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.

This is a borrowed slide

Neil deGrasse Tyson, from Nova Now http://www.pbs.org/wgbh/nova/space/origins-solar-system.html