An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30
The Origin of the Solar System
The present-day properties of our Solar System hold
important clues to its origin.
Primordial Solar Nebula:
- Process of the Sun's formation
- Condensation of grains & ices
From Planetesimals to Planets:
- Aggregation of small grains into planetesimals
- Aggregation of planetesimals into 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.
- Orbits of the planets and asteroids.
- Rotation of the planets and the Sun.
- Composition of the planets, especially the strong
distinction between Terrestrial, Jovian, and Icy planets.
Clues from planetary motions:
- Planets orbit in nearly the same plane.
- Planet orbits are nearly circular.
- Planets & Asteroids orbit in the same direction.
- Rotation axes of the planets tends to align with the
sense of their orbits, with exceptions.
- Sun rotates in the same direction in the same sense.
- Jovian moon systems mimic the Solar System.
Clues from planet composition:
Inner Planets & Asteroids:
- Small rocky bodies
- Few ices or volatiles
- Deep Hydrogen & Helium atmospheres rich in volatiles.
- Large ice & rock cores
Outer solar system moons & icy bodies:
- Small ice & rock mixtures with frozen volatiles.
Formation of the Sun
Stars form out of interstellar gas clouds:
- Large cold cloud of H2 molecules and dust
gravitationally collapses and fragments.
Rotating fragments collapse further:
- Rapid collapse along the poles, but centrifugal forces
slow the collapse along the equator.
- Result is collapse into a spinning disk
- Central core collapses into a rotating proto-Sun
surrounded by a rotating "Solar Nebula"
Primordial Solar Nebula
The rotating solar nebula is composed of
- ~75% Hydrogen & 25% Helium
- Traces of metals and dust grains
Starts out at ~2000 K, then cools:
- As it cools, various elements condense out of the gas
into solid form as grains or ices.
- Which materials condense out when depends on their
||All elements are
||Al, Ti, Ca
||Iron & Nickel
||H & N
||Ices (H2O, CO2, NH3,
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
Inner Solar System:
- Too hot for ices & carbon grains.
Outer Solar System:
- Carbon grains & ices form beyond the "frost
From Grains to Planetesimals
Grains that have low-velocity collisions can stick together,
forming bigger grains.
- Beyond the "frost line", get additional growth
by condensing ices onto the grains.
- Grow to where their mutual gravitation assists in the
aggregation process, accelerating the growth rate.
Can form km-sized planetesimals after a few 1000 years
of initial growth.
Only rocky planetesimals inside the frost line:
- Collisions between planetesimals form small rocky bodies.
- It is hotter closer to the Sun, so the proto-planets cannot
capture H and He gas.
- Solar wind is also dispersing the solar nebula from the
inside out, removing H & He.
- Form rocky terrestrial planets with few ices.
The addition of ices to the mix greatly augments the masses of
These collide to form large rock and ice cores:.
- Jupiter & Saturn: 10-15 MEarth rock/ice
- Uranus & Neptune: 1-2 MEarth rock/ice
As a consequence of their larger masses & colder temperatures:
- Can accrete H & He gas from the solar nebula.
- Planets with the biggest cores grow rapidly in size,
increasing the amount of gas accretion.
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
- Form large Jovian planets with massive rock & ice
cores and heavy H and He atmospheres
Moons & Asteroids
Some of the gas attracted to the proto-Jovians forms a rotating disk
- Get mini solar nebula around the Jovians
- Rocky/icy moons form in these disks.
- Later moons added by asteroid/comet capture.
- Gravity of the proto-Jupiter keeps the planetesimals in
the main belt stirred up.
- Never get to aggregate into a larger bodies.
Icy Bodies & Comets
Outer reaches are the coldest, but also the thinnest parts of the
- Ices condense very quickly onto rocky cores.
- Stay small because of a lack of material.
Gravity of the proto-Neptune also plays a role:
- Assisted the formation of Pluto-sized bodies in
3:2 resonance orbits (Pluto and Plutinos)
- Disperses the rest of the icy planetesimals into the Kuiper Belt (30-50AU)
to become Kuiper Belt Objects.
Comets and other Trans-Neptunian objects are the leftover icy
planetesimals from the formation of the Solar System.
The whole planetary assembly process probably took about 100 Million
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 disk's rotation was imprinted on the orbits of the planets.
- Planets share the same sense of rotation, but have been
perturbed from perfect alignment by strong collisions
The Sun "remembers" this original rotation:
- Rotates in the same direction with its axis aligned with
the plane of the Solar System.
Planetary compositions reflect the different
conditions in which they formed.
Terrestrial planets are rock & metal:
- They formed in the hot inner regions of the Solar Nebula.
- Too hot to capture Hydrogen/Helium gas from the Solar
Jovian planets contain ice, H & He:
- They formed in the cool outer regions of the Solar
- Grew large enough to accrete lots of H & He.
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
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
Readings in Universe:
Return to [
Unit 6 Index
Astronomy 161 Main Page
Updated: 2006 November 5
Copyright © Richard W. Pogge,
All Rights Reserved.