CH4 - Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson...

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Unformatted text preview: Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson & McMillan, Astronomy Chapter 4 The Solar System How was the Solar System made? the How are planetary system made in general ? (around other stars too) general There is a lot of evidence to look at There A good theory should address all the key evidence key Key Observational Evidence Key 1. The Solar System: Planets, asteroids, comets … 2. Star-forming regions: 2. forming Nebulae, Dusty Disks, Young Stars Young 3. Extra-solar planets: 3. solar Planets around other stars Planets Theories of Solar System Formation: Theories Historical development Historical Historically there was only the Solar System to explain – a sample of one Many theories were devised before star forming regions or extra-solar planets forming solar were observed were This history influences current theories This 1. The Solar System 1. 4 Rocky Planets plus junk: Asteroids plus 2 Gas Giants: Jupiter & Saturn Gas 2 Ice Giants: Uranus & Neptune Ice big junk: Pluto & Kuiper Belt (comets) Kuiper Oort Cloud (long period comets) Oort Table 4-2 Comparison Between the Terrestrial and Jovian Planets Solar System Solar Features The orbits of the planets: Nearly Circular Nearly All in a nearly flat plane All All revolve in the same direction which is the same as the way the Sun rotates is Solar System Features Solar More on Rotation: Most planets rotate in the way the Sun rotates rotates Planets’ moons almost all rotate this Planets moons way way Conclusion: There is a preferred Conclusion There rotation direction for the Solar System rotation Solar System Features: Solar Planets are highly differentiated: As you go further out: As Rocky Gaseous Icy Rocky Icy Comets and Asteroids fit the same pattern: They appear to be old and may represent the formation era! may Solar System Features: Solar Assuming planets, asteroids and comets are currently close to where they formed… formed Conclusion: Different formation Conclusion: Different conditions existed at different distances from the young Sun from Solar System Explanations: Solar Rotation Laplace 1796 realized that a slowly rotating cloud will form a flat disk as it collapses as This is due to conservation of angular momentum angular Angular Momentum Angular Angular Momentum is always perfectly conserved for an isolated system conserved Rotation x radius2 = constant Rotation ½ size means 4 times the rotation rate Flattening of the disk Flattening Angular Momentum only conserves motion around one axis one The gas is free to cool and collapse and get very flat in the other direction in When the rotation of the gas (centrifugal force) exactly balances the pull of gravity then contraction stops stops Nebular Contraction Nebular The formation from a gas cloud is called nebular contraction nebular The end result is the “solar nebular”, a flat spinning disk of gas flat Can we parcel up gas disk to make Can planets? planets? The Minimum Solar Nebula The Gas clouds in space contain little dust (dust includes rocky material) (dust Hydrogen 74% Hydrogen Helium 24% Helium Everything else (dust) 2% Everything Perhaps the rocky inner solar system once had all this gas too? once The Minimum Solar Nebula The Assume the solar nebula was once mostly gas and 1/50th dust mostly dust The theoretical “minimum solar nebula” has enough material at each distance to has make the planet there make Do we see dusty gas disks like this? Do 2. Young Stars 2. HST Images of the Orion Nebula Orion contains a new star cluster star Young stars have dusty gas disks around them with enough material to make planets make Dust disks around Dust other stars Some dust disks appear to have structure structure It might be due to planets! to Gas disks to planets Gas Can gas disks break-up into clumps that Can up collapse to planets? collapse What about all that gas near the rocky planets? planets? Gas disks to planets Gas Can gas disks break-up into clumps that Can up collapse to planets? collapse Not if the gas is too hot to compress What about all that gas near the rocky planets? planets? Rocky planets form from only the dust Planet formation: Planetesimal Theory Planetesimal Dust particle in the solar nebula disks can stick together together This clumping can go on indefinitely… This 1 km sized in 100,000 years 100,000 Origin of asteroids and comets! Planet Formation: Planet Planetesimal Theory At 1 km and up gravity is important At Larger objects quickly gobble up most material until there are many moon sized “planet embryos” Planet Formation: Planet Planetesimal Theory It takes a long time but eventually most of the embryos collide to form well separated planets well Loose Ends: Rotation Loose Most planets revolve and rotate in the same way around the sun way BUT Venus rotates very slowly backwards BUT The inner planets had very large collisions during their creation, enough to change their rotation a lot rotation Venus’ llast big collision set it spinning slightly Venus ast backwards backwards Differentiation Differentiation The different composition of planets, asteroids and comets is related to the temperature in the solar nebula solar Differentiation Differentiation Only rocks can condense close in to form inner planets and asteroids asteroids Ices can condense further out to form icy planets and comets comets What about the gas? What Hydrogen and Helium never condense Hydrogen BUT: If a planet is 10 times Earth mass its gravity can hold onto gas its Core Accretion theory says all giant Core theory planets have a big solid core necessary to get gas The Solar Wind eventually pushes away all the leftover gas all Planetesimal Theory: Problems Problems It takes 100 Million years+ to make outer planets with planetesimals but planetesimals but disks like those in Orion are gone in a fraction of that time fraction Jupiter might have no 10 Earth mass core core Extra-Solar Planets … Extra Solar 3. Extra-Solar Planets 3. Since the early 1990’s over 400 planets have Since over been found around other stars! been They are all big and gaseous They They have very elliptical orbits They or are right next to the star “hot Jupiters” hot Jupiters Perhaps the Solar System is unusual? Extra-solar Planets: Detection Extra Radial Velocity Method: Doppler shift of spectral lines of star of For a big planet the star moves in response to its orbit response Multiple Planet Multiple System Upsilon Andromedae Upsilon Andromedae 3 detected planets detected Motion of star superposition of 3 periodic signals periodic Extra-solar Planets: Detection Extra Transits: If edge on, planets pass before Transits: and behind star and Detect actual light from planets – can can measure composition and size! measure Extra-solar Planets: Detection Extra Direct Imaging: Direct Fainter, young stars Fainter, Can see bright young Planets nearby Star covered with coronagraph, special image subtraction techniques used techniques Key workers in Canada: C. Marois, (UdeM, NRC) C. Marois NRC) HR8799 – 3 planets 2008 Making Giant Planets a different way different Hot gas will not collapse but the outer solar nebula was quite cold, less than 50 K (-223 C) 50 We need a way to make large planets fairly fast fairly Gravitational Instabilities Gravitational A heavy and cold gas disk can fragment because gravity can overwhelm gas pressure overwhelm This is tricky to prove mathematically – best to use computer simulations My research includes supercomputer models of giant planet formation from gravitational instabilities gravitational Gas Giant Formation Gas in 1000 years… MOVIE: Giant Planets Fast Giant Gravitational instability in gas can form large giant planets with elliptical orbits large Extra-Solar Planets fit this model Extra The Solar system can fit too… The Planet Formation: Planet The big picture Gas Giants can form directly only in the outer solar nebula Terrestrial Planets form from planetesimals in any case planetesimals These models are simplified extremes: These Gas and planetesimals are both present Gas and are around young stars around Planetesimals: Planetesimals Cleaning up Large planets eject remaining planetesimals via Large planetesimals via gravity to Oort cloud (long period comets) Oort Comets and Asteroids: Comets Leftovers Asteroids are planetesimals that failed planetesimals that to go further, probably because of Jupiter’s gravity Jupiter Comets are icy planetesimals planetesimals Kuiper Belt Objects: Hard to build Kuiper Belt planets beyond Neptune Oort Cloud Objects: Thrown out by Oort Cloud giant planets giant Asteroids & Comets Comets Comet TempeI Comet TempeI Asteroid Belt Asteroid Between Mars Between and Jupiter and Total mass Total much less than a moon than Some orbits Some quite eccentric eccentric Asteroids – Why so few? Suspect large population originally similar to Suspect numbers to make terrestrial planets numbers Culprit: Jupiter Culprit: Jupiter’s gravitational tugs create eccentricity in gravitational orbits and kick out objects that get too close – explains low current total mass explains current Jupiter may have prevented a planet forming Jupiter between it and Mars between Asteroids close up Asteroids Potato like Potato Craters Craters Large chunks missing Large Very low density – lless ess than rock (1 g cm-3) than cf. Water is 1 g cm-3 Asteroids – composition? Asteroids contain materials like those in planets, Iron rich (expect 5-8 g cm-3) planets, and getting rockier (expect 2-3 g cm-3) and as you get further out as How do you get 1 g cm-3 ? Answer: Asteroids full of holes (porous), not rocks but sand piles not You can take chunks out of sand Comets Comets Comets Comets contain volatile material: ices Comets Within 3 AU the Sun melts the ices which sublimate (go directly to gas) and flow off Gas is pushed directly away Gas Dust particles are pushed off by the flow of gas gas We don’t know if it’s a comet for sure We comet until its gets inside 3 AU, some asteroids may be “dead” comets with no ice left Tails Tails Ion tail: Gas is ionized – then then pushed back by radiation and solar wind solar Dust tail: Dust just orbits and tends to lag tends Halley’s Comet Halley Comet Orbits Comet Comets are typically on long elliptical orbits Comets Comets ices run out eventually – we need we fresh comets! fresh Some come from any direction (long periods), Oort Cloud, put there by Jupiter Oort Some come from beyond Neptune (short period), Kuiper Belt, leftovers from Kuiper Belt, Solar system formation Solar Comet Orbits Comet Comets and Meteors Comets Comets leave dust along their orbit along When Earth passes through the orbital ellipse of a comet the dust enters the atmosphere as meteors atmosphere Table 4-3 Some Prominent Meteor Showers Comets, Asteroids and Extinctions Extinctions Comets are massive and move fast, they impact with the power of millions of atom bombs of Tunguska 1908 Barringer Crater Discovery 4-1 What Killed the Dinosaurs? ...
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This note was uploaded on 04/10/2011 for the course ASTRONOMY 1f03 taught by Professor Wadsley during the Spring '11 term at McMaster University.

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