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Formation of the Solar System © Sierra College Astronomy Department Formation of the Solar System The Formation of the Solar System What properties must a planetary formation theory explain? It must explain the patterns of motion of the present solar system (last week). 2. It must explain why planets form into 2 groups. 3. It must explain the huge existence of asteroids and comets. 4. It must allow for possible exceptions to the rules. The theory may be able to be used on other solar systems in the Galaxy 1. © Sierra College Astronomy Department 2 Formation of the Solar System The Formation of the Solar System Evolutionary Theories All evolutionary theories have their start with Descartes’s whirlpool or vortex theory proposed in 1644. Using Newtonian mechanics, Kant (in 1755) and then Laplace (around 1795) modified Descartes’s vortex to a rotating cloud of gas contracting under gravity into a disk. The Solar Nebular Hypothesis is an example of an evolutionary theory. © Sierra College Astronomy Department 3 Formation of the Solar System The Formation of the Solar System Catastrophic Theories Catastrophic theory is a theory of the formation of the solar system that involves an unusual incident such as the collision of the Sun with another star. The first catastrophic theory - that a comet pulled material from the Sun to form the planets - was proposed by Buffon in 1745. Other close encounter hypotheses have been proposed too. Catastrophic origins for solar systems would be quite rare (relative to evolutionary origins) due to the unusual nature of the catastrophic incident. © Sierra College Astronomy Department 4 Formation of the Solar System Solar Nebula Hypothesis Origin of the Solar Nebula Galactic recycling – Most of the universe started as Hydrogen and Helium. All other heavy elements (loosely called “metals” by astronomers) were formed in stars – When stars die they release much of the content into space – While this has been going on for 4.6 billion years, only 2% of all the have been converted to “metals” Evidence from other gas clouds – All new systems that we can observed formed within interstellar clouds, such as the Orion Nebula © Sierra College Astronomy Department 5 Formation of the Solar System Solar Nebula Hypothesis Towards a Solar Nebula Hypothesis A supernovae shock wave likely triggered the events which led to the birth of our solar system The nebular cloud collapsed due the force of gravity on the cloud. But the cloud does not end up spherical (like the sun) because there are other processes going on: – Heating – The cloud increases in temperature, converting gravitational potential energy to kinetic energy. The sun would form in the center where temperatures and densities were the greatest – Spinning – as the cloud shrunk in size, the rotation of the disk increases (from the conservation of angular momentum). – Flattening – as cloud starting to spin, collisions flattened the shape of the disk in the plane perpendicular to the spin axis © Sierra College Astronomy Department 6 Formation of the Solar System Testing the Model If the theory is correct, then we should see disks around young stars Dust disks, such as discovered around beta-Pictoris or AU Microscopii, provide evidence that conditions for planet formation exist around many Sun-like stars. © Sierra College Astronomy Department 7 Formation of the Solar System Solar Nebula Hypothesis The Formation of Planets As the solar nebula cooled and flattened into a disk some 200 AU in diameter, materials began to “freeze” out in a process called condensation (changing from a gas to a solid or liquid). The ingredients of the solar system consist of 4 categories (with % abundance): – – – – Hydrogen and Helium gas (98%) Hydrogen compounds, such as water, ammonia, and methane (1.4%) Rock (0.4%) Metals (0.2%) Since it is too cool for H and He to condense, a vast majority of the solar nebula did not condense Hydrogen compounds could only condense into ices beyond the frost line, which lay between the present-day orbits of Mars and Jupiter © Sierra College Astronomy Department 8 Formation of the Solar System Solar Nebula Hypothesis Building the Terrestrial Planets In the 1940s, Weizsächer showed that eddies would form in a rotating gas cloud and that the eddies nearer the center would be smaller. Eddies condense to form particles that grow over time in a process called accretion. Materials such and rock and metal (categories #3 and #4). These accreted materials became planetesimals which in turn sweep up smaller particles through collision and gravitational attraction. These planetesimals suffered gravitational encounters which altered their orbits caused them to both coalesce and fragment. Only the largest planetesimals grew to be full-fledged planets. Verification of this models is difficult and comes in the form of theoretical evidence and computer simulations. © Sierra College Astronomy Department 9 Formation of the Solar System Solar Nebula Hypothesis Building the Jovian Planets Planetesimals should have also grown in the outer solar system, but would have been made of ice as well as metal and rock. But Jovian planets are made mostly of H and He gas… The gas presumably was captured by these ice/rock/metal planetesimals and grew into the Jovian planets of today. © Sierra College Astronomy Department 10 Formation of the Solar System Solar Nebula Hypothesis A Stellar wind is the flow of nuclear particles from a star. Some young stars exhibit strong stellar winds. If the early Sun went through such a period, the resulting intense solar wind would have swept the inner solar system clear of volatile (low density) elements, molecules and compounds. The giant planets of the outer solar system would then have collected these outflowing gases. © Sierra College Astronomy Department 11 Formation of the Solar System Solar Nebula Hypothesis An object shrinking under the force of gravity heats up. High temperatures near the newly formed Sun (protosun) will prevent the condensation of more volatile (low density) elements. Planets forming there will thus be made of nonvolatile, dense material. Farther out, the eddies are larger and the temperatures cooler so large planets can form that are composed of volatile elements (light gases). © Sierra College Astronomy Department 12 Formation of the Solar System Solar Nebula Hypothesis Problem: The total angular momentum of the planets is known to be greater than that of the Sun, which should not occur according to conservation laws (i.e. the present Sun is spinning too slowly). Solution: As the young Sun heated up, it ionized the gas of the inner solar system. – The Sun’s magnetic field then swept through the ions in the inner solar system, causing ions to speed up. – As per Newton’s third law, this transfer of energy to the ions caused the Sun to slow its rate of rotation. © Sierra College Astronomy Department 13 Formation of the Solar System Solar Nebula Hypothesis Explaining Other Clues Over millions of years the remaining planetesimals fell onto the moons and planets causing the cratering we see today. This was the period of heavy bombardment. Comets are thought to be material that coalesced in the outer solar system from the remnants of small eddies. The Asteroid belt formed from debris that could not coalesce into a planet due to the gravitational influence of Jupiter © Sierra College Astronomy Department 14 Formation of the Solar System Solar Nebula Hypothesis The formation of Jovian planets and its moons must have resembled the formation of the solar system. Jupiter specifically: – Moons close to Jupiter are denser and contain fewer light elements; – Moons farther out decrease in density and increase in heavier elements. © Sierra College Astronomy Department 15 Formation of the Solar System The Exceptions to the Rule Captured Moons – satellites which go the opposite way were likely captured. Most of these moons are small are lie far away from the planet. Giant impacts – may have helped form the Moon and explain the high density of Mercury and the Pluto-Charon system. Furthermore, the unusual tilts of Uranus and Venus can also be explained by giant impacts. © Sierra College Astronomy Department 16 Formation of the Solar System Solar System Destiny The nebular hypothesis accounts for all major features in the solar system It does not account for everything, however It probably took about a few tens of million of years, about 1% of the current age of the solar system The solar system was probably not completely predestined from the collapse of the solar nebula, though the initial were orderly and inevitable The final stage of accretion and giant impacts were fairly random in nature and made our solar system unique © Sierra College Astronomy Department 17 Formation of the Solar System Radioactivity Radioactivity Certain isotopes (elements which contain differing number of neutrons) are not stable and will decay into two or more lighter elements The time it takes for half of a given isotope to decay is called the half-life By noting what percentage a rock (or human body) has left of a radioactive element can enable us to estimate the age of that object. This process is called radioactive dating. See Mathematical Insight 8.1 © Sierra College Astronomy Department 18 Formation of the Solar System Radioactivity Radioactivity - examples Potassium-40 decays into Argon-40 with a half-life of 1.25 billion years – Since Argon-40 is an inert gas, it is very unlikely to have formed inside a rock as the solar nebula condensed, so it must have formed via decay Uranium-238, after a series of decays, turns into Lead206 with half-life of 4.5 billion years – Lead and Uranium have very different chemical behaviors – Some minerals have nearly no lead to begin with, so when uranium is mixed with lead, we can assume that the lead formed via decay © Sierra College Astronomy Department 19 Formation of the Solar System Radioactivity Radioactivity The general formula for the age of a radioactive material is (see Mathematical Insight 8.1): current amount log10 original amount t thalf 1 log10 2 © Sierra College Astronomy Department 20 Formation of the Solar System Radioactivity Earth rocks, Moon rocks, and meteorites The oldest Earth rocks date back to 4 billion years and some small grains go back to 4.4 billion years. Moon rock brought back from the Apollo mission date as far back as 4.4 billion years. – These tell us when the rock solidified, not when the planet formed The oldest meteorites, which likely come form asteroids, are dated at 4.55 billion years, marking the time of the accretion of the solar system © Sierra College Astronomy Department 21 The End © Sierra College Astronomy Department 22