Solar System Formation: The Frost Line and Composition of Planets

Solar System Formation: 

The Frost Line and Composition of Planets 

(picture credit to GnosticWarrior.com)

It is crucial to remember that “we are all star stuff!” In other words, the planets form as a result of galactic recycling.  Galactic recycling is the phenomena where the elements that form our planets are created within starts and recycled through space as the stars shed their layers.

As a result of galactic recycling, we know that all the elements that makeup our planets can be found within the Sun.

To understand the formation of our planets, we must go back to the very beginning!  During the formation of our solar system as the solar nebula collapsed, gravity was able to pull together the material to create the Sun, however, the other planets were not immediately formed.   Instead, seed formation occurred as a result of condensation out of the nebular gas. 

For those who don't know what the solar nebula is, check out Katie's blog accessed here --> 

In short, the solar nebula is the spinning molecular cloud in which our solar system formed.

However, the condensation of different materials depends on temperature.  The materials that make up our solar nebula can be placed into four broad categories:

1)   Hydrogen and Helium Gas: these gases do not condense in the solar nebula

2)   Hydrogen Compounds: these compounds can condense into ices below 150 K

3)   Rock: these turn from gas form into solid rock between 500 K and 1300 K.

4)   Metals: these turn from gas form into solid metals between 1000 K and 1600 K

Based on these differing temperatures needed for condensation, we know that distance from the Sun, which determines temperature, must be the deciding factor.  Here is where the frost line becomes crucial.

(picture credit to Pearson Education and Addison Wesley, accessed through www.cfa.harvard.edu)

What is the frost line?

The frost line is the boundary where ices can freeze.  It is the divider between the warmer inner region of our solar system, closest to the Sun where terrestrial planets form and the cooler regions further away from the Sun where jovian planets form.

The Formation of Planets:

Within the frost line bits of metal and rock began to condense into "seeds" which were small solid particles that acted as a building piece for gravity to build planets around.  Outside of the frost line "seeds" began to form from metal, rock and ice.  The formation of these seeds is the basis for the forming of the two types of planets.  

The Formation of Terrestrial Planets:

The seeds made up of metal and rock developed into planets through accretion.  Accretion is the process where the seeds began joining together as a result of electrostatic forces, and once they were large enough, their gravitational pull towards one another joined the pieces into large rocks which can be called planetesimals.  Once the planetesimals reached a certain large size, more growth was rare as collisions would just lead to rocky pieces breaking off.  From those planetesimals that did survive the collisions, terrestrial planets formed.  However, terrestrial planets tend to be quite small as they have less material to be built from since ice cannot form within the frost line.  This smaller size leads to less gravitational pull, therefore, less ability to capture gases and therefore, less frequent formation of moons.

The Formation of Jovian Planets:

A similar process occurred to form icy planetesimals, however, from there, the biggest of those icy planetesimals were so massive that their gravity could capture increasing amounts of helium and hydrogen gas.  These two gases were extremely plentiful.  Eventually, these gaseous planets had so much gas that their disks of gas ended up spinning, heating and flattening like the solar nebula which resulted in accretion of moons from the icy planetesimals that were already present earlier. 

References: "The Essential Cosmic Perspective" 7th Edition