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Notes - Formation of the Earth & Solar System
2015
I. Three Hypotheses on the Formation of the Solar System
A. Encounter Hypothesis (Buffon - 1745) - another star passed so close to the sun that its
gravity caused a tidal bulge to pull away from the sun and form the planets.
Weaknesses:
1) Very unlikely (calculated as a 1 in 100 billion billion (1/1020) chance!)
2) Planets should be closer to the sun than they are.
3) Sun and planets should have almost identical compositions, but the Sun has no
Deuterium ( 1H2 ) at all, but the planets do have 1H2 !
4) The sun spins too slowly (slower than it should at the center)
B. (Old) Nebular Hypothesis (Kant – 1755 and LaPlace 1796)
1) a huge cloud of dust & gas (called a solar nebula) collapses due to gravity.
2) To conserve angular momentum, the nebular cloud spun faster as it shrank.
3) As the cloud spun faster, centrifugal force caused it to flatten into a disk-like shape.
4) The spinning cloud disk threw off rings of matter, which compressed into planets.
5) The central cloud became the sun.
Weaknesses:
1) There would not be enough matter thrown off in rings to form planets.
2) The sun’s rotation is slower than expected – it should be rotating much faster.
***C. Protoplanet (Condensation) Hypothesis - modifies the original Nebular Hypothesis:
1) The Solar System started as a nebular cloud 4.6 billion years ago (bya).
2) 4.6 billion years ago (b.y.a.) - shock waves from two nearby supernovas swept
through our part of the Milky Way Galaxy & created turbulence, which…
3) triggered the initial gravitational collapse of the solar nebula and…
4) caused spinning eddies of dust and particles to form within the nebular cloud.
5) The supernovas also splattered the nebular cloud with radiation and rare, heavy
elements such as lead, gold and uranium.
6) The dense core of the nebular cloud heated up to become the sun.
7) The swirling eddies of dust and gas began to cling together to form asteroid-sized
planetesimals.
8) Gravity caused planetesimals to collide and stick together into protoplanets. This
process is called accretion. These eventually grew into full-sized planets.
9) The Protoplanet (Condensation) Hypothesis explains the weaknesses of the first
2 hypotheses and also explains all of the known evidence about the solar system.
10) A good hypothesis or theory also leads to successful predictions:
 Astronomers went looking for evidence of a nearby supernova that could have
caused all of the turbulence 4.6 bya.
 Recently, they located two supernovas of the right age and distance!
 This ability to predict further supports the Protoplanet Hypothesis.
11) As a result, the Protoplanet Hypothesis is currently the most popular hypothesis.
II. The Protoplanet/Condensation Hypothesis in Detail (from Ch.
1, Sec. 3, Ch. 12, Section 2 & Ch. 22, Sec. 1-3)
A. 4.6 bya - From Planetesimals to Protoplanets
1) The sun and planets formed from a rotating cloud of interstellar gases and dust called
a solar nebula, which consisted of Hydrogen and Helium gas along with microscopic
dust grains containing heavier elements produced by earlier stars and supernovas.
2) Shockwaves from supernovas may have triggered the collapse of the nebular cloud,
which rotated faster as it contracted in order to conserve angular momentum.
3) The inward pull of gravity along with the outward push of centrifugal force turned
the cloud into a flattened, rotating disk.
4) As gravitational energy turned into heat energy, the center of the solar nebula
heated up enough to start nuclear reactions and become a star (The Sun).
5) Repeated collisions caused gas & dust particles to stick together into larger chunks
that eventually became asteroid-sized planetesimals – a process called accretion.
6) Gravity accelerated the accretion process and caused planetesimals to collide and
stick together into protoplanets. These eventually grew into full-sized planets:
• The inner solar system had higher temperatures, so the inner planetesimals were
made of metals (Fe & Ni) and rocky materials (Si) with high melting points.
• The outer solar system had extremely low temperatures (328˚ below zero F), so
planetesimals contained high %s of ices of water (H2O), carbon dioxide (CO2),
ammonia (NH3) and methane (CH4) along with the metallic and rocky grains.
7) The inner regions were too hot for ices and gases to condense: the atmospheres of
the inner planets are just a thin layer of heavier gases such as CO2, N2 and O2.
8) The accumulation of ices caused the outer planets to be much larger, but less dense.
As a result, the outer planets had enough gravity to hold on to lighter, volatile gases:
• Jupiter and Saturn formed quickly enough to also hold on to a large amount of H2
and He, the two lightest gases.
• Uranus and Neptune took longer to form- a strong solar wind swept most of the
available H and He before these planets could accumulate much of these 2 gases.
Two factors contribute to the difference in atmospheres:
Solar heating (temperature)
Gravity
[See SmartFigure 22.3]
9) It took about ¾ billion years for the planets to “sweep up” most of the debris. During
this time, the planets were bombarded by millions of meteorite and asteroid impacts.
10) Jupiter’s gravity grabbed many incoming asteroids and comets and “flung” them
back outward to form Kuiper Belt Objects such as Pluto and the Oört Cloud.
11) The tidal pull of Jupiter’s gravity is probably responsible for lowering the number
of asteroids in the asteroid belt, which prevented asteroids’ accreting to form a planet.
12) A 2 million mph solar wind and deadly U - V radiation stripped away gas layers of
the 4 inner planets (they’re now rocky). Outer planets were able to hold on to their
gas envelopes because they were: a) larger to start with, with more gravitational
pull and b) farther away from the sun, so the solar wind was weakened.
13) Streams of charged particles called the solar wind were captured by the long arms
of the sun’s magnetic field, which slows down the sun’s rotation by a process called
magnetic braking.