lecture9and10_ch6 - Solar System(s) What does the solar...

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Unformatted text preview: Solar System(s) What does the solar system look like? Sun Mercury Over 99.9% of solar systemʼs mass A hot rock 98% H and He Venus Earth The cloudy planet The water planet Surface is hotter and smoother than Mercuryʼ s The double planet Itʼs another rock 1 Earth Mars The water planet A frozen planet The double planet Also a rock Two more rocks Asteroid Belt Jupiter Lots more rocks A gaseous planet, H & He Not a rock! 40 km Jupiter Jupiter A gaseous planet, H & He A gaseous planet, H & He Not a rock! Not a rock! 2 Jupiterʼs Moons Io Europa Largest moons in solar system are similar in size to Earthʼs Moon Four largest discovered by Galileo Worlds of ice and rock Ganymede Callisto Ioʼs sulfur volcanoes Artistʼs conception of the view from Europaʼ s icy surface Saturn Saturn Giant and gaseous like Jupiter Rings are made of ice chunks Artist’s conception 3 Saturn Huygens probe landed on Titan Jan 2006 Cassini probe arrived July 2004 (Launched in 1997) Radar shows liquid (ethane or methane) lakes on Titan Uranus Uranus The tilted planet: 90 degrees!! & a tiny moon discovered with Hubble Space Telescope Also giant and gaseous Has many moons Be a scientist: observe the patterns in the orbits. Neptune Pluto Almost a twin of Uranus (except for the tilt) A dirty ball of ice Plutoʼs moon Charon is not much smaller 4 Planet X = Eris Comets Also dirty balls of ice Spend most of their time in solar systemʼs outer reaches Have elongated orbits Comet Hale-Bopp Comet Wild 2 • 2003 UB313, discovered at Caltech in 2003 • 97 AU from the Sun now, gets as close as 38 AU. • 45-degree inclination of its orbit • 1.5 times larger than Pluto, covered with methane ice • It has a moon! Terrestrial Planets: Four rocks close in Jovian Planets: Four gas balls farther out Pluto: An icy oddball What features of our solar system provide clues to how it formed? Rocky asteroids between Mars & Jupiter Icy comets in vicinity of Neptune including Pluto Virtually all rotational motion in same direction More icy comets far beyond Pluto (Right-hand rule) 5 Exceptions to the basic patterns What theory best explains the features of our solar system? According to the nebular theory our solar system formed from a giant cloud of interstellar gas (nebula = cloud) As gravity forced the cloud to become smaller, it began to spin faster and faster Why is that? A. Newtonʼ s Law of Gravitation B. Conservation of Momentum C. Conservation of Energy D. Conservation of Angular Momentum As gravity forced the cloud to become smaller, it began to spin faster and faster Conservation of angular momentum As gravity forced the cloud to become smaller, it began to spin faster and faster Conservation of angular momentum Gas settled into a spinning disk because spin slowed collapse perpendicular to spin axis 6 We see plenty of evidence for spinning gas disks around other newly forming stars Disks around newborn stars in the Orion Nebula Solar system formation is a stage of galactic recycling Where did the solar system come from? Fig 6.14 Four Big Questions Composition of Milky Way gas: 70% H, 28% He, 2% other stuff 1) Why does virtually everything in the solar system rotate in the same direction? 2) Why are the inner worlds rocky and the outer ones gaseous or icy? 3) Where did the comets and asteroids come from? 4) How can we explain the exceptions? [Orbit and rotation planets IF] 7 As gravity causes cloud to shrink, its spin increases •Conservation of angular momentum •Collisions between gas particles in cloud gradually reduce random motions •A spinning cloud flattens as it shrinks •Collisions between gas particles also reduce up and down motions Virtually all rotational motion in same direction What caused the orderly patterns of motion in the solar system? Why are there two types of planet? Conservation of Angular Momentum Jovian Planets = gas balls (H, He, hydrogen compounds) Jovian Moons = mostly ices Terrestrial Planets = rocks (C, O, Si, Fe, etc.) 8 •As gravity causes cloud to contract, it heats up Conservation of energy Inner parts of disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice. Tiny solid particles stick to form planetesimals. Gravity draws planetesimals together to form planets. This process is called accretion. Gravity of rock and ice in jovian planets draws in H and He gases Fig 6.17 Inside the frost line: too hot for hydrogen compounds to form ices. Outside the frost line: cold enough for ices to form. Summary Why two types of planet? 1. Outer planets get bigger because they can accrete hydrogen ices. 2. Because they get bigger, outer planets accrete and keep H & He gas. Moons of jovian planets form in miniature disks around the jovian planets Where did asteroids and comets come from? Comets and asteroids are leftover planetesimals. Outflowing matter from the new Sun -- the solar wind -blew away the leftover gases 9 How do we explain exceptions like the existence of the Moon? Earthʼs moon was probably created when a big planetesimal tore a chunk out of the newly forming Earth. Other large impacts may be responsible for other exceptions like rotation of Venus and Uranus Earthʼs moon was probably created when a big planetesimal tore a chunk out of the newly forming Earth. How do we explain exceptions like the existence of the Moon? Collisions and impacts. Review of nebular theory= Fig 6.24 Solar System(s) Data/Observe: Patterns Exceptions Explanation: Nebular theory 10 Today’s topics 1) The age of the solar system (radioactive dating) 2) Discovering planets outside our own solar system How do we know when the planets formed? Your $1 account loses 50% a day Day 0: $1.00 Day 1: $0.50 Day 2: $0.25 Day 3: $0.125 (rounds to $0.13) Day 4: $0.06 Day 5: $0.03 Day 6: $0.015 (rounds to $0.02) Day 7: $0.0075 (rounds to $0.01) Day 8: Too small for bank to care. • • • • • • • • • Day 0 $0.00 Day 1 $0.50 Day 2 $0.75 Day 3 $0.87 Day 4 $0.94 Day 5 $0.97 Day 6 $0.98 Day 7 $0.99 Day 8 $1.00 The half-life of your money is one day. Plot of your money and the other account’s money $1.20 $1.00 Dollars • • • • • • • • • Say your money went to some other account… $0.80 $0.60 $0.40 $0.20 $0.00 0 1 2 3 4 Days 5 6 7 8 Example: Half of all potassium-40 nuclei decay into argon-40 every 1.25 billion years 11 Measurements of decaying elements in meteorites indicate age of solar system: Know: • How we know how old the Earth is. • How to do a radioactive decay problem. 4.6 billion years We detect planets around other stars by looking for a periodic wobble How do we detect planets around other stars? We measure the wobble using the Doppler shift in the starʼs spectrum Doppler shift: measures velocity “now” Change in doppler shift = change in velocity = acceleration We can also detect planets if they eclipse their star The size of the wobble (in velocity) tells us the planetʼs mass Fraction of starlight blocked tells us planetʼs size The period of the wobble tells us the radius of its orbit 12 Kepler Mission • Monitor 100,000 stars for occultations to search for Earth-sized planets for 4 years. • 0.01% variations • Expect to find ~100 What have other planetary systems taught us about our own? Over 313 known extrasolar planets as of 24 Sept 2008. Most are gas giants more massive than Jupiter and closer to their star than Earth is to Sun! Revisions to the basic nebular theory are necessary! Is the nebular theory wrong? • Stars and planets DO form from interstellar gas clouds -- we’ve caught them in the act. • The clouds do collapse under their own weight and form disks. • Exactly how the disks form planets is unsure, but there are several possibilities, all being tested by observations and simulations. The nebular theory needs revision. More planet facts • • • • Planets seem to be common. Planets can form in multiple-star systems. Short-lived, very hot stars can have planets. Stars with more heavy elements have more planets. • 20% of metal-rich stars have planets. • One of the smallest planets discovered so far (>6 Earth masses) orbits its sun in 2 days. • One of the smallest one so far was discovered 3 years ago with the help of an MSU undergrad (Tui Britton) ! Migrating planets? • Recall: planets can only change their orbits if they interact gravitationally with something else. • A planetary disk, if present, could interact w/planets. • No interactions, no migrating. • Disk goes away, interactions stop so migration stops • Our planets’ orbits are stable. • It would have been bad for us if Jupiter had migrated toward the Sun. Why didn’t it? 13 Is Earth Unusual? • No Earth-like planets found yet. • But data aren’t good enough to tell if they exist! • Available methods can most easily detect BIG planets. exosolar planet review • Why we can’t just take a picture of another planet. • How and why the velocity curve method for detecting planets works. • How the eclipse-method works. • That most of the new planets are gas giants very close to their star, and why that was a surprise. • For these systems, planet orbits changed over time (migrated), and a plausible mechanism (gravitational interactions with the protostellar disk) has been proposed. 14 ...
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This note was uploaded on 11/13/2011 for the course ISP 205 taught by Professor Donahue during the Fall '08 term at Michigan State University.

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