Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 31: Origin of the Solar System

Key Ideas:

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 and ices
From Planetesimals to Planets: Aggregation of small grains into planetesimals; Aggregation of planetesimals into planets; Terrestrial vs. Jovian planet formation

Clues from Motions

Orbital motions:: Planets all orbit in nearly the same plane; Most planets 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 the planets orbit it

Clues from Planet Compositions

Inner Planets and Asteroids: Small and rocky (silicates and iron); Few ices or volatiles, 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 is composed of: ~75% Hydrogen; ~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

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 "Front Line"
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)

From Grains to Planetesimals

Grains stick together after low-velocity collisions, forming bigger grains: Beyond the frost line there is additional growth from ices condensing onto the grains
Grow until their mutual gravitation assists in aggregation, accelerating the growth rate:
Form km-sized planetesimals after few 1000 years of initial growth

Terrestrial Planets

Only rocky planetesimals inside the frost line:: Collide to form small, rocky bodies
Hotter closer to the Sun:: Inner proto-planets cannot capture or retain H and He gas; Solar wind also disperses the solar nebula from the inside out, removing H and He

The result is that terrestrial planets form with few ices

Jovian Planets

Ices augment the masses of the planetesimals
These collide to form large rock and ice cores:: Jupiter and Saturn: cores 10-15 times the mass of the Earth; Uranus and Neptune: cores 1-2 times the mass of the Earth
Larger masses and colder temperatures:: Accrete H and He gas from the Solar Nebula; Planets with the biggest cores grow rapidly

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

Fast Forward to the Future

The whole planetary assembly process took about 100 Million years
This was followed by ~1 Billion years of heavy bombardment of the planets by the remaining rocky and icy pieces
Sunlight dispersed the remaining gas in the Solar Nebula into the interstellar medium
Many small pieces of rock and icy were pushed out into the Kuiper Belt or the Scattered Disk
Sunlight dispersed the remaining gas into the interstellar medium
Planetary atmospheres evolved or were lost

Planetary Atmospheres

Formation of an atmosphere is set by the balance between surface temperature and escape velocity
Massive planets and planets with colder surfaces can more easily hold onto lighter atmos and molecules in the gas phase.
The Planets Today: Can they hold onto their original material?: Mercury and the Moon are too small and too hot to retain any atmosphere; Venus and Mars can retain oxygen and carbon dioxide, but not water; The Earth can not retain hydrogen and helium; The Jovian planets are both massive enough and cold enough to hold onto everything

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