The Origin of the Solar System

Survey of Solar System

Planetary orbits lie in almost the same plane
- planes of Venus, Mars, Jupiter, Saturn, Neptune, Uranus orbits lie within $3.4^\circ$ of plane of Earth orbit
- Mercury: plane of orbit $7^\circ$ from plane of Earth's orbit, Pluto: plane of orbit $17.^\circ 2$ from plane of Earth's orbit
$\rightarrow$ Solar system almost a plane

Most planets and Sun have rotation axes almost perpendicular to plane of Earth's orbit
- eg. Sun - axis tilted $7.^\circ 25$, Earth - axis tilted $23.^\circ 5$, Mars: axis tilted $24^\circ$
- Exceptions:
-- Uranus & Pluto rotate on their sides: axes tilted by $\approx 90^\circ$ - ie axes lie almost in plane of Earth's orbit
-- Venus' axis upside down, axis tilted $\approx 180^\circ$ - ie. rotates backwards

$\rightarrow$ Almost everything in solar system rotates AND revolves in same direction: Counter-Clockwise when viewed from above Earth's North pole looking down bigskip So: Organization of Solar system suggests a disk

Two kinds of planets

1) Terrestrial (Earth-like) planets:
- Mercury, Venus, Earth & Mars
- located in inner solar system (closer to Sun)
- small, rocky, dense - density ($\rho$) $\approx 4$ g/ cm$^3$
- metallic core surrounded by rocky mantle
- density decreases with increasing distance from Sun
- thin atmospheres
- few, if any, satellites (eg. Earth -1 moon, Venus - no moons)

2) Jovian (Jupiter-like) planets:
- Jupiter, Saturn, Uranus, Neptune
- located in outer solar system (further from Sun)
- large, gaseous, low density - $\rho\le 1.75$ g/cm$^3$
- composed mostly Hydrogen (H) & Helium (He) (lightest elements)
- However: large $\rightarrow$ more massive than terrestrial planets
- large satellite systems (eg. Jupiter - 16 moons)
- have ring systems (rings made of rocks, ice) - Saturn is most obvious example

Minor bodies (Space debris):
- smaller than planets and most moons

1) Asteroids & Meteoroids
- made of rock & metal
- found in inner solar system
-asteroid belt: most asteroids in zone 2.8 AU from Sun (between orbit of Mars & Jupiter) lying in plane of Earth's orbit

2) Comets
- made of ice & rock
- found mostly in outer solar system, BUT, occasionally some enter inner solar system

Age of Solar System:

Rock composed of many chemical elements - some are radioactive
Radioactive elements gradually decay into daughter elements
Half-life ($T_{1/2}$): time needed for half of radioactive element to become daughter element
To date a rock:
- Must know original amount of radioactive and daughter elements
- Measure current amount $\rightarrow$ Get age

e.g. Uranium (U) radio-actively decays into Lead (Pb)
U $~\rightarrow$ Pb, Half-life ($T_{1/2}$) $=4.5$ billion yrs.
So: If have rock with half of original U turned to Pb
$\rightarrow$ Age = 4.5 billion yrs

Ages from radioactive dating:
1) Earth - oldest rocks have age of 3.9 billion years
- But: Earth's surface geologically active - ie ``re-paved'' by volcanoes, earthquakes, etc.
$\rightarrow$ rocks today probably not original
2) Moon - oldest rocks 4.48 billion years old
3) Meteorites from space - 4.6 billion year olds

$\rightarrow$ Common ages - solar system about 4.5 billion years old

Formation of Solar System

Solar Nebula Theory
- Main idea: Planets form as a by-product of star formation
- ie. Planets formed from gas left over from formation of Sun

Sequence:
1) Large Rotating cloud of gas & dust in space collapses due to gravity
2) proto-Sun (ie. ``baby'' Sun) forms at center of collapsing cloud
3) Cloud is rotating $\rightarrow$ forms gas disk around proto-Sun
4) Planets gradually form in rotating disk
5) Fully formed Sun eventually blows remaining gas & dust away

Note: Planet formation should be common

Evidence from other stars - observed in Infrared (IR) band:
- Forming stars embedded in clouds of gas & dust
- Young stars surrounded by disks of gas & dust
- Recently, planets found around other stars
$\rightarrow$ 1995: First planet found around another star - star 51 Pegasi, planet with half the mass of Jupiter

Planet Building:

Interstellar clouds composed mostly of Hydrogen (H) & Helium (He)
Sun & Jovian planets are also composed mostly of H & He
BUT: terrestrial planets composed mostly of rock & metal
$\rightarrow$ So, where did terrestrial planets come from??

Condensation:
In solar nebula - dust and gas condense to form grains of solid matter
Condensation: gas atoms stick together to form grains
- allows smallest grains to grow quickly
- less effective as grains gets larger
Type of matter that can condense depends on temperature of solar nebula

Condensation Sequence:
Which types of materials can condense from a gas depends on the temperature
- the lower the temperature, the lower the density of material that can condense
- $T < 1500^\circ$K, ($1250^\circ$C) - only refractory (ie. high melting point) materials can condense
$\rightarrow$ high density materials
$\rightarrow$ e.g. metals, metal oxides
- $T<1000^\circ$K, ($750^\circ$C) - high & medium melting point materials condense
$\rightarrow$ medium and high density materials
$\rightarrow$ e.g. silicates (rocky material)
- $T<150^\circ$K ($-100^\circ$C) - volatile (ie. low melting point) AND refractory materials can condense
$\rightarrow$ low, medium and high density materials
$\rightarrow$ e.g. ices of water, ammonia, methane

Temperature of solar nebula decreases with increasing distance from proto-Sun
$\rightarrow$ Close to proto-Sun: only refractory materials (eg. metallic grains) condense
$\rightarrow$ Medium distance from proto-Sun: silicate (rocky) & metallic grains condense
$\rightarrow$ Furthest from proto-Sun: volatile and refractory materials ie. ice, silicate & metallic grains condense

Also: solar nebula cools with time:
Close to proto-Sun:
- First metallic grains condense
- Later metallic AND silicate grains condense

Planet formation:

I. Growth of dust grains by Accretion
Accretion: two or more grains collide and stick together
- grains stick by chemical bonding
- can grow into planetesimals, ie. objects of diameter ($D$) $<$ 1 km

II. Planetesimals grow

- Planetesimals attract each other by gravity
$\rightarrow$ planetesimals concentrated into thin disk around Sun
$\rightarrow$ planetesimals concentrated into clumps in disk
$\rightarrow$ increases the space density of planetesimals (ie. number of planetesimals per cubic kilometer)

- planetesimals orbit Sun at $\approx$ 30 km/s
- However: everything is orbiting in same direction (counter-clockwise)
$\rightarrow$ planetesimals moving at low speed relative to each other So: Planetesimals can coalesce (ie. stick together) into larger planetesimals of diameter ($D$) up to $100$ km
- Stick together by chemical and electrical bonding

III. Proto-planets (ie. ``baby'' planets)

Largest planetesimals grow by gravitationally attracting smaller planetesimals
$\rightarrow$ grow into proto-planets
Proto-planets grow by gravitationally attracting planetesimals
$\rightarrow$ grow into planets

IV. Planets

Terrestrial planets:

Differentiation:
Planets have differentiated structure
- heavy metallic minerals (Iron (Fe), Nickel (Ni)) concentrated at center
- light silicates (ie. rocky material) near surface

Differentiation occurred because newly formed planets are very hot:
- Heat of formation: in-falling planetesimals release energy
- Natural radio-activity (from, for example, Uranium) releases energy
$\rightarrow$ planet melts
$\rightarrow$ heavy metals sink to center
$\rightarrow$ lighter silicates rise to surface
- later, planet cools down and ``re-freezes'', locking in differentiated structure
Another cause of differentiation:
- solar nebula cools with time
$\rightarrow$ later grains added to planet are less dense than earlier grains

Planetary Atmospheres:

Terrestrial planets:
First atmosphere: gravitationally attracted gas from solar nebula - Hydrogen (H) & Helium (He)
- this atmosphere driven off by heat
Secondary atmosphere:
- gases baked out of rock (out-gassing)
- gases evaporated from icy planetesimals that hit planet

Jovian planets:
Massive $\rightarrow$ gravitationally attract LOTS of Hydrogen (H) & Helium (He)
$\rightarrow$ large primordial atmosphere
- explains why Jovian planets much more massive than terrestrial planets

When did planet building stop?

Four mechanisms that stop planet formation:
1) Radiation pressure from Sun:
Sun formed at same time planets were forming
- light from Sun pushes away small grains & atoms of gas
$\rightarrow$ sweeps away solar nebula
2) Solar Wind: stream of gas ions from Sun (wind ``blows'' at 400 km/s)
$\rightarrow$ push away gas & dust - ``blows'' away solar nebula
3) Young planets swept up remaining debris (small planetesimals)
- cratering of planet & moon surfaces show record of heavy bombardment $\approx$ 4 billion years ago
4) Gravitational ejection: remaining small planetesimals flung out of solar system by Jovian planets' gravity

Solar Nebula Theory Explains Characteristics of
Solar System:

1. Common age:
- everything formed at same time from solar nebula

2. Planets orbit in plane, and revolve & rotate in same direction
- planets all formed from disk shaped rotating nebula
- exceptions (Venus & Uranus) - large impact during heavy bombardment pushed over rotation axes

3. Terrestrial planets in inner solar system, Jovian planets in outer solar system
- due to condensation sequence:
$\rightarrow$ only metallic and rocky minerals condensed in inner solar system
$\rightarrow$ minerals and LOTS of ice condensed in outer solar system
$\rightarrow$ outer proto-planets more massive $\rightarrow$ gravitationally attracted LOTS of gas

4. Density of terrestrial planets decreases with increasing distance from Sun
- due to condensation sequence
$\rightarrow$ closer to Sun only higher density materials condensed

5. Asteroid belt
- planet that failed to form
- due to planetesimals being disrupted by Jupiter's gravity

6. Jovian planets have many moons
- LOTS of ice condensed in outer solar system $\rightarrow$ lots of building material for moons
- SOME moons formed around Jovian proto-planets
- Jovian planets are large $\rightarrow$ have large gravity $\rightarrow$ SOME moons are planetesimals captured into orbit

7. Jovian planets have rings
- Jovian planets have large gravity $\rightarrow$ grains and small bodies captured into orbit $\rightarrow$ forms rings
- inner solar system: small particles swept away by radiation pressure & solar wind $\rightarrow$ no material to form rings

8. Minor bodies (debris) - asteroids, meteoroids, & comets
- small planetesimals left over from planet building
- too heavy to get swept away by solar wind and radiation pressure
- asteroids and meteoroids: left over rocky planetesimals in inner solar system
- comets: left over icy planetesimals in outer solar system