Formation of Solar System – Introduction
The formation of the Solar System is a story of ourselves. It is believed from studies of meteorites – remnants of the early Solar System – that our Sun and planets formed some 4.6 billion years ago from what is called a giant molecular cloud. The gas and dust making up this cloud had been created over billions of years by the processing of primeval hydrogen and helium in stars to create heavier elements that are then ejected into space at the end of their lives.
The Nebulae Hypothesis
The nebula hypothesis, which explains how the Sun and planets were created, was first proposed by Emanuel Swedenborg in 1734 and then, independently, by Pierre-Simon Laplace in 1796. It is now thought that the initial cloud of dust and gas would have been about 1 pc (3.26 Ly) across and that its collapse was triggered by the shock waves from one or more supernovae (the explosive end to giant stars). These would have produced regions of higher density than their surroundings which would then collapse under their own gravity. The region that spawned our Solar System would have had a diameter of perhaps 13 000 AU with a total mass perhaps twice that of our Sun. Its composition would have been similar to that of our Sun, with 98% made up of hydrogen (∼74%) and helium (∼24%) and about 2% of heavier elements.
It is interesting to get a feel of its composition from a list of some of the most common isotopes of the elements in nuclei per million.
- Iron-56 1169
Notice that those elements whose nuclei have an integral multiple of four nucleons are relatively common. We will see when we study stellar evolution that these elements are those produced by the build-up of helium nuclei and thus their atomic numbers are integral multiples of four. Iron is also common as it is the element with the most stable nucleus and the virtual endpoint of the nuclear fusion processes during the lifetime of a star. Elements of a higher atomic number are only created when a star explodes at the end of its life and is thus comparatively rare.
Collapsing The Nebulae
It is likely that the solar nebula would have had some rotational energy. As the nebula collapsed, conservation of angular momentum meant that it would have begun to spin faster and, as it became denser, collisions within it would cause the gas to heat up. This would increase the pressure within the nebula so tending to make it expand and thus preventing further collapse. The term ‘Giant Molecular Cloud’ used for the initial gas and dust cloud from which our Solar System formed implies that it contained many molecules. These played an important role in allowing the nebula to condense in that transitions between their vibrational states can emit long-wavelength infrared photons. These could escape through the dust and so carry energy away from the cloud so preventing a build-up of heat that would have prevented further collapse.
Sir James Jeans showed that a nebula would only collapse if it was sufficiently massive. A gas cloud would have to exceed what is termed the Jeans Mass in order to collapse – a dense, cool cloud being able to collapse more easily than a less dense, warmer cloud. The initial mass required, even for a dense, cool cloud, has to be many solar masses so it is virtually impossible for a single star to form on its own and so young stars are seen in clusters like the Hyades and Pleiades clusters in the constellation Taurus.
It is believed that the Pleiades cluster is passing through a dust cloud and this gives rise to the scattered light that is reflected from regions surrounding the brighter stars – known as a reflection nebula.
Due to the forces of gravity and the net angular momentum of the nebula, the nebula flattened into a spinning protoplanetary disc. Its overall diameter was roughly 200 AU with a dense central region that rapidly increased in temperature to form what is called a protostar.
From Protostar to Star
1st step of formation of the solar system
After around 100 million years, the temperature and pressure at the core of the protostar became so great (∼10 million K) that its hydrogen began to fuse into helium and the pressure produced by the resultant gamma rays became able to counter the force of gravitation. The fledgling star went through a turbulent phase, throwing off perhaps half its mass until it finally stabilized. The protostar had become a star, our Sun.
Formation of Planets
From the gas and dust surrounding the nascent star, the various planets were formed through a process known as accretion. Dust grains in orbit around the protostar clumped together and formed what are called planetesimals, between 1 km and 10 km in diameter, which then gradually increased in size by further collisions. Due to the Sun’s radiation, the inner Solar System was too warm for volatile molecules like water and methane to condense, so the planetesimals which formed there were relatively small and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets: Mercury, Venus, Earth, and Mars. As a result of the gravitational effects of Jupiter, the formation of a planet between Mars and Jupiter was disrupted leaving rocky objects that are known as minor planets or asteroids in what is called the asteroid belt. The largest of these, Ceres, has recently been given the status of a ‘dwarf planet’.
2nd step of formation of the solar system
As the temperature fell further away from the Sun, volatile icy compounds could remain solid – beyond what is called the frost line. Jupiter and Saturn were able to gather far more material than the terrestrial planets and overlaying their icy/rocky cores were layers of metallic and molecular hydrogen. They became the gas giants and contain the largest percentages of hydrogen and helium. Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be mostly made of ice which is overlain by molecular hydrogen and gases such as ammonia, methane, and carbon monoxide.
As our young Sun settled down as stable hydrogen-burning star it went through a phase – called the T-Tauri phase – when there was a major outflow of material ‘boiling off ’ from its surface. This outflow still continues, but at a far slower rate, and is called the solar wind. As a result, the protostar lost much of its original mass. This strong solar wind cleared away all the remaining gas and dust in the protoplanetary disc into interstellar space, thus ending the growth of the planets.
Formation of Moon
Last Step of formation of the solar system
The majority of the moons probably formed at the same time as their parent planets. However, it seems that our own Moon probably formed later when a body several times as massive as Mars collided with our planet. The giant impact blasted molten rock into orbit around Earth which then cooled to form the Moon.
Conclusion – Formation of the Solar System
From radiometric dating, we believe that the oldest rocks on Earth are approximately 3.9 billion years old. We expect that the Solar System is older than this as the Earth’s surface is constantly evolving as the result of erosion, volcanism and plate tectonics. It is believed that meteorites were formed early on within the solar nebula so estimates of their age should give us an age of the Solar System. The oldest meteorites are found to have an age of ∼4.6 billion years, giving us a minimum age of the Solar System. And in short, this is the story of the formation of our solar system.