Download Lecture #33: Solar System Origin I The Main Point What is a

Lecture #33: Solar System Origin I
• How did the Solar System Form?
The Main Point
• Observational Constraints:
– Planetary motions.
The currently-favored model for the
formation of the solar system is the solar
nebula theory, which postulates that the Sun
and planets were formed from a spinning
cloud of gas & dust.
– Planet, asteroid, comet compositions.
– Meteorite and cosmic dust ages.
– Other solar systems?
• The Solar Nebula Model.
• Reading: Chapters 8.1 to 8.5.
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What is a scientific theory?
Observational Constraints
• Theories of solar system formation.
• Theory of general relativity.
– Extraordinary explanatory and predictive power in
the pphysical
y
sciences.
– Perihelion advance of Mercury, deflection of light
by stars.
• Theory of evolution.
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• For a theory of the formation of our solar system
to be viable, it must be consistent with our
presently-available data.
• Theories must fit 3 classes of constraints:
(1) Motions: The observed spin and orbital motions of the
planets, asteroids, and comets.
– Extraordinary explanatory and predictive power in
the biological sciences.
(2) Composition: The measured or inferred compositions
of planets, asteroids (meteorites), and comets.
– The existence of biological evolution is a fact. The
precise mechanisms are active areas of research.
(3) Age: The measured ages of primitive meteorites.
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"Motion" Constraints
•
•
•
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All of the planets orbit the Sun in roughly the same plane (the ecliptic),
which is very close to the Sun's equatorial plane.
The orbits of the major planets are nearly circular.
Planets, asteroids, and most comets circle the Sun counter-clockwise as
viewed from “above” (exceptions: some comets).
The Sun and most of the planets generally spin in this same
counterclockwise direction and have very small tilts relative to the plane
of the solar system (exceptions: Venus, Uranus).
Nearly all the planets and asteroids spin with the same period, roughly
20±10 hours per "day" (exceptions: Mercury, Venus).
The Sun contains 99.9% of the mass of the solar system, but the planets
contain 99.7% of the conserved angular momentum.
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Top view
Edge on
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Compositional Constraints
• The compositions of the planets vary with distance
from the Sun:
–
–
–
–
The inner planets are dense and rocky/metal-rich.
The outer planets are primarily gas (H, He) and ices.
Asteroids in the main belt are transitional.
Comets and TNOs are primarily ices.
• Most of Jupiter and Saturn, and the atmospheres of Uranus and
Neptune, have approximately the same composition as the Sun.
• Many meteorites appear chemically and mineralogically
different (more "pristine") than planets and moons.
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Major Characteristics of our
Solar System: Summary
Age Constraints
(1) Large bodies have orderly motions.
(2) Planets fall into two main categories:
• The primitive (unaltered) carbonaceous and
ordinary chondrite meteorites ALL have ages of
approximately 4.6 billion years.
• Tiny fractions of some very primitive meteorites
and cosmic dust particles have ages even slightly
older than this.
• The oldest rocks from the lunar highlands are
about 4.4 billion years old.
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• Small, rocky inner planets.
• Giant, gas- and ice-rich outer planets.
(3) Swarms of asteroids and comets abound!
(4) There are notable exceptions to the trends:
• Obliquity of Venus and Uranus, spin of Mercury and
Venus, orbit of Triton, ...
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Historical Models for the Formation of
the Solar System
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The Solar Nebula Theory
• The leading model today is called the solar nebula theory:
• Catastrophic Models:
– Previous generations of stars lived and died before our solar system
was formed. The violent death of a previous star or stars contributed
material to the present solar system ("cosmic recycling").
– A large comet collided with the Sun and ejected material that
cooled to form the planets [Buffon, 1745].
– Another star made a close pass by the Sun, tidally pulling material
out that cooled to form the planets [Bickerton,
[Bickerton 1880].
1880]
y
was born from an enormous cloud of hydrogen,
y g ,
– The solar system
helium, tiny amounts of heavier elements, and interstellar dust
particles in this part of our galaxy.
• Nebular Models:
– Sun and planets formed from a vast nebula of gas [Descartes, 1644]
that swirled around and condensed into planets [Kant, 1755].
– Nebula contracted into a fast spinning disk, shedding "rings" of
material that formed the planets [Laplace, 1796].
– This cloud collapsed under its own gravity and spun into a disk.
– Most of the mass fell into the center of the disk and formed the Sun.
– A tiny fraction of the leftovers formed the planets, moons, etc.
• All have problems with the basic physics or matching
some of the constraints, but nebular models are “best”.
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– Some day (maybe 5 billion years from now) the Sun will die and
redistribute most of this matter back into an interstellar cloud...
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The Solar Nebula Theory
Cosmic Recycling
Ours may not be
the first solar
system to
occupy this part
of the galaxy...
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The Violent Young Sun
"accretion"
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The
"Standard Model":
From gas clouds to gas giants...
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Original nebula may have had ~2
solar masses of material; half of it
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by the young Sun's strong
"bipolar outflow"
More detail in Lecture 35...16
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How long did this take?
Putting the age of the solar
system into perspective...
• FAST according to computer models:
– Contraction, spin-up of original cloud of gas takes only
a few million years.
– Condensation of dust and settling of the dust into a
rotating
t ti disk
di k may have
h
taken
t k only
l ~ 100 years!!
– Clumping of dust by self-gravity into 1-5 km
planetesimals may have taken only ~ 1000 years!
– Accretion of planetesimals into ~1000 km objects may
have occurred in only ~1 million years.
– Growth of a few objects to even larger Moon, Mars,
Earth sizes in only about 10-100 million years.
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Implications...
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Our galaxy forms
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Testing the Solar Nebula Theory
• How do we decide if this is right? (we weren't there!)
• The original catastrophic theories assumed
that the Sun formed first and the planets
formed later, by accident:
– Do the models rely on the laws of physics and chemistry or
do they require miracles or ad hoc assumptions?
– Do the models accurately predict the present nature and
distribution of the planets?
– Implies that planets are rare.
• Nebular theories assume that the Sun and
planets form together:
– Are there clues in the compositions of the planets or comets
or meteorites that could help to support this model?
– Can we look elsewhere in the galaxy for evidence for this
kind of process of solar system formation?
– Implies that planetary systems are common.
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The Cosmic Calendar
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Constraints Revisited
Compositional Evidence
•Motion constraints:
–Planets orbit in a plane with circular trajectories and the same sense of
revolution. Expect from flat, spinning disk.
•Composition constraints:
–Jovian
Jovian atmospheres have solar composition,
composition unprocessed meteorites have solar
abundances. Expect from collapse of single large cloud.
•Age constraints:
–Meteorites and oldest rocks 4.6-4.4 billion years old. Consistent with rapid
condensation and differentiation from nebula.
•Asteroids and TNOs:
–Consistent with leftover planetesimals.
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Orion
Compositions of Meteorites
• A few special "primitive"
carbonaceous chondrite meteorites
contain grains of dust with ages
greater than 4.5 billion years.
Sun'ss "parent
parent body
body"??
• Samples of the Sun
• Confirms that meteorites (and by
inference, asteroids and comets)
provide the best way to study the
"building blocks" of the solar system.
• Some of these ancient stones also
have complex organic molecules and
even amino acids.
• The solar nebula theory predicts
a specific chemical condensation
sequence
q
as the hot g
gaseous
nebula cooled...
• In general, this sequence is
consistent with the observed
changes in planetary composition
with solar distance.
• "Lewis Model"; more details in
Lecture 34.
Allende
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Observational evidence elsewhere:
Nebulae, Star-forming Regions
Dusty disks around nearby stars!
"Brown Dwarfs" and
Planets around other stars!
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Summary
Next Lecture...
• Solar System Origin II
• Observations of the Sun, planets, asteroids, and
comets can be used to constrain models for the
formation of the solar system.
• The
Th currently-favored
tl f
d model
d l is
i the
th solar
l nebula
b l
theory, which postulates that the Sun and planets
were formed from a spinning cloud of gas & dust.
• This theory explains many of the observations:
– Formation of the Terrestrial Planets.
– Formation of the Giant Planets.
• Chemical Condensation ("Lewis") Model.
– Tests and Predictions.
– Planetary Evolution.
– Motions of the planets.
– Compositions of the Sun, planets, and meteorites.
– Ages of meteorites and cosmic dust particles.
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• Reading: Chapters 8.1 to 8.5.
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