Bring: Density demonstrations - bottles with coins, water, sand [Pluto-Charon on a stick.] eraser on string density bottles spinning table glass dish w/pepper
The Planets at a Glance Small Inner Rocky Planets Misfit Planets Giant Outer Gas Planets
Size, Density, Composition TIFF (Uncompressed) decompressor
Terrestrial (silicate) planets Venus Earth Mars Mercury Moon Io Ganymede Consist mainly of silicates ((Fe,Mg)SiO4) and iron (plus FeS) Mercury is iron-rich, perhaps because it lost its mantle during a giant impact (more on this later) Volatile elements (H2O,CO2 etc.) uncommon in the inner solar system because of the initially hot nebular conditions Some volatiles may have been supplied later by comets Satellites like Ganymede have similar structures but have an ice layer on top (volatiles are more common in the outer
Gas and Water Giants 90% H/He 75% H/He 10% H/He 10% H/He Jupiter and Saturn consist mainly of He/H with a rockice core of ~10 Earth masses Their cores grew fast enough that they captured the nebular gas before it was blown off Uranus and Neptune are primarily ices (CH4,H2O,NH3 etc.) covered with a thick He/H atmosphere Their cores grew more slowly and captured less gas
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Note how they spin! TIFF (Uncompressed) decompressor Orbital Inclinations
Main Belt Asteroids Near Earth Objects TIFF (Uncompressed) decompressor Trojan Asteroids Jupiter
Hubble s Best Pictures Smoothed and modeled images
Kuiper Belt Object Detection Digital cameras and computers make this much, much easier.. GIF decompressor GIF decompressor
http://cfa-www.harvard.edu/iau/lists/tnos.html Uranus Saturn The Kuiper Belt TIFF (Uncompressed) decompressor Jupiter 1,342 as of 1 October 2007 Neptune
2003 UB313 Now calle d Eris Xena TIFF (Uncompressed) decompressor TIFF (Uncompressed) decompressor Images 1.5 hrs apart Oct 21, 2003. TIFF (Uncompressed) decompressor
Eris aka XENA 557 year orbit a=68 AU Diameter ~3000 km TIFF (Uncompressed) decompressor TIFF (Uncompressed) decompressor
TIFF (Uncompressed) decompressor 2003 EL61 & UB313 have moons TIFF (Uncompressed) decompressor
48 Kuiper Belt Objects have moons. Why is the presence of a moon VERY, VERY useful?
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Keeping Neptune fixed and watching paths of other planets for millions of years Pluto now But orbital calculations show that Pluto and Neptune s orbits are in a 3:2 resonance - they dance together - but never get close. Neptune fixed
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Three Swarms 1. Asteroids TIFF (Uncompressed) decompressor 2. Kuiper Belt Objects 3. Oort cloud
TIFF (Uncompressed) decompressor Orbit of Sedna P=10,000 yr a=450 AU
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TIFF (Uncompressed) decompressor Our solar system 1. 2. 3. 4. Patterns of motions 2 types of planets Asteroids and comets Exceptions
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TIFF (Uncompressed) decompressor How does a solar system form from a cloud of gas? TIFF (Uncompressed) decompressor
Formation: Sources of Evidence TIFF (Uncompressed) decompressor TIFF (Uncompressed) decompressor Star-forming regions Chemistry of source material TIFF (Uncompressed) decompressor Our solar system 1. Patterns of motions 2. 2 types of planets 3. Asteroids and comets 4. Exceptions Other solar systems Similarities and differences
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As of Oct 6 2007 186 systems 218 planets http://www.princeton. edu/~willman/planetary_systems/ TIFF (Uncompressed) decompressor
Collapse of the Solar Nebula Formation of the Sun seems a good place to start. Theories of star formation are based on observing millions of stars of different ages. Start with a nebula of gas and dust. Nebula = noun = "cloud" (plural = nebulae) Nebular = adjective = "cloud-like" Section could have been called Collapse of Nebular Solar Nebula.
How Big Was Solar Nebula? ~1% efficiency (guess) start with ~100 Mass of Sun = 1032 kg If Temperature of cloud ~1000K density ~ 10-12 kg m-3 R~2,500 AU If Temperature of cloud ~10K density ~ 10-18 kg m-3 R~250,000 AU We are not addressing the how/why of this - take-home message = initial cloud was LARGE & had low density
What was the composition of solar nebula? WHEN did it collapse?
H2O, NH3, CH4 Water, Ammonia, Methane Hydrogen compounds Ignore inert gases He, Ne, Ar
Elements made in star explosions super novae TIFF (Uncompressed) decompressor
Interstellar Organic Molecules - lots of them! C H O N
Collapse of the Solar Nebula Spins up Forms a disk Heats up
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Where did the angular momentum come from??? Small random motions averaging out to a tiny bulk motion - this bulk motion is then amplified (due to conservation of angular momentum) as the cloud collapses
Why a Disk? TIFF (Uncompressed) decompressor As the cloud collapses (due to gravity) the gases, dust and stuff orbit the central mass On the timescale of an orbit gravity still balances the centrifugal force - the disk is not formed by being flung out into a disk. Nor does gravity pull the material into a disk. These are common misconceptions.
Go with the flow or crash to oblivion - Extreme Conformism! TIFF (LZW) decompressor
TIFF (Uncompressed) decompressor Explains how everything ends up orbiting - and spinningthe same way
TIFF (Uncompressed) decompressor 98% of material - hydrogen & helium - does not condense - anywhere - stay as gases. Inside frostline only refractory materials condense rocks & metals Outside frostline volatiles also condense - WAM AND rocks & metals too.
Refractory = melts/evaporates at higher temperatures, tends to be a solid at reasonable temperatures Volatile = melts/evaporates at lower temperatures, tends to be a gas at reasonable temperatures
An Artist s Impression The young Sun solid planetesimals gas/dust nebula
TIFF (Uncompressed) decompres Accretion of Planetesimals Many smaller objects collected into just a few large ones The bigger get bigger - Oligarchic growth
Hyperion Moon of Saturn Size: 180 x 140 x 112 km Typical planetestimal?? Core material for giant planet? http://ciclops.org/view_event.php?id=37 http://saturn.jpl.nasa.gov
TIFF (Uncompressed) decompres Accretion of Planetesimals Many smaller objects collected into just a few large ones REALLY big planetesimals (~20 Mearth) gravitationally pull in hydrogen - the most abundant gas - and become GIANT.
The Giant Planets Hydrogen envelopes over cores of rock, metals and Water, Ammonia, Methane
Why Only 2 Types of Planets? 1. Cosmic Abundance of Elements - H, O, N, C 2. Temperature Colder Farther from Sun Abundance ices condense beyond frost line Snowballs -> bigger snowballs Giant snowballs have enough gravity to hold H - most abundant element - > giant planets Small amounts of rock & metal-> terrestrial planets Ice dwarfs, comets, asteroids = leftovers
Dust grains Timescale Summary Runaway growth ~Moon-size (planetesimal) ~0.1 Myr Orderly growth ~Mars-size (embryo) ~1 Myr Late-stage accretion (Giant impacts. Gas loss?) ~Earth-size (planet) ~10-100 Myr
Nebula collapse <1 Million Years - fast!! Planetesimal formation < 1MY Jupiter, Saturn <2 MY Terrestrial Planets <4 MY Uranus & Neptune?? Accretion is SLOW in the outer solar system Less material Material orbits the sun slowly Few collisions, slower accretion Too slow for Uranus & Neptune to have formed in their current locations
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3 swarms of small bodies: Asteroid Belt, Kuiper Belt, Oort Cloud of comets
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Q u ickt im e ﾪ a n d a d e co m p re s s o r a re n e e d e d to s e e th is p ic tu re.
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Oldest Meteorite TIFF (Uncompressed) decompressor Allende - fell to Earth near Chihuahua, Mexico at 1:05am on February 8, 1969. Age: 4.5 BY old How do we know it s that old? TIFF (Uncompressed) decompressor
Fission of atomic nucleus + bits Potassium-40 "mother" Argon-40 "daughter" Probability of "splitting up": Expect half the material to decay in 1.25 billion years Half-Life = 1.25 billion years
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How do we date rocks?! Measure the ratio: Mother Isotope Daughter Isotope Potassium - 40 TIFF (Uncompressed) decompressor Argon - 40 Half-Life = 1.25 billion years
Mineral grains (zircon) Uranium/Lead (U/Pb) ratios suggest age ~4.4 billion years sedimentary rocks in west-central Australia. 4.4 Billion Years Old Oldest Earth Rocks TIFF (Uncompressed) decompressor
Oldest Solar System Rocks The oldest dated moon rocks, however, have ages between 4.4-4.5 billion years and provide a minimum age for the formation of Moon Meteorites - therefore Solar System formed - 4.53 and 4.58 billion years ago http://pubs.usgs.gov/gip/geotime/radiometric.html#table 4.54 BY to <1% accuracy
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Where is everything? Note logarithmic scales! V E Me Ma Terrestrial planets J S U N P KB Gas giants Ice giants 1 AU is the mean Sun-Earth distance = 150 million km Nearest star (Proxima Centauri) is 4.2 LY=265,000 AU Me V E Ma Inner solar system 1.5 AU Note log scales!5 A U AU 0 3 Outer solar system
Basic data Sun ρ (days) (1024kg) Radius (km) 24.7 2x106 695950 1.41 Distance (AU) Porbital (yrs) Protation - - Mass (g cm-3) Mercury 0.38 0.2458.6 0.33 2437 5.43 Venus 0.72 0.62243.0R 4.87 6052 5.24 Earth 1.00 1.001.00 5.97 6371 5.52 Mars 1.52 1.881.03 0.64 3390 3.93 Jupiter 5.20 11.860.41 1899 71492 1.33 Saturn 9.57 29.600.44 568 60268 0.68 Uranus 19.19 84.060.72R 86.6 24973 1.32 Neptune 30.07 165.90.67 102.4 24764 1.64 Pluto 39.54 248.66.39R 0.013 1152 2.05 See e.g. Lodders and Fegley, Planetary Scientist s Companion
What does the Solar System consist of? The Sun 99.85% of the mass (78% H, 20% He) Nine Eight Planets Satellites A bunch of other junk (comets, asteroids, Kuiper Belt Objects etc.)
Three kinds of planets... Nebular material can be divided into gas (mainly H/He), ice (CH4,H2O,NH3 etc.) and rock (including metals) Planets tend to be dominated by one of these three end-members Proportions of gas/ice/rock are roughly 100/1/0.1 The compounds which actually Gas-rich condense will depend on the local nebular conditions (temperature) Rock-rich E.g. volatile species will only be stable beyond a snow line. This is why the inner planets are Ice-rich rock-rich and the outer planets gas- and ice-rich
Solar System Formation - Overview Some event (e.g. nearby supernova) triggers gravitational collapse of a cloud (nebula) of dust and gas As the nebula collapses, it forms a spinning disk (due to conservation of angular momentum) The collapse releases gravitational energy, which heats the centre; this central hot portion forms a star The outer, cooler particles suffer repeated collisions, building planet-sized bodies from dust grains (accretion) Young stellar activity (T-Tauri phase) blows off any remaining gas and leaves an embryonic solar system These argument suggest that the planets and the Sun should all have (more or less) the same composition Comets and meteorites are important because they are relatively pristine remnants of the original nebula
Complications 1) Timing of gas loss Presence of gas tends to cause planets to spiral inwards, hence timing of gas loss is important Since outer planets can accrete gas if they get large enough, the relative timescales of planetary growth and gas loss are also important 2) Jupiter formation Jupiter is so massive that it significantly perturbs the nearby area e.g. it scattered so much material from the asteroid belt that a planet never formed there Jupiter scattering is the major source of the most distant bodies in the solar system (Oort cloud) It must have formed early, while the nebular gas was still present. How?
Observations (1) Early stages of solar system formation can be imaged directly dust disks have large surface area, radiate effectively in the IR Unfortunately, once planets form, the IR signal disappears, so until very recently we couldn t detect planets (~150 so far) Timescale of clearing of nebula (~1-10 Myr) is known because young stellar ages are easy to determine from mass/luminosity relationship. Thick disk This is a Hubble image of a young solar system. You can see the vertical green plasma jet which is guided by the star s magnetic field. The white zones are gas and dust, being illuminated from inside by the young star. The dark central zone is where the dust is so optically thick that the light is not being transmitted.
Observations (2) We can use the present-day observed planetary masses and compositions to reconstruct how much mass was there initially the minimum mass solar nebula This gives us a constraint on the initial nebula conditions e.g. how rapidly did its density fall off with distance? The picture gets more complicated if the planets have moved.. The observed change in planetary compositions with distance gives us another clue silicates and iron close to the Sun, volatile elements more common further out
Cartoon of Nebular Processes Disk cools by radiation Polar jets Dust grains Infalling material Nebula disk (dust/gas) Cold, Hot, low ρ high ρ Stellar magnetic field (sweeps innermost disk clear, reduces stellar spin rate) Scale height increases radially (why?) Temperatures decrease radially consequence of lower irradiation, and lower surface density and optical depth leading to more efficient cooling
What is the nebular composition? Why do we care? It will control what the planets are made of! How do we know? Composition of the Sun (photosphere) Primitive meteorites (see below) (Remote sensing of other solar systems - not yet very useful) An important result is that the solar photosphere and the primitive meteorites give very similar answers: this gives us confidence that our estimates of nebular composition are correct
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Kepler s 3 Laws of Planetary Motion 1. Planets move on elliptical orbits with the Sun at one focus 2. Planets move faster when closer to the Sun, slower when farther from the Sun 3. (Orbital Period)2 = (Semi-major axis)3 = P2 a3 Years A.U.
Sequence of events 1. Nebular disk formation 2. Initial coagulation (~10km, ~104 yrs) 3. Runaway growth (to Moon size, ~105 yrs) 4. Orderly growth (to Mars size, ~106 yrs), gas loss (?) 5. Late-stage collisions (~107-8 yrs)
Debris Swarm 1 TIFF (Uncompressed) decompressor The Asteroid Belt 100s of thousands of asteroids 100s Near Earth Objects 100s Trojans ± 60 of Jupiter http://cfa-www.harvard.edu/iau/animations/animations.html
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Note: distance scale is in factors of 10! TIFF (Uncompressed) decompressor Sizes of objects not to scale
Leftovers: Three Kinds of Small Bodies 1. Jupiter s gravity stirs up rocky planetesimals between Mars & Jupiter: the Asteroid Belt 2. Jovian planets stir up orbits of icy planetesimals in their vicinity, flings em out into the Oort Cloud. 3. Icy planetesimals slowly form from nebula outside Neptune s orbit: The Kuiper Belt 4. All these objects rain down on the planets: making impact craters & bringing good stuff