Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 30: Origin of the Solar System

Key Ideas:

The present-day Solar System holds clues to its origin
Primordial Solar Nebula:: Formation of the Sun; Temperature and planetary building blocks
Growth of the Planets:: Small grains to planetesimals to planets; Temperature, gravity, and atmospheres
The Present and Future Solar System

Clues from Motions

Orbital motions:: Planets all orbit in nearly the same plane; Planet orbits are nearly circular; Planets and Asteroids orbit in the same direction
Rotation:: Axes of the planets tend to align with the sense of their orbits, with notable exceptions; Sun rotates in the same direction as planets orbit it

Clues from Planet Compositions

Inner Planets and Asteroids: Small and rocky (silicates and iron); Few ices or volatiles (compounds with low boiling points), no Hydrogen or Helium
Jovian Planets: Large ice and rock cores; Hydrogen atmospheres rich in volatiles
Outer solar system moons and icy bodies: Ice and rock mixtures with frozen volatiles

Formation of the Sun

Stars form out of interstellar gas clouds: Large cold cloud of molecular hydrogen and dust 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 "Solar Nebula"

Primordial Solar Nebula

The rotating solar nebula was composed of: About 75% Hydrogen and 25% Helium; Traces of metals and dust grains
Starts out at ~2000 K, then cools:: Which elements condense out when depends on their condensation temperature (when they solidify); Lots of material close to the Sun, progressively less further away

The "Frost Line"

Rock and metals condense out anywhere that the gas becomes cooler than 1300 K
Carbon grains and ices condense out only when the gas is cooler than 300 K
Inner Solar System:: Too hot for ices and carbon grains
Outer Solar System:: Carbon grains and ices form beyond the "frost line"
Because there is so much Hydrogen, there is a lot more solid material anywhere ices form
Condensation Temperatures:: Above 2000K all elements are gaseous; At 1600K: Al, Ti, Ca form mineral oxides; At 1400K: Iron and Nickel can form metal grains; At 1300K: Silicon can form silicate grains; At 300K: Carbon can form carbonaceous grains; At 100 - 300K: H and N can form ices (water, carbon dioxide, ammonia)
Most of the (solid) mass is composed of ices because they include Hydrogen

From Grains to Planetesimals

Grains stick together after low-velocity collisions, forming bigger grains: Beyond the frost line the grains grow faster because of the condensing ices
Grains grow until their mutual gravitation assists in aggregation and accelerates the growth rate:
Form km-sized planetesimals after a few 1000 years of initial growth

Inner Solar System

Close to the Proto-Sun the temperature is high:: Rock and metals condense into solids; Ices remain in gaseous form; Planetesimals are too low in mass and too hot to capture H and He

Terrestrial planets form with few ices and are high density (but low in mass)

Outer Solar System

Further from the Proto-Sun the temperature is lower:: Rocks and metals condense; LOTS of solid ices also form; Planetesimals grow much faster and much larger than those closer in; The most massive are large enough to capture H and He gas

Jovian planets grow much larger, but are less dense

Moons and Asteroids

Gas gets attracted to the proto-Jovians and forms rotating disks of material: Get mini solar nebulae around the Jovians; Rocky and icy moons form in these disks; Later moons added by asteroid and comet capture
Asteroids: Gravity of the proto-Jupiter keeps the planetesimals in the main belt stirred up; Never get to aggregate into a larger body

Icy Bodies and Comets

Outer reaches are the coldest and thinnest parts of the Solar Nebula: Ices condense very quickly onto rocky cores; Stay small because of a lack of material
Gravity of the proto-Neptune: Assisted the formation of Pluto-sized bodies in 3:2 resonance orbits (Plutos and Plutinos); Disperses the others into the Kuiper Belt

The Last 4.5 Billion Years

The planetary assembly process took about 100 million years
Over the next billion years, most of the remaining rocky and ice pieces crashed into the planets or Sun (period of heavy bombardment)
Many rock and ice pieces were pushed out into the Kuiper Belt or Scattered Disk
Sunlight dispersed the remaining gas into the interstellar medium
Atmospheres evolved or were lost
Uranus and Neptune likely formed closer to the present locations of Jupiter and Saturn and then migrated outwards

Planetary Atmospheres

Formation of an atmosphere is set by the balance between temperature and surface gravity
Massive planets and planets with colder surfaces can hold onto lighter elements and molecules

The Planets Today

Can they hold onto an atmosphere?: Mercury and the Moon are too small and too hot; Venus and Mars retain oxygen and CO₂, but no water; The Earth can not retain H and He; The Jovian planets hold onto everything

The Future

The Sun will slowly brighten over the next 5 billion years
In about 1 billion years, it will be 10% brighter, which will make the surface of the Earth uninhabitable
Within 3 billion years, the Earth's surface temperature will be close to that of Venus' today

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.