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The Origin of Our Solar System The Great Chain of Origins and Origin Hypotheses (Nebular Hypothesis)
Objectives • SWBAT evaluate varying hypotheses and their evidence about the origin of the solar system. • SWBAT compare, and contrast hypotheses about the origin of our solar system.
Early Hypotheses - Examples • René Descartes (Early 17th century) • Created first recorded physical theory. • Did not recognize gravitation as the universe’s central force. • Believed force was exerted by contact betwn physical entities and the universe was filled with vortices of “whirling invisible particles.” • Posited that the sun and planets formed when a large vortex contracted and condensed.
Early Hypotheses - Examples • Louis de Buffon (mid to late 17th century) was a French naturalist. • Proposed that the planets formed when a comet collided with the sun and pulled matter out of it. • Buffon knew nothing of the actual size of a comet; however, later astronomers took his idea and replaced the comet with a passing star. • Matter ripped from the two stars condensed to form planets.
Catastrophic V. Evolutionary (these ideas are found throughout science) • Descartes’ Hypothesis – is an “evolutionary” type hypothesis. • An evolutionary hypothesis involves gradual processes to produce the sun and planets. • If an evolutionary perspective were correct, and the origin of planets, solar systems, and galaxies were through gradual, common, process, then stars and planets would be very common.
Catastrophic V. Evolutionary • Buffon’s hypothesis depends on highly improbable, sudden events. • If a catastrophic hypothesis were true, then planetary systems should be very rare. • The catastrophic versus evolutionary debate has mostly been settled. • Scientists have mostly sided with the gradualist perspective; however, catastrophism does play a real role and should not be dismissed.
Evolutionary Processes Are Favored • The modern theory for the origin of the planets, which is based on large amounts of data, is without a doubt evolutionary. • The modern theory began to form in the late 17th early 18th centuries when Pierre-Simon de Laplace combined Decartes’ vortex idea with Newton’s laws of gravity.
Laplace’s Nebular Hypothesis • Many of science’s greatest advances come when there is a synthesis of two seemingly different disciplines (astronomy and physics in this case. Laplace posited that vortices had been acted on by gravity – applying Newton’s laws. Gravitational forces pulled on the vortices and flattened them out into a disk. The center of the vortex became the sun.
Nebular Hypothesis Continued • Disks grew smaller as gravity acted on them. • In order to maintain angular momentum the sun, the source of the spin, and the resulting disks had to spin faster: • Angular momentum = moment of inertia x angular velocity. • The increased “spin” was necessary to keep the evolving disk rotating.
Nebular Hypothesis Continued • When our cloud of spinning matter was spinning as “fast as it could,” the disk began to shed layers – leaving behind rings of matter. This is a painting of the early solar system, according to Laplace, from NASA’s website.
Nebular Hypothesis Continued • As the cloud spun, it created many rings. • The rings would become the newborn planets orbiting a new sun at the center of what was the cloud of gas (which remember, is matter). • As with most hypotheses in science, this one was a very good try but the evidence did not support it.
Nebular Hypothesis Continued • The hypothesis depended on the sun having most of the angular momentum in the solar system. • Angular momentum (re-stated in easier to understand form): the tendency of a rotating object to continue to rotate. • In fact, the sun rotates relatively slowly – having little angular momentum. • The planets rotate much faster and have greater angular momentum.
Nebular Hypothesis Continued • The Nebular Hypothesis could not adequately explain the angular momentum problem (sun low, planets high). • Therefore it was rejected, however, it was absolutely key because of its incorporation of Newton’s laws of motion. • It was the first “modern” theory. • In the 1940s, the Solar Nebula Hypothesis was proposed.
Solar Nebula Theory (SNT) • Proposes that the planets, including Earth, formed in a rotating disk of gas and dust that surrounded the sun as it, the sun, formed. Laplace’s nebular hypothesis included a disk but depended on rings of matter left behind by disk contraction. He did not take into account how gas and dust behave in such a disk. In the SNT, the planets grew within the disk by carefully described physical processes – an evolutionary process (called uniformitarianism in geology.
Solar Nebula Theory (SNT) • Disks of gas and dust are common around young stars. • Modern astronomical tools can image the disks directly and evidence is strong that our own planetary system formed in such a disk shaped cloud. • So, evidence comes from direct observation as well as the laws of physics and chemistry – and now biology and geology.
Solar Nebula Theory (SNT) • If planet formation is a natural part of star formation, from the disks of gas and dust, then most stars should have planets orbiting them.
Neil deGrasse Tyson, from Nova Now • http://www.pbs.org/wgbh/nova/space/origins-solar-system.html