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Lecture 15

AS101 Lecture 15

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Patrick Mc Graw

Lecture 15 4/10/2013 12:24:00 PM Features and Formation of the Solar System—and Are we Alone? Notable feature of the Solar system:  Most of the solar system (except the Oort cloud) has a disclike shape: most planets and other objects orbit in the same plane.  Most orbital and rotational motion is in the same direction: counterclockwise when viewed from above the north pole.  Two types of planets with different sizes and chemical composition. Closer to sun-- denser and more rocky/metallic. Farther away: less dense, with more Hydrogen, helium, other gases and ices.  Most objects in the solar system appear to have formed at about the same time, 4.6 billion years ago. How did the Solar System form?  Theories can be divided into catastrophic (sudden) vs. evolutionary (gradual).  Most widely accepted accepted theory is the Nebular Hypothesis, a.k.a. solar nebula theory which is an evolutionary explanation.  Nebular hypothesis has a history going back to Kant and Laplace in the 18 thcentury. Nebular Hypothesis:  Solar system begins with a large cloud of gas and dust (mostly hydrogen and helium) that begins collapsing. (Balance between gravity and pressure gets upset by a shockwave from a nearby exploding star, or some other trigger.)  As the cloud collapses, its original very slight spin speeds up (like a figure skater pulling in her arms, or like a planet speeding up in its orbit as it gets closer to the sun.)  Most of the matter concentrates in the centre, rotation flattens the rest out into a disc (like pizza dough being spun).  Meanwhile, the protosun at the centre heats up due to the energy released in collapse.  Colliding dust grains in the disc begin to stick together and form planetesimals, some of which eventually grow into protoplanets large enough to start pulling in more planetesimals gravitationally.  The growing protoplanets begin to sweep out clear lanes around their orbits.  Once the sun gets hot enough, a nuclear fusion reaction begins in its core, and the sun begins producing its own energy. When the sun turns on, solar wind (a stream of high-energy particles) blows most of the remaining gas out into space. This essentially stops the process of planet formation. Notable Features of the Solar System: Explanations:  Most of the solar system (except the Oort cloud) has a disclike shape: most planets and other objects orbit in the same plane.  Nebular hypothesis explains how the matter that formed the solar system collapsed into a disk  Most orbital and rotational motion is in the same direction: counterclockwise when viewed from above the north pole.  Nebular hypothesis: all of the angular momentum (i.e., circular motion and rotation) in the solar system came from the initial spin of the collapsing cloud.  Two types of planets with different sizes and chemical compositions.  In the inner solar system, temperatures were too hot for water, ammonia, methane, etc. to solidify. Only rocky and metallic materials were able to form solid grains.  Farther out, beyond the ice line, more solids were available so those protoplanets grew faster. They grew large enough to begin pulling in gaseous matter (hydrogen, helium) before the sun heated up enough to drive away the gases.  All the planets seem to have formed at about the same time.  In the solar nebula theory, there is a natural expiry date for the formation of planets: as soon as the protosun becomes hot enough to start nuclear fusion and become an actual star, the solar wind drives away most of the remaining dust and gas, so the process of planetary formation mostly stops. Nebular Hypothesis: Summary:  Collapsing cloud of gas and dust spins faster as it collapses (like a figure skater pulling in her arms).  Rotation flattens the cloud into a disc with a central bulge containing most of the matter.  Central bulge forms protosun  Matter in the outer disc condenses in planetesimals, which collide to form protoplanets.  Protoplanets grow into planets by attracting more planetesimals.  The process stops when the Sun “turns on.” The Encounter Hypothesis: Strengths:  Easily explains the angular momentum as due to the encounter itself.  Explains the different chemical compositions of the inner and outer solar system: They came from two different stars. The Encounter Hypothesis: Problems:  Most simulations show gas pulled away from the two stars would have been too hot to condense into planets: it should have expanded instead.  Close encounters between stars are extremely rare: not likely to happen very often within the age of the universe. The Nebular Hypothesis: Strengths:  Explains many key features of the solar system, as described above.  Explains planetary formation as a normal part of the process that forms a star--- does not rely on rare events.  Discovery of planets around other stars suggests that planets are common, not rare.  There is direct evidence of disc-shaped clouds around other newly forming stars. The Nebular Hypothesis: Problems:  If all
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