Formation of the Solar System. Ch. 6

Transcription

1 Formation of the Solar System Ch. 6

2 Things to Explain Similarities in the patterns of motion of the large objects in the solar system Planetary dichotomy (terrestrial vs Jovian) Asteroids and comets Peculiarities

3 Orbital Motions Orbits are nearly circular and lie in same plane All the planets orbit the sun in the same direction Most planets rotate in the same direction as the sun Most of the moons have similar orbital properties as their planets

4 Planetary Dichotomy Terrestrial planets Jovian planets Located in the inner solar system Located in the outer solar system Small and dense Large and low density Abundance of metals Abundance of H compounds Few, if any, moons Many moons No rings All have rings

5 Asteroids & Comets Asteroids Smaller and less massive than planets Composed of rocks and metals Located in the asteroid belt Inner solar system Comets Smaller and less massive than planets Composed of ices Ammonia, water and methane Located in the Kuiper belt and Oort Cloud Outer solar system

6 Uranus rotates nearly on its side Venus rotates opposite to its orbital motion Earth has a one of the largest Moons in the solar system Peculiarities Other terrestrial planets either don t have moons or have very small, likely captured moons

7 Peculiarities COSMIC CONTEXT FIGURE 6.1. THE SOLAR SYSTEM The solar system s layout and composition offer four major clues to how it formed. The main illustration below shows the orbits of planets in the solar system from a perspective beyond Neptune, with the planets themselves magnified by about a million times relative to their orbits. 1 Large bodies in the solar system have orderly motions. All planets have nearly circular orbits going in the same direction in nearly the same plane. Most large moons orbit their planets in this same direction, which is also the direction of the Sun s rotation. Seen from above, planetary orbits are nearly circular. Saturn Neptune Jupiter Uranus Venus White arrows indicate the rotation direction of the planets and Sun. Mercury Mars Earth Red circles indicate the orbital direction of major moons around their planets. 2 Planets fall into two major categories: Small, rocky terrestrial planets and large, hydrogen-rich jovian planets. terrestrial planet Terrestrial Planets: small in mass and size close to the Sun made of metal and rock few moons and no rings jovian planet Jovian Planets: large mass and size far from the Sun made of H, He, and hydrogen compounds rings and many moons 3 Swarms of asteroids and comets populate the solar system. Vast numbers of rocky asteroids and icy comets are found throughout the solar system, but are concentrated in three distinct regions. Asteroids are made of metal and rock, and most orbit in the asteroid belt between Mars and Jupiter. Comets are ice-rich, and many are found in the Kuiper belt beyond Neptune s orbit. Even more comets orbit the Sun in the distant, spherical region called the Oort cloud, and only a rare few ever plunge into the inner solar system. Kuiper belt Each planet s axis tilt is shown, with small circling arrows to indicate the direction of the planet s rotation. 4 Several notable exceptions to these trends stand out. Some planets have unusual axis tilts, unusually large moons, or moons with unusual orbits. Uranus s odd tilt Earth s relatively large moon Orbits are shown to scale, but planet sizes are exaggerated about 1 million times relative to orbits. The Sun is not shown to scale. Jupiter Venus Asteroid belt Sun Mercury Earth Mars Saturn Uranus Uranus rotates nearly on its side compared to its orbit, and its rings and major moons share this sideways orientation. Our own Moon is much closer in size to Earth than most other moons in comparison to their planets. Neptune Orange arrows indicate the direction of orbital motion. Copyright 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley

8 Nebular Theory

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10 Clouds of gas are very large Need a catalyst Gravity is very weak and can t collapse the cloud on its own

11 Conservation of Energy & Angular Momentum Heating - cloud gets hot due to conservation of energy Spinning - cloud starts to spin very quickly due to conservation of angular momentum Flattening - cloud changes shape from spherical to a disk due to collisions during the collapse

12 Spinning-Flattening Disk Orbits are in the same plane Planets all orbit the Sun in the same direction Orbits are nearly circular Highly eccentric orbits would be reduced due to collisions

13 Planetary Dichotomy Condensation - transition from gas to liquid or solid Temperature of solar nebula decreases with radius Metals could condense in inner solar system Hydrogen compounds could condense in outer solar system

14 Frost line - boundary beyond which H compounds can form ice Inside the frost line only rocks and metals can condense Outside the frost line ices (from H compounds) can condense

15 Terrestrial Planets Formed inside the frost line Condensates stuck together due to electrostatic forces Particles accrete material due to collisions As they gain in mass, gravity becomes stronger and accelerates the process Planetesimals accrete more material to become planets Not much material to work with so planets don t grow massive enough to accrete H and He gas

16 Jovian Planets Formed outside the frost line Ice condensates stuck together due to electrostatic forces Particles accrete material due to collisions As they gain in mass, gravity becomes stronger and accelerates the process Icy planetesimals become large enough to accrete H and He gas Lots of gas in the solar nebula for massive planetesimals to accrete Jovian planets become very massive

17 Formation of Jovian planets Jovian moons formed from the same disks of gas that formed the Jovian planets

18 Clearing out the excess material Young sun forms and begins pouring out radiation and charged particles Young stars have strong solar winds Radiation and solar wind blow out the remaining H and He Clearing out the excess H and He stops planet formation Planets are now stuck with their masses

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20 Comets and Asteroids Comets and asteroids are a fraction of the leftover planetesimals from the formation of the planets Some of the leftovers were kicked out of the solar system through close encounters Others were lost due to impacts with the planets Period of heavy bombardment

21 Giant Impacts Formed the Moon Tilted the rotational axis of Uranus Flipped the rotational axis of Venus

22 Formation of the Moon

23 When did the solar system form? Radiometric dating (ratios of radioactive isotopes) of rocks shows that the Earth formed about 4.5 billion years ago Half life - the length of time it takes for half of the material to decay

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25 More evidence from other systems

26 Exoplanets

27 Detection Methods Direct - direct proof of the planet s existence from images or spectra of the planet Indirect - precise measurements of the host star that point to the existence of a planet Astrometric Technique - measurement of star s wobble in the sky due to presence of a planet Doppler Technique - measurement of star s wobble due to presence of a planet with doppler shifts in star s light Transits & Eclipses - measurement of changes in star s brightness as a planet transits or is eclipsed by the star

28 Direct Detection Very difficult with present technology Star s light overwhelms planet s Need extremely high angular resolution

29 Gravitational Wobble Center of mass - the balancing point of the system All objects orbit the center of mass The center of mass is very close to the star Star appears to wobble whereas the planet appears to orbit

30 Doppler effect due to wobble of a star Doppler effect - the change in wavelength due to motion The faster you go, the more the light or sound is shifted Blue shift - the shift to shorter wavelengths due to motion toward from the observer Red shift - the shift to longer wavelengths due to motion away from the observer

31 Transits & Eclipses

32 Kepler Mission

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34 Major Discoveries Kepler 10-b - First rocky exoplanet (1.4 REarth, 20 times closer than Mercury) Kepler 11 - system 6 rocky planets larger than Earth ( AU) Kepler 16b - Planet orbiting two stars Kepler 20 - First Earth-sized planets

35 Kepler 22b MEarth in a habitable zone Kepler 37 - A planet only slightly larger than the Moon & a planet slightly smaller than the Earth Kepler 62 & 69 - Super-Earths in a habitable zone

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37 Selection Effect / Bias Massive planets Small orbits or elliptical orbits Outdated! Nebular theory says Jovian planets should form far out...

38 Planetary Migration Explaining proximity of Jovian-like planets to stars Planetary migration a planet loses or gains angular momentum due to a gravitational interaction and moves to a different orbit Waves of material Close encounter

39 The Sun Ch. 10

40 Properties M = kg = 333,000 MEarth R = 700,000km = 109 REarth Tsurf = Teff = 5780 K Tcore = K ρavg = 1.4g/cm 3 98% H and Helium L = W (Joules / second) Solar Dynamics Laboratory

41 Hydrostatic Equilibrium Hydrostatic equilibrium - the balance between the force of gravity and internal pressure Pressure, temperature and density increase with depth

42 Energy Balance Energy escaping from Sun s surface equals to the energy generated by nuclear fusion in the core Otherwise, the sun will expand or contract

43 Solar Structure Core - the hottest part of the sun and where nuclear fusion occurs Radiation zone - hot plasma where photons are trapped for hundreds of thousands of years Convection zone - energy is transferred to the surface through convection Photosphere - the visible surface of the Sun and where photons finally escape Chromosphere - hot part of the atmosphere where most of the UV light is radiated Corona - extremely hot part of Sun s atmosphere and where the x-rays are generated

44 Nuclear Reactions Fission - the breaking up of a nucleus into smaller nuclei Fusion - the merging of nuclei to make a larger nucleus Requires extremely high temperatures & pressures

45 High Temperature & Pressure Need very high kinetic energies to overcome electrostatic repulsion Strong force dominates only over very small distances Fusion possible when temperature and pressure are very high

46 Mass of 4 protons greater than 4 He Difference in mass is the energy output of the proton-proton chain (E = mc 2 ) 600 megatons/s 596 megatons/s = 4 megatons of mass lost per second Rate of PP chain is highly sensitive to core temperature & pressure

47 Solar Thermostat Contraction causes heating Expansion causes cooling

48 Random walk creates conditions necessary for convection Escape of Energy Photons created in core must escape Interior is extremely dense Photons scatter off electrons Random walk for hundreds of thousands of years

49 Convection Transports energy to the photosphere Hot gas rises Cool gas falls Each granule lasts only a few minutes

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