form around 4.6 billion years ago within a rotating disc of gas and dust. This is known as the Solar Nebula. The planar shape and the shared sense of rotation is strong evidence of how the Solar System formed and by measuring the products of radioactive decay of meteorites, the age of the Solar System can be determined.

The solar nebula was itself formed when a cloud of interstellar gas and dust was disturbed, possibly by the shock waves from a nearby supernova, and the cloud began to contract and rotate. The rotation speeded up as the nebula contracted (the Conservation of Angular Momentum) and as the cloud collapsed it grew denser and it began to heat up, the centre became hot enough to form a protostar. The gas continued to flow around the central protostar with most of it flowing inwards and into the protostar, thus adding to its mass. Eventually the temperature reached 10 million ºC, hot enough for nuclear fusion and the protostar became a new young star, our Sun. All the original dust in the solar nebula was vaporised and this added to the gas surrounding the Sun.

The solar nebula
Artist's rendering of the solar nebula. Image Credit: NASA

The gas rotating around the Sun was prevented from totally falling into the Sun by centrifugal force and this created an accretion disc around the Sun. As the gas in the disk began to cool new tiny particles of dust began to form. These tiny particles (consisting of metal, rock and ice) collided with each other forming larger particles. Due to the solar nebula being denser nearer the centre, the majority of collisions occurred within a few AU of the Sun (an astronomical unit (AU) is the mean distance between the Earth and the Sun, about 150 million km or 93 million miles). After a period of about 10 000 years these particles formed globules about 1 cm across. These larger particles continued to collide, forming ever larger particles, until eventually they were the size of boulders. After around 10 000 years these boulders collided and formed objects about 10 km in diameter, known as planetesimals. These planetesimals had their own noticeable gravity and this attracted more of the smaller objects. A run away process had now begun and collisions became more common, with the largest objects exerting the greater pull. Because the larger objects experienced more collisions than their smaller counterparts did, they grew much more rapidly.

The size of these planetary embryos depended upon their distance from the Sun and the composition of the solar nebula. It is thought that the sizes of these objects in the inner solar system were between the size of a large asteroid and our moon. In the outer solar system, (beyond what is now the asteroid belt) the sizes of the objects were between one and 15 times the size of the Earth. At this time, collisions were now much less frequent and due to the size of the objects involved when collisions did occur many were giant impacts causing almost total devastation. Sometimes the colliding objects were completely fragmented but usually the fragments of the smaller object would fall onto the surface of the larger object increasing its size even further. The heat liberated in these giant impacts would melt the object causing the denser materials to segregate inwards and the lighter, less dense material would flow outwards. This process is known as differentiation and it accounts for why the terrestrial planets have an iron core surrounded by a rocky mantle.

The formation of the outer planets was slower than that of the inner rocky planets and the process finished much later. The planets that formed in the outer solar system were giant planets. Unlike in the inner solar system where the heat from the Sun kept ice as a vapour, the temperature at a distance of 5 AU and over from the Sun was low enough for frozen water to condense directly from the solar nebula. These outer planets began as rocky bodies several Earth masses in size and surrounded by ice but once they had reached a point of being 10 Earth masses their gravity was great enough to pull in hydrogen and other gases directly from the solar nebula, increasing their size even further. Jupiter and Saturn were particularly suitable to capture a greater mass of hydrogen than both Uranus and Neptune. The mass of captured hydrogen around Jupiter and Saturn is far greater than their rock and ice cores and they are correctly known as gas giants. Uranus and Neptune being further away from the Sun grew more slowly, were less massive, and were unable to collect so much gas as Jupiter and Saturn. Eventually after around 1 million years the Sun entered a phase known as the T Tauri phase (named after the young star T Tauri which is currently going through this period). This phase produces violent winds and these winds blew away the remnants of the solar nebula thus ending any chance of Uranus and Neptune capturing any more gas. Uranus and Neptune are therefore correctly known as giant planets not gas giants.

It is believed that the accretion process that formed the early planetesimals took around a few hundred thousand years in the inner solar system and up to 20 million years for the outermost objects. It is also believed that it took somewhere between 10 and 100 million years before the solar system was formed to something similar to the structure that we see today.

Our Solar System (diagram)
Diagrammatic view of our Solar System (not to scale)

The T Tauri wind also swept away any early atmospheres that had been collected by the terrestrial planets, and it is believed that the atmospheres now present are the result of the escape of gases from within (mainly due to volcanic activity). The T Tauri phase is thought to have lasted around 10 million years after which the solar wind became more tenuous. The speed of the solar wind experienced by the Earth at present is around 250 km per second. However its density is very slight, and in fact less than one ten thousandth of the Sun’s total mass has been lost this way.

The giant planets also collected gas and dust, which formed a disc around them similar to the disc in the solar nebula. In these discs the satellites of the giant planets grew. In fact two of these satellites are larger then the planets Mercury and Pluto. Each giant planet also has its own ring system although Saturn has by far the most spectacular.

Between Mars and Jupiter, there are vast amounts of rocky bodies known as the asteroids and they orbit the Sun in what is known as the asteroid belt. The asteroids were formed because Jupiter’s gravitational field caused the planetesimals in this area to collide violently and fragment. Growth here was prevented and most of the material was ejected into space by close encounters with Jupiter. The remnants left in the asteroid belt have a mass less than one thousandth of the Earth.

Beyond the orbit of Neptune, collisions were rare and no large bodies grew, however there are thousands of icy bodies orbiting the Sun between 30 and 50 AU. This area is known as the Kuiper Belt. There are estimated to be around 70,000 Kuiper Belt Objects (KBOs) greater than 100 km in diameter. Pluto was the largest of these objects until the discovery of 2003 UB313 was made. This object is larger than Pluto is and confirmation from the International Aastronomical Union (IAU) is yet to be made confirming this object as the 10th planet! Triton - the moon of Neptune - is believed to be a captured KBO.

Beyond the Kuiper Belt is the Oort Cloud where the comets reside. Comets are made from ice, carbonaceous material and rock dust. Some are a few km in diameter whilst others may be tens of km in diameter. These comets exist in highly eccentric orbits and are most likely to be objects that were flung out of the solar system by the giant planets. The Oort Cloud is about 40 000 AU away from the Sun.

It is worth noting that the theory of the origins of solar system formation is still evolving and this is particularly true since the discovery of exosolar planets. So far the discoveries made have failed to match the current theory, however this could be due to observational bias or possibly errors in the current theory. It is to be expected that as more discoveries are made then the theory will need to be revised or indeed even rejected. There is a theory that the solar system was created more rapidly in violent star forming processes but as of yet the evidence has not been fully gathered for this.