Dating the Universe Lecture 6: Formation of the Solar System Astro 202 Prof. Jim Bell ([email protected]) Spring 2008 But first... Graded Paper 1 returned today... Paper 2 is due at beginning of class on Feb. 14 Don’t wait until the last minute! Paper 3 to be posted online next Tuesday 2/12 How did the Solar System Form? ! Observational Constraints – Planetary motions – Planet, asteroid, comet compositions – Meteorite and cosmic dust ages – Other solar systems? ! The Solar Nebula Model – Formation of the Terrestrial Planets – Formation of the Giant Planets • Chemical Condensation ("Lewis") Model !Planetary Evolution Astro 202 4 Observational Constraints ! 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 (2) Composition: The measured or inferred compositions of planets, asteroids (meteorites), and comets
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How did the Solar System Form? Observational Constraintsastrosun2.astro.cornell.edu/academics/courses/... · The Solar Nebula Theory! The leading model today is called the solar nebula
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– Higher eccentricity means higher impact velocities
– Higher impact velocities mean less "sticking"
– Planetesimals cannot accrete and grow very well
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Caveats
!The Chemical Condensation Model is a simplistic view of a complex process!
– Was chemical equilibrium reached
everywhere?
– Many of the "leftovers" that we see today from
solar system formation have undergone
complex evolutionary histories
! Thus it is important to find and study the most
primitive solar system bodies (asteroids, comets)
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Planetary Evolution
!After only 100 to 500 million years, most of the "debris" in the solar system was either swept up by the planets or ejected
!The planets started to cool and assume their present appearances
!Each planet or moon has followed it own evolutionary course, governed by its size, composition, solar distance, and the occasional random catastrophic impact...
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How Do Interiors Get Hot?
Heat In
Heat Out
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Initial "inventory" of internal heat governs a
planet's evolution
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Summary
!Observations of the Sun, planets, asteroids, and
comets can be used to constrain models for the
formation of the solar system
! The currently-favored model is the solar nebula
theory, which postulates that the Sun and planets
were formed from a spinning cloud of gas & dust
! This theory explains many of the observations
– Motions of the planets
– Compositions of the Sun, planets, and meteorites
– Ages of meteorites and cosmic dust particles
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Summary
! The solar system formed about 4.5 billion years ago
– More soon on where that number comes from...
! The leading theory is that it formed rapidly from a spinning cloud (nebula) of gas and dust
! Condensation and accretion of solids occurred quickly: much of the "action" over in ! 108 years
! The outer planets were massive enough (especially Jupiter and Saturn) to accrete gases from the nebula
! But the inner planets were not!
! Jupiter may have prevented the formation of another terrestrial planet in the asteroid belt
Next Time...How do we know how old space rocks are?