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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 32:
The Origin of the Solar System

Key Ideas:

The present-day properties of our Solar System hold important clues to its origin.

Primordial Solar Nebula:

From Planetesimals to Planets:

Terrestrial vs. Jovian planet formation.

The Birth of the Solar System

The present-day properties of the Solar System hold important clues to its formation history.

Relevant Observations:

Clues from planetary motions:

Clues from planet composition:

Inner Planets & Asteroids:

Jovian Planets:

Outer solar system moons & icy bodies:

Formation of the Sun

Stars form out of interstellar gas clouds:

Rotating fragments collapse further:

Primordial Solar Nebula

The rotating solar nebula is composed of

Starts out at ~2000 K, then cools:

Condensation Temperatures
Temp (K) Elements Condensate
>2000 K All elements are gaseous
1600 K Al, Ti, Ca Mineral Oxides
1400 K Iron & Nickel Metallic Grains
1300 K Silicon Silicate Grains
300 K Carbon Carbonaceous grains
300-100 K H & N Ices (H2O, CO2, NH3, CH4)

The "Frost Line"

Rock & Metals can form anywhere it is cooler than about 1300 K.

Carbon grains & ices can only form where the gas is cooler than 300 K.

Inner Solar System:

Outer Solar System:

From Grains to Planetesimals

Grains that have low-velocity collisions can stick together, forming bigger grains.

Can form km-sized planetesimals after a few 1000 years of initial growth.

Terrestrial Planets

Only rocky planetesimals inside the frost line:


Jovian Planets

The addition of ices to the mix greatly augments the masses of the planetesimals

These collide to form large rock and ice cores:.

As a consequence of their larger masses & colder temperatures:


The largest gas giants grow to such size that their gravity acts to either scatter or accrete any remaining planetesimals and protoplanets in the region, shutting off further planetary formation in the outer solar system.

Moons & Asteroids

Some of the gas attracted to the proto-Jovians forms a rotating disk of material:


Icy Bodies & Comets

Outer reaches are the coldest, but also the thinnest parts of the Solar Nebula:

Gravity of the proto-Neptune also plays a role:

Comets and other Trans-Neptunian objects are the leftover icy planetesimals from the formation of the Solar System.

Mopping up...

The whole planetary assembly process probably took about 100 Million years.

Followed by a 1 Billion year period during which the planets were subjected to heavy bombardment by the remaining rocky & icy pieces leftover from planet formation.

Light from the Sun dispersed the remaining gas in the Solar Nebula gas into the interstellar medium.

Planetary motions reflect the history of their formation.

Planets formed from a thin rotating gas disk:

The Sun "remembers" this original rotation:

Planetary compositions reflect the different conditions in which they formed.

Terrestrial planets are rock & metal:

Jovian planets contain ice, H & He:

Final Remarks

These are the basic outlines of our current understanding of how our Solar System formed. The basic picture seems sound, but there are many details to be worked out. For example, we are still not sure how Jupiter grew as large as it did, or what influence it had on the formation of the rest of the solar system. The location of the "frost line" is also a matter of some debate, but current thinking holds that it is probably about 4 AU from the Sun. A great deal depends on how much solar radiation can penetrate deep into the outer parts of the primordial Solar Nebula.

The real test will be to observe other planetary systems in the process of formation, and draw information from those to apply to unraveling the history of our own system. Research in this area has begun to take off in exciting new directions in recent years, and we are starting to find support for and against some of the ideas presented here. This will remain an important and exciting area of study for years to come. Return to [ Unit 6 Index | Astronomy 161 Main Page ]

Updated: 2007 November 4
Copyright Richard W. Pogge, All Rights Reserved.