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Lecture 32:
The Origin of the Solar
System
Astronomy 161 – Winter 2004
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 & 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
preserves its formation history.
Relevant Observations:
• Orbits of the planets and asteroids.
• Rotation of the planets and the Sun.
• Compositions of the planets
Clues from motions
Orbital Motions:
• Planets all orbit in nearly the same plane.
• Most planet's 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 notable exceptions.
• Sun rotates in the same direction as planets orbit.
Jovian moon systems mimic the Solar System.
Pluto
Clues from planet composition
Inner Planets & Asteroids:
• Small & rocky (silicates & iron)
• Few ices or volatiles, no H or He
Jovian Planets:
• Large ice & rock cores
• Hydrogen atmospheres rich in volatiles.
Outer solar system moons & icy bodies:
• Small ice & rock mixtures with frozen volatiles.
Icy
Pluto
Rocky
Planets
Giant Gas Planets
Mostly H, He, & Ices
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 “Solar Nebula”.
Cold Interstellar H2 Cloud
Interstellar Cloud
of H2 and Dust
Stellar-mass fragment
Gas & dust disks observed around
young stars
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 elements condense out when depends on
their “condensation temperature”.
Condensation Temperatures
Temp (K)
>2000 K
1600 K
1400 K
1300 K
Elements
all are gaseous
Al, Ti, Ca
Iron & Nickel
Silicon
300 K
Carbon
300-100 K H, N
Condensate
Mineral oxides
Metal Grains
Silicate grains
Carbonaceous
Grains
Ices (H2O, CO2,
NH3, CH4)
The “Frost Line”
Rock & Metals form anywhere the gas cooler
than 1300 K.
Carbon grains & ices only form when the gas is
cooler than 300 K.
Inner Solar System:
• Too hot for ices & carbon grains.
Outer Solar System:
• Carbon grains & ices form beyond the “frost line”.
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 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 &
He gas.
• Solar wind also disperses the solar nebula from the
inside out, removing H & He.
Result:
• Form rocky terrestrial planets with few ices.
Formation of a Terrestrial planet
Jovian Planets
Ices augment the masses of the planetesimals.
These collide to form large rock and ice cores:
• Jupiter & Saturn: 10-15 MEarth rock/ice cores.
• Uranus & Neptune: 1-2 MEarth rock/ice cores.
Larger masses & colder temperatures:
• Accrete H & He gas from the Solar Nebula.
• Planets with the biggest cores grow rapidly.
Formation of Jupiter
Solar Nebula
ProtoSun
Moons & Asteroids
Gas gets attracted to the proto-Jovians & forms
rotating disks of material:
• Get mini solar nebulae around the Jovians
• Rocky/icy moons form in these disks.
• Later moons added by asteroid/comet capture.
Asteroids:
• 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 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 (Pluto & Plutinos)
• Disperses the others into the Kuiper Belt.
Mopping up...
• The whole planetary assembly process took
about 100 Million Years.
• Followed by ~1 Billion years of heavy
bombardment of the planets by the remaining
rocky & icy pieces.
• Sunlight 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 were
perturbed from perfect alignment by strong
collisions during formation.
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 environments of formation.
Terrestrial planets are rock & metal:
• Formed in the hot inner Solar Nebula.
• Too hot to capture and retain Hydrogen & Helium.
Jovian planets contain ices, H, & He:
• Formed in the cool outer Solar Nebula
• Grew large enough to accrete lots of H & He.