Part 4A: FORMATION OF THE SOLAR SYSTEM Reading: Chaisson & McMillan, Chs. 6 & 15

Any theory for the formation of the Solar System must deal with the dynamical regularities of the planets and the existence of the Terrestrial, Jovian, and Icy planets (and other Solar System debris). Recall:

DYNAMICAL REGULARITIES

• All planetary orbits are roughly in the same plane -- low inclinations
• All planetary orbits are roughly circular -- small eccentricities
• All planetary orbits are in the same sense -- CCW as viewed from the North

Secondary regularities are:

• Most planets rotate in the same sense as they orbit
• The rotation axes of most planets are roughly perpendicular to the ecliptic
• The orbits of the major moons of most planets are in the CCW sense

The dynamical regularities can be explained if the planets all formed from a large, rotating disk.

REGULARITIES IN THE PROPERTIES OF THE PLANETS

The planets also show distinct regularities in that they can be divided into three classes: Terrestrials, Jovians, and Icy planets ( Pluto, and some other large moons [ Triton] of the Jovian planets).

The regularity of the properties of the planets is tied to how far they are from the Sun. The Terrestrial planets are closer to the Sun than are the Jovian planets.

---

SOLAR SYSTEM FORMATION: CONDENSATION THEORY

Schematically, the process goes as follows:

Star formation in our Galaxy occurs in Interstellar clouds known as Giant Molecular Clouds (see Topic 7: Star Formation in ASTR 122). The Solar System formed from a cold, rotating clump inside a GMC. The initial cloud was roughly 90 % hydrogen, 9 % helium with small amounts of everyting else (like iron, carbon, oxygen, ...). This large swirling cloud that formed the Solar System is referred to as the Solar Nebula . As the cloud shrinks, it starts to spin faster (in order to conserve angular momentum). As the cloud spins faster, the centrifugal force causes it to flatten into a disklike shape. Eventually the central region of the solar nebula forms the Sun while the planets form in the disk of the solar nebula. This simple idea of flattening as the solar nebula collapses leads to a natural explanation for several of the dynamical regularities of the planets. Namely, the orbits are in the same sense, the orbits are roughly coplaner, the rotations of the planets are the same sense as the orbital motions, and the orbits of moons are in the same sense as the planet's orbits. The circularization of the orbits occurs later.

---

The planets form in the midplane of the Solar Nebula as follows: small dust grains (~10-5 m in size) embedded in the cloud collide and coalesce. This process of collision and coalescence continues until the clumps are a few hundred kilometers across. At this time the objects are referred to as planetesimals. The gravity of the planetesimals is large enough to start attracting other planetesimals and so form larger bodies referred to as protoplanets. The larger protoplanets may attract and then hold onto the hydrogren and helium gas in the surrounding Solar Nebula. The preceding process takes equires on the order of 100,000,000 years.

Around the same time the larger protoplanets start to capture gas, the Sun ignites nuclear fusion in its core on and generates a strong outward flow of material (an extreme Solar Wind). The Sun enters the T Tauri stage. The strong outward wind cleans out the gas from the Solar System arresting planet formation. The wind, however, does not clear out solid material. The large chunks of solid debris have a substantial impact on the evolution of the young planets.

---

Okay, but how does the CONDENSATION THEORY explain existence of the 3 distinct classes of planets, the Terrestrials, Jovians, and Icy planets?

The existence of different classes of planets is a natural product of the planet formation process.

• After the initial phase of Solar System formation, we have a rotating disk of gas, dust, and ice-covered dust. The gas makes up the bulk of the disk. The gas is primarily hydrogen and helium, the dust is made of heavier elements and materials (silicates, iron, carbon, ...), and the ice is water, methane, and ammonia.

• The Protosun is at the center of the disk and the planets will form in the disk.

• The initial phases of planet formation are essentially the same for all planets (the Terrestrials, the Jovians, and the Icy planets). The solid material (the dust particles, and the ice-covered dust) collide and stick together and form planetesimals (up to asteroid size).

• When the planetesimals become large enough to start attracting each other gravitationally, the largest ones grow rapidly in size and are renamed protoplanets.

  It is from here that the process branches. The protoplanets eventually:

  a. become Terrestrial planets;

  b. start to attract and hold onto the hydrogen and helium gas of the Solar Nebula to form the Jovian planets; or

  c. become objects like the Icy planets.

Why the different outcomes for the process?

The key point which explains why the Terrestrials are rocky and in the central portions of the Solar System and the Jovians are more gaseous and in the outer portions of the Solar System is that different materials can be in the solid phase only under well-defined conditions. For example, water is liquid at room temperature on the Earth and becomes a solid below 32 F. Iron and other heavier elements are clearly solid at room temperature on the Earth and can remain solid even to very high temperatures. The exact temperatures to which materials remain solid depends the temperature, pressure, and the type of material. In general, the lighter the element, the lower the temperature at which it will vaporize. (Materials that vaporize at high temperature are referred to as refractory elements. Materials that vaporize at low temperature are referred to as volatile elements. .)

In the Solar Nebula, the inner regions of the disk are warmer than the outer regions of the disk and only relatively massive elements, the rocky materials can be solid and participate in the planet formation process. In the outer regions of the disk, the temperature decreases and becomes low enough for water to solidify (around 3-4 A.U.), ammonia to solidfy, and methane to solidify, and the outer planets start to form from rock/ice mixtures.

Jupiter, in fact, forms just beyond where water ice is first able to form. Since water is made up of hydrogen (the most abundant element in the Universe) and oxygen (one of the most abundant of the heavy elements--carbon and nitrogen are the others), the amount of planet forming material greatly increases once water forms ice. Because of this, Jupiter was able to form a giant rock/ice core (more than 10 times the mass of the Earth). This extra-large core gave Jupiter a larger gravitational pull than the Earth and allowed Jupiter to capture and then hold onto the abundant hydrogen and helium gas. Saturn also followed this route.

Uranus and Neptune started down this path, but before they could finish the job at hand, the Sun turned on and blew out the gaseous material from the Solar Nebula and stunted their growth.

The reason Uranus and Neptune were a little bit slower than Jupiter and Saturn is that since they formed in a region a little farther from the Protosun than did Jupiter and Saturn, the particles were a little farther apart and the particles were moving around a little less quickly. Consequently, the coalesence process moved along more slowly.

The Icy planets are thought to be smaller objects which formed in the outer portions of the disk around Saturn ==> Neptune. Some people believe that as many 1,000 Pluto-type objects were formed out there and that only Pluto has managed to survive. The brothers and sisters of Pluto were probably ejected from the inner Solar System by encounters with the large planets (Jupiter, Saturn, Uranus, and Neptune) forming the Kuiper Belt, a region which extends from ~30 A.U. (around Neptune's orbit) to 55 A.U. (outside Pluto's orbit) which contains many low-mass ice/rock objects. Since their first detection, around 1,000 Kuiper Belt Objects (KBOs) have been discovered and perhaps as many as 70,000 KBOs exist. Neptune's moon Triton may be a captured KBO.

A Few Odds and Ends

• Origin of the Moon
• Origin of the Asteroids