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Part 4A:
FORMATION OF THE SOLAR SYSTEM

Reading: Formation of the Solar System, Chapter 6

Any successful theory for the formation of the Solar System must explain the dynamical regularities of the planets and the existence of the three distinct classes of planets, the Terrestrial, Jovian, and Rocky/Ice planets (and other Solar System debris).

SOLAR SYSTEM FORMATION: NEBULAR THEORY

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 several light years across and was composed of material with compostion of roughly 90 % hydrogen, 9 % helium and small amounts of everyting else (like iron, carbon, oxygen, ...). (see the abundance plot below.) This large swirling cloud that formed the Solar System is referred to as the Solar Nebula . 4.6 billion years ago, an event triggered the slowly spinning cloud to start to collapse. As the nebula constracted, it started to spin faster to conserve angular momentum (to see another example of this, look at the YouTube video showing ice skaters YouTube video, start around 0:44). As the nebula spun faster, because of centrifugal forces, it (flattened. Eventually the central region of the solar nebula formed the Sun while the planets formed in the rotating disk of gas and dust, the Solar Nebula.

This simple idea of flattening into a disk as the Solar Nebula contracted leads to a natural explanation for most of the important dynamical regularities of the planets. Namely, (i) the orbits are in the same sense, (ii) the orbits are roughly coplaner, (iii)mthe rotations of the planets are the same sense as the orbital motions, and (iv) the orbits of moons are in the same sense as the planet's orbits. The circularization of the orbits occurs later.

Schematically, the process goes as follows:

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 large enough to be held together by gravity; this occurs when they are a few kilometers across. At this time the objects are referred to as planetesimals.

When the gravity of the planetesimals becomes large enough to start attracting other planetesimals and so form larger bodies, they are referred to as protoplanets. Protoplanets have sizes in the range 100 km to thousands of kilometers.

The larger protoplanets (after reaching a critical size) may then grab and hold onto the hydrogen and helium gas in the cooler regions of the Solar Nebula. These objects become the Jovian planets. The objects in the inner part of the Solar Nebula where it is hot are unable to capture the hydrogen and helium gas and become the Terresrials.

The process is slow, it takes 3 to 10 Million years to form the Jovian planets.
Around the same time the larger protoplanets start to capture hydrogen and helium gas, the Sun ignites nuclear fusion in its core and generates a strong outward flow of material (an extreme Solar Wind). The Sun enters the T Tauri stage. The strong wind clears the gas from the Solar System arresting Jovian planet formation. The wind does not clear out solid material, however. The left-over pieces of rocks and rock/ice are the rock/ice objects and the material left-over in the Asteroid Belt and Kuiper Belt. Large chunks of solid debris can also have a substantial impact on the evolution of the young planets as well as eventually forming the Terrestrial planets.

Some nice pictures of planet forming disks are shown below:

HL Tau (ALMA observatory)
PDS 70 (Subaru telescope)

CONDENSATION THEORY

Okay, can the NEBULAR THEORY further elucidate details of how the 3 distinct classes of planets, the Terrestrials, Jovians, and rocky/icy planets come about? As described above, the general existence of the Terrestrial and Jovian planets is a natural by-product of the planet formation process, that is,

after the initial phase of Solar System formation, we have a rotating disk of gas and dust. The gas makes up the bulk of the disk. The gas is primarily hydrogen and helium, and the dust is made of heavier elements and materials (silicates, iron, carbon, ...).
Now, let us introduce a little tweak that explains why the Jovians form where they do in the Solar System.
As you move outward in the Solar Nebula, the environment gets cooler and ice coatings around the dust particles start to form. The ice coatings form from molecules like water, methane, and ammonia. Because there is so much hydrogen and oxygen in the disks (and indeed in the Universe), a large amount of ice can form and enhances the amount of particulate material, the particulate material becomes much more abundant and can lead to faster planetesimal formation and to the formation of much more massive planetesimals. This in term leads to the formation of much more massive protoplanetary objects and likely marks the boundary between where Terrestrials and Jovians form.
The radius beyond which ice can form in the disk is called the Snowline. In our Solar System, the Snowline falls around 3-4 Astronomical Units. This crucial boundary is expected to show up in oextra-Solar planetary systems if our understanding of Solar System formation is correct.

Are there thoughts why the Jovians show two distinct types, Jupiter and Saturn versus Uranus and Neptune? Perhaps. The Protosun forms at the center of the disk while the planets form in the surrounding disk.
The initial phases of planet formation are essentially the same for the Terrestrials, Jovians, and Rock/Ice objects. 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. combine and form Terrestrial planets in the inner part of the disk where it is warm;

or

b. start to attract and hold onto the hydrogen and helium gas of the Solar Nebula to form the Jovian planets where it is cooler

or

c. never do this and become objects like the rocky/icy planets over the region where the Jovian planets form

What Detemines the Boundary Between Where Jovians form Compared to Terrestrials?

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 at which materials remain solid depends on the local pressure and the type of material. In general, the lighter the element, the lower the temperature at which it vaporizes. (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., the Snowline), 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.
The Earth was not massive enough to capture and hold the hot hydrogen and helium gas in the inner part of the Solar Nebula; see the chemical composition of the Earth of the plot at lower left.
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. It is suggested that as many 1,000 Pluto-type objects formed there but only Pluto remains in this region. 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) into 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.

New Horizons flyby of Ultima Thule (MU 69)

Ultima Thule

A Few Odds and Ends

Origin of the Moon
Solar System simulator
Origin of the Asteroids (for your amusement, see Titius-Bode Law)