Origin of Our Solar System

Earth and Space Science Objectives and Expectations: TEK Objective 5: Earth in space and time. The student understands the solar nebular accretionary disk model. The student is expected to: a) analyze how gravitational condensation of solar nebular gas and dust can lead to the accretion of planetesimals and protoplanets;

Where did Earth come from? The origin of the planets in our Solar System has been debated since 1755 when Immanuel Kant proposed the first theory Scientists debated whether the origin of our Solar System was usual, or a common consequence of star formation During the 1900s, many believed Earth was not a unique occurrence because of the recent discovery of other planetary systems around other stars

The Protoplanet Hypothesis Two Main Theories The Nebular Hypothesis The Protoplanet Hypothesis

Nebula Sun-like stars usually take around 100 million years to form. Nebula are star “nurseries”, where stars are born. This nebular photograph was taken by the Hubble Space Telescope The Eagle Nebula is an emission nebula famous for its gaseous star making regions, like the the Pillars of Creation shown here. Next, see the Orion Nebula. The Orion nebula is a tapestry of star formation, from the dense pillars of gas to the hot, young, massive stars that have emerged from their gas-and-dust cocoons. The next nebula looks like the head of an animal. Probably the most recognizable nebula to most people, The Horsehead Nebula is a dark, easily visible nebula in the constellation Orion. This is the blue lagoon nebula. You can see where it gets its name! The crab nebula is named after the cluster it is found near, the Cancer constellation. A nebula is the product of a supernova event. The death of one system, may well be the birth of another!

The Nebular Hypothesis In 1755 Immanuel Kant proposed that a solar nebulae was pulled together by gravity so that it collapsed into a flat, rotating disk that eventually coalesced into the Sun and planets Pierre Laplace expanded the theory in 1796 and proposed that the planets were formed by rings of matter split off a rotating nebulae by centrifugal force Remember…solar nebulae are huge clouds of dust and gas The Nebular Hypothesis is where a star(s) and planet(s) form because nebular debris cools and contracts under the force of gravity, eventually flattening into a spinning disk. With enough accumulation of certain elements like Hydrogen and Helium and enough pressure, a fusion process can begin a protostar. A protostar (early-stage “baby” star) forms in the center of the nebula. As matter condenses around the protostar in the middle, planets are formed from the spinning matter in the outer areas of the disk. This idea is widely accepted to account for the formation of stars and planetary systems such as ours.

The Nebular Hypothesis Laplace stated that after the matter split off, it coalesced into a planet. The process repeated itself, resulting in a planet each time. The matter left over was the Sun.

The spinning nebula formed planets because as the masses became more concentrated, the faster they began to spin. This is called conservation of momentum Much like a figure skater. You may have noticed that skaters can spin much faster if they pull their arms closer to their bodies. The more concentrated their body masses are, the faster they'll be able to rotate. As the sun formed in the middle, the cloud started to flatten out into a disc, sort of like a Frisbee or a pancake, with tiny dust grains making up the rest of the disc. The hypothetical explosion squeezed the unformed gas and dust together, which began to spin faster and faster in a circle

Problems With the Nebular Hypothesis Used a flat, rotating nebula as the Solar System's origin to explain why all the planets orbit in nearly the same plane and in the same direction All planets revolve around the Sun in a counterclockwise direction within a 7° band of the equatorial region of the Sun, and nearly all of them also turn on their individual axes in a counterclockwise direction as well. Contradicted the observation that the Sun contains most (99%) of the Solar System's mass but only a small fraction of its angular momentum (rotates slowly) Remember, angular momentum is the measure of the tendency of a rotating body to remain rotating

Also… If rings of matter were split off, as Laplace stated, they wouldn't be pulled together to form planets but would disperse into space.

How Does Accretion Occur? Accretion – gradual growth of planets by the accumulation of other smaller bodies Heavy bombardment period on Mercury Protostar forms with opaque core Energy is given off by protostar causing a cooling Cooling causes gas to condense into tiny specs of metal, rock, & ice “Stellar Debris” Stellar Debris begins to stick together to form Planetesimals Accrection of Planetesimals forms Protoplanets Some Planetsimals will form into asteroids, comets, and moons Planetesimal is a “baby” planet. A protoplanet is a small body that could later become a planet. In the meantime, the heat and radiation released by the protostar and the gas flow vaporize the dust grains of the cloud. The protostar begins its evolution into a star, accreting the gas. The disk starts cooling through energy radiation. Depending on the amount and distribution of gas, it can be gravitationally stable or unstable and form one or more new protostars. In such a way, a binary or multiple stellar system is formed. Far from the star, the gas is cold enough so that part of the gas condenses into dust and ice. The dust grains merge due to collisions until they form small pieces of rock called planetesimals. Planetesimals merge and form protoplanets. The maximum size of protoplanets depend on their distance from the star and on the chemical composition of the primordial nebula. It is much smaller in the inner regions than in the outer regions since the protostar tends to disrupt and vaporize dust. The later evolution of the planetary system is governed by the collisions between its constituent bodies. Impacts of meteorites and planetesimals upon protoplanets and satellites, generate craters on their surfaces. Many of craters from this time are still visible today on planets within our own Solar System. A good example of this can be found on Mercury. When impacts are especially violent, they can even move the bodies out of their original orbit. Our Solar System went through this phase roughly 4-4.5 billion years ago. This period is known as Heavy Bombardment. Ev idence of it is seen on Mercury.

Review of Planet-Forming Scenarios Nebular Hypothesis

Accretion Disk Formation

The Protoplanet Hypothesis Because of faults in the nebular hypothesis, other explanations of planet formation were sought. After many failures, such as the encounter hypothesis, astronomers returned to the nebular hypothesis to find improvements during the mid 1900s. Encounter theory:

Encounter Hypothesis What’s wrong with that? Hot gases expand, not condense…so hot gas would not form clumps and planets. Also…star encounters are extremely rare, so rare in fact that it should not happen in the lifetime of the Universe! Rogue star passes close to the Sun. Gas is “tidally” removed from both rogue star and Sun Rogue star material is less dense and becomes outer Solar System planets Inner Solar System material is more dense and becomes terrestrial planets

Improvement to Solar Nebular Theory: The Protoplanet Hypothesis Formed independently by Carl von Weizsacker Gerard Kuiper

The Protoplanet Hypothesis The solar system begins to form as a rotating cloud, or nebulae, collapses Instabilities in the nebulae cause dust particles to stick together and accrete into billions of planetesimals with diameters of about 10 meters. The planetesimals then collide and form protoplanets. Meanwhile, the protosun in the center of the nebular disk becomes massive and hot enough to "turn on" by fusing hydrogen.

The Sun begins to radiate energy and vaporize dust in the inner part of the Solar System. The remaining gas is blown away by solar winds.