The formation of the solar system

2.1 The formation of the solar system

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 nebula hypothesis , which explains how the Sun and planets were created, was fi rst 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 trig- gered 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 surround- ings 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:

40 Introduction to Astronomy and Cosmology

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 end point of the nuclear fusion processes during the lifetime of a star. Elements of higher atomic number are only created when a star explodes at the end of its life and are thus comparatively rare.

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 allow- ing 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 suffi ciently 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

Our Solar System 1 – The Sun

Figure 2.1 The Pleiades cluster. Image: Digitized Sky Survey NASA/ESA/AURA/Caltech.

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.

The Pleiades cluster, shown in Figure 2.1, contains about 500 stars. It is believed that the Pleiades cluster is passing through a dust cloud and this gives rise to the scattered light that is refl ected from regions surrounding the brighter stars – known as a refl ection nebula .

Due to the forces of gravity and the net angular momentum of the nebula, the nebula fl attened 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 (Figure 2.2).

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 fl edgling star went through a turbulent phase, throwing off perhaps half its mass, until it fi nally stabilized. The protostar had become a star, our Sun.

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

42 Introduction to Astronomy and Cosmology

Figure 2.2 Artist’s impression of the solar nebula. Image: Ames Research Centre, NASA.

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 gravi- tational 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’.

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 a stable hydrogen burning star it went through a phase – called the T-Tauri phase – when there was a major outfl ow of material ‘boiling off ’ from its surface. This outfl ow still continues, but at a far slower rate, and is called the solar wind . As a result the protostar lost much of

Our Solar System 1 – The Sun

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.

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.

From radiometric dating, we believe that the oldest rocks on Earth are approxi- mately 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.