The Formation of the Solar System

Introduction

In this lecture, I briefly review the principal characteristics of the Solar System, and examine the current theory of how these characteristics came about. More information can be found in Universe, Sections 7.7 & 7.8.

The Characteristics of the Solar System

Any theory put forward to explain how the Solar System formed must account for a number of important characteristics which it possesses.

1. The planets are divided into two distinct classes, with quite different natures. The terrestrial planets (being Mercury, Venus, Earth and Mars) are small, of high density, and mainly comprised of heavy elements such as iron, nickel, silicon, magnesium and sulphur. They have shallow or non-existent atmospheres. In contrast, the Jovian planets (being Jupiter, Saturn, Uranus and Neptune) are large, of low density, and mainly comprised of light elements, primarily hydrogen and helium. Their atmospheres are very deep, which is why the Jovian planets are often termed 'gas giants'.
2. The two classes of planets are found in different parts of the Solar System: the terrestrial planets are all situated closer to the Sun than the Jovian planets.
3. The orbits of the planets are all close to circular.
4. The orbits of the planets all lie approximately in the same plane

Note: I have left out Pluto here, since there is still debate whether it is truly a planet, or whether it is actually a captured asteroid or comet.

The planets of the Solar System

The Condensation Model

Given the above characteristics, the best theory put forward so far to explain the formation of the Solar System is the Condensation Model. This model maintains that the Sun, and all planets, asteroids and comets, formed 5 billion years ago out of a huge cloud of material known as the Solar Nebula.

The Solar Nebula was comprised primarily of hydrogen, with some helium and traces of heavier elements; table 7.3 of Universe (p. 162) gives a breakdown of the relative abundances of some of the more significant elements. When the Universe began in the Big Bang, only hydrogen was created; all of the other material making up the the Solar Nebula was created by nuclear processes in stars which lived during the earlier stages of the Universe (between 5 and 10 billion years ago).

The Solar Nebula would have begun at a temperature of around 50K. Therefore, although the hydrogen and helium would have remained gaseous, the heavier elements would have condensed into grains of dust, about 0.1 micrometres across and comprised of metallic elements, water ice, methane, ammonia, carbon dioxide and more complex molecules.

Under its own gravitational pull, the Solar Nebula began to contract, and the following important processes took place:

1. Associated with the contraction was a conversion of gravitational energy into kinetic energy, which in turn was converted into thermal energy. This input of thermal energy means that the Solar Nebula heated up as it contracted, with the greatest degree of heating at its dense centre.
2. Initially, the Solar Nebula would have had a small degree of rotation (completely artificial conditions would have been required to ensure no rotation). During contraction, this rotation would have picked up speed, due to conservation of angular momentum (much like a spinning ice skater picks up speed as they pull their arms in).
3. As the rotation became faster, the centrifugal force caused the Solar Nebula to flatten. This resulted in the formation of a protoplanetary disk, with material moving in approximately-circular orbits around the dense, hot protosun at the centre.

Impression of the contraction of the Solar Nebula and formation of a protoplanetary disk

The Formation of Planetesimals

Within the protoplanetary disk, there existed a temperature gradient, such that the regions closest to the protosun were hottest, and those further out were cooler. This gradient meant that:

• Near the protosun, the protoplanetary disk was so hot (up to a few thousand Kelvin) that only refractory elements with high melting points (such as iron, nickel and silicon) were able to remain solid. The grains of dust made from lighter compounds (such as ice, ammonia and methane) would have evaporated.
• Further away from the protosun the disk was cool enough that, in addition to the dust comprised of refractory elements, that made from from lighter compounds were also able to remain solid. However, temperatures were still too high for hydrogen and helium to condense.

The net result was that, out to about 4 AU from the Sun, the protoplanetary disk became devoid of dust made from lighter compounds, which had evaporated; only the refractory elements remained in solid form.

As they orbited the protosun, grains of dust collided with one another at low velocities, and became stuck together by electrostatic forces. Through this process, the grains grew in size to become planetesimals, chunks of material up to about a kilometre across. The composition of these planetesimals reflected the composition of the dust from which they formed:

• In the inner regions, mainly refractory elements (e.g., iron, nickel, silicon),
• In the outer regions, a combination of refractory elements and lighter compounds (e.g., ice, ammonia and methane).

From Planetesimals to Planets

The planetesimals were large enough that they could exert a significant gravitational force on one another. Through this attraction, they combined with one another to build planets, made from refractory elements near the protosun, and from a combination of refractory elements and lighter compounds further out. Such planet building ceased once all of the available nearby planetesimals had been swept up, leaving an empty gap in the protoplanetary disk.

Being formed not only from refractory elements, but also lighter compounds, the planets further away from the protosun were large enough to then pull in material (via gravity) from neighbouring parts of the protoplanetary disk which had not been swept clean. This material was predominantly hydrogen and helium (the most abundant elements in the Solar Nebula), which although still gaseous was cool enough to accrete onto the outer planets as dense, thick atmosphere.

The formation of the Solar System finally stopped when the protosun at the centre became hot and dense enough for nuclear reactions to commence, at which point it switched on. A strong solar wind would have blown from the surface of the newly-born Sun, sweeping up all remnants of the protoplanetary disk and leading to the Solar System as it appears today. This would have occurred a few hundred million years after the Solar Nebula first began to contract.

Those planetesimals which never grew large enough to be considered a planet remain in the solar system today as comets and asteroids. The comets, which are mainly found a great distance from the Sun in the Oort Cloud, were spread too thinly in space for them to combine to form planets. In contrast, the asteroids which make up the Asteroid belt were probably prevented from coming together by the perturbing gravitational influence of nearby Jupiter. In both cases, the composition of these planetesimal relics represents the composition of the dust from which they formed.

Distribution of temperature and condensates within the protoplanetary disk

Summary

The Condensation Model is able successfully to account for the principal characteristics of the Solar System, discussed in the Introduction:

1. The differentiation between terrestrial and Jovian planets reflects the different composition of dust in the inner and outer parts of the protoplanetary disk, which came together to first form planetesimals and then form planets. In the inner parts, the dust was predominantly made from refractory elements (such as iron, nickel, silicon, magnesium and sulphur), which were able to remain solid at the high temperatures prevailing. In the outer parts, were the temperature was lower, the dust also contained lighter compounds such as ice, ammonia and methane. Because more planet-building material was available in these outer parts, the planets formed there were large enough subsequently to accrete thick atmospheres made mainly from hydrogen and helium, the most abundant elements in the Solar Nebula.
2. See above.
3. The orbits of the planets are close to circular because the dust in the protoplanetary disk moved around the central protosun in orbits themselves close to circular.
4. The orbits of the planets all lie approximately in the same plane because they formed from a protoplanetary disk, which itself had a planar configuration.

Rich Townsend
Last Modified Date: 27 January 2003