ASTA01H3 Lecture Notes - Lecture 1: Planetesimal, Protoplanet, Chemical Element
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Condensation of Solids
As the porto-planetary disc loses some of its heat and cools, material start to condense.
The condensation of material depends on the temperature of the disc.
Close to the Sun, the temperature is very high and only metals might condense but at
sufﬁciently far distances many volatiles condense into ices.
This condensation sequence is observed in the uncompressed density of the planets
The closer the planet to the Sun, the higher the uncompressed density.
The Formation of Planetesimals Planetesimals came into existence primarily through
condensation and accretion.
Planetesimals start as dust particles, condensed in the original solar nebula.
Each microscopic particle in the solar nebula can grow by condensation (adding one atom
or molecule at a time form the surrounding gas).
The dust grains grew to a few centimetres via condensation when they start growing by
accretion. Dust particles frequently collided and from time to time got stuck to each other
via static force.
Condensation depends on the temperature of the disc.
Making of a Planet
An object is considered a planetesimal when the size exceeds 1 km.
Once the seeds (planetesmials) grow large enough in size (a kilometer or so), gravity will
start playing a role in accretion and eventually form protoplanets.
4 Stages in the Formation of a Planet
(1) Dust Condenses out of hot gaseous disk.
(2) Dust grains grow to planetesimals by accretion
(3) Planetesimals grow into proto-planets by accretion but gravity is the dominant force
(4) Runaway Accretion terminates due to lack of material in the feeding zone. Protoplanets
turn into planet by long-term and long-distance cumulative gravitational interactions.
Two Types of Planets
The condensation sequence implies that two types of planetesimals could form:
Planetesimals made of metal and rock in the inner solar system
Planetesimals made of ices and rock in the outer solar system
The Formation of Terrestrial Planets
Planetesimals made of metal and rock in the inner solar system:
Grew rapidly Collided with other Planetesimals frequently at high speeds
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