The Main Point. What is a scientific theory? Observational Constraints. Lecture #33: Solar System Origin I

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1 Lecture #33: Solar System Origin I 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. Reading: Chapters 8.1 to 8.5. The Main Point 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. Astro 102/104 1 Astro 102/104 2 What is a scientific theory? Theories of solar system formation. Theory of general relativity. Extraordinary explanatory and predictive power in the physical py sciences. Perihelion advance of Mercury, deflection of light by stars. Theory of evolution. Extraordinary explanatory and predictive power in the biological sciences. The existence of biological evolution is a fact. The precise mechanisms are active areas of research. Astro 102/104 3 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. (3) Age: The measured ages of primitive meteorites. Astro 102/

2 "Motion" Constraints 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. Top view Edge on Astro 102/104 5 Astro 102/104 6 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. Astro 102/104 7 Astro 102/

3 Age Constraints 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. Astro 102/104 9 Major Characteristics of our Solar System: Summary (1) Large bodies have orderly motions. (2) Planets fall into two main categories: 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,... Astro 102/ Historical Models for the Formation of the Solar System Catastrophic Models: 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, 1880]. 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]. All have problems with the basic physics or matching some of the constraints, but nebular models are best. Astro 102/ The Solar Nebula Theory The leading model today is called the solar nebula theory: 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"). The solar system was born from an enormous cloud of hydrogen, helium, tiny amounts of heavier elements, and interstellar dust particles in this part of our galaxy. 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. Some day (maybe 5 billion years from now) the Sun will die and redistribute most of this matter back into an interstellar cloud... Astro 102/

4 Cosmic Recycling The Solar Nebula Theory Ours may not be the first solar system to occupy this part of the galaxy... Astro 102/ Astro 102/ The Violent Young Sun "accretion" The Astro 102/104 "Standard Model": 15 From gas clouds to gas giants... Original nebula may have had ~2 solar masses of material; half of it Astro lost 102/104 by the young Sun's strong More detail in Lecture "bipolar outflow" 4

5 How long did this take? 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 ti disk may have taken only ~ 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 million years. Astro 102/ Putting the age of the solar system into perspective... The Cosmic Calendar Astro 102/104 Our galaxy forms 18 Implications... The original catastrophic theories assumed that the Sun formed first and the planets formed later, by accident: Implies that planets are rare. Nebular theories assume that the Sun and planets form together: Implies that planetary systems are common. Testing the Solar Nebula Theory How do we decide if this is right? (we weren't there!) 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? 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? Astro 102/ Astro 102/

6 Constraints Revisited Motion constraints: Planets orbit in a plane with circular trajectories and the same sense of revolution. Expect from flat, spinning disk. Composition constraints: Jovian atmospheres have solar composition, unprocessed meteorites have solar abundances. Expect from collapse of single large cloud. Age constraints: Meteorites and oldest rocks billion years old. Consistent with rapid condensation and differentiation from nebula. Asteroids and TNOs: Consistent with leftover planetesimals. Astro 102/ The solar nebula theory predicts a specific chemical condensation sequence as the hot 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. Compositional Evidence Astro 102/ Compositions of Meteorites A few special "primitive" carbonaceous chondrite meteorites contain grains of dust with ages greater than 4.5 billion years. Samples of the Sun's "parent body"? 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. Allende Astro 102/ Orion "Brown Dwarfs" and Planets around other stars! Observational evidence elsewhere: Nebulae, Star-forming Regions Dusty disks around nearby stars! Astro 102/

7 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 lis 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. Astro 102/ Next Lecture... Solar System Origin II Formation of the Terrestrial Planets. Formation of the Giant Planets. Chemical Condensation ("Lewis") Model. Tests and Predictions. Planetary Evolution. Reading: Chapters 8.1 to 8.5. Astro 102/