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Today ● ● Today: – Review of Parts 4 & 5 of the text (Weeks 8-12) – Cover the last of the material Next week – Assignment covering Weeks 8-13 due – Projects also due next week – Class summary – Any student presentations Review: Weeks 8-12 ● History of our Planetary system ● Planets Other than Our Own in our Solar System ● Habitability of places other than Earth ● Finding Planets outside of the Solar System ● Visiting or Communicating? Our Solar System ● Orbits and Gravity ● Planetary System Formation Orbits ● ● ● ● ● Planets are falling towards Sun due to gravitational acceleration Moving toward the side fast enough that they miss Moving too fast – escape entirely, leave Sun Move too slowly – fall into Sun Same with satellites circling Earth, or Sun orbiting in our galaxy, or... Gravity ● ● ● ● ● Gravity acts between all massive objects Gravitational force is equal on both objects If orbiting, both objects move, not just one, since both are being acted on by gravity Both orbit the center of mass of the system Equal mass objects; center of mass is at the center of the two objects Gravity ● ● ● ● ● If one body is more massive, then gravitational force is increased Center of mass tilts towards more massive body Forces still equal Equal force on lighter body moves it more than the same force on the heavier body Lighter object moves larger distance than heavier object Gravity ● ● Force of gravity also increases as objects get nearer Inverse Square Law (same as light) Orbits ● Kepler's Laws: (EMPERICAL) – Planets travel in ellipses, with sun at one focus of ellipse – Area swept out by radius is equal over any equal amount of time – Square of the planet's period (the `year' for that planet) proportional to the distance to the sun cubed. – P2 ~ a3 Planets ● ● ● ● ● Almost all planets are in same plane All planets (except Uranus) rotate more or less in the same plane, as does Sun Very suggestive of the idea that planets, Sun formed from a disk, as we discussed before Suggested by Laplace in 1600s. Disk near star is depleted in Hydrogen, Helium by evaporation Planet Formation ● ● ● ● As disk cools, gas/dust disk can begin condensing Grains form, which themselves agglomerate to larger particles Regions where disk is originally dense condense faster, gravitationally attract more material Process of continued agglomeration can form planets Instability ● ● ● Some processes are naturally stable – Burning in main sequence stars – Core heats up – outer layers puff up – core cools down – Automatically stabilizes itself – Ball in a right-side-up bowl Once there's a region of high density in a gas cloud or disk, increase in gravitational attraction to that region... Unstable – Ball on an up-side-down bowl Planet Formation ● ● ● ● Proto-planetary-core starts sweeping out material and planetesimals at its radius Accrete material streams in from just outside or inside its radius There is a limit to this process; if there are planets forming on either side, eventually the gaps collide – no more new material This process of slowly sweeping up and accreting material can take millions of years Mystery: `Hot Jupiters' ● ● A Jupiter couldn't form at 1AU; evaporation would prevent such a gas giant from forming Many of the extra-solar planets observed are gas giants at distances ~ 1AU ● What happened? ● Two possibilities: – Migration – Different formation mechanism Planet Formation ● ● ● Migration is possible As planets form and accrete material, they experience a drag force Drag takes energy from planets motion and they fall inwards Planet Formation ● ● ● Fast formation is also possible In sufficiently massive disk, instabilities can occur much faster, and on larger scales Can happen quickly enough that perhaps giants can form near star Our Solar System ● Other Bodies – Mercury – The Moon – Venus – Mars – Gas Giants – Gas Giant Moons The Moon ● No atmosphere ● No geological activity ● No water ● -> no erosion ● Can provide information about formation of solar system that is absent from Earth Mercury ● Similar to moon ● Similar size ● Small, empty, simple ● Very close to Sun ● No atmosphere to mediate temperature swings: – +750o F in sun – -230o F in shade Moon's Cratering ● ● ● Nothing to alter surface Complete history of cratering in Moon's history From predicted cratering rate, one expects that crust of moon formed very quickly in solar system history Possible Moon Formation Scenario Possible Moon Formation Scenario ● Explains similar Oxygen abundances – ● Very different from meteorites Explains fewer volatiles – If Earth's iron core had already settled, impact would have dislodged crust material – Heat of impact would have vaporized volatiles Venus ● ● ● ● Closest to Earth ¾ as far away from Sun as Earth is Very similar to Earth's size, density Covered by thick, opaque clouds Venus ● Runaway greenhouse effect ● Hot: very near sun ● Water begins to evaporate ● ● Water vapor is a greenhouse gas! Surface gets hotter, more water evaporation ● Surface is hundreds of degrees ● No liquid water Mars ● ● Red planet between Earth and Asteroid Belt Half again as far away from Sun as the Earth is – Expect it to be ~100o F colder than Earth on average – Average too cool for water – Peak temps ~ 70o F (but -130 at night!) Mars ● Near asteroid belt – ● Large impacts can blow off significant rocky material – ● Likely more collisions than Earth Meteorites As well as gases (atmosphere) Mars ● ~1/2 radius of Earth ● ~1/10 mass ● ~40% surface gravity – Force of a 1 lb weight less than ½ lb on Mars – Less gravity holding the atmosphere in place Mars ● ● Too little gravity to be able to hold onto a significant atmosphere Atmospheric pressure less than 1% of Earth's Evaporation ● What causes evaporation of liquid, and what prevents it? Evaporation ● ● ● ● What causes evaporation of liquid, and what prevents it? Fastest moving water (say) molecules can escape into atmosphere Water molecules in atmosphere can collide into water and become part of the liquid Balance is reached when evaporating water = condensing water Evaporation ● Can change balance: – Little water in atmosphere, evaporation happens faster ● – (Why feel so sticky on a humid day) If air pressure is very low, evaporated water molecules can move very far away from pool of water ● Fewer around to condense ● Faster evaporation Evaporation Boiling Point at Alt it ude ● 220 Boiling Pt of Wat er (F) 210 200 190 ● 180 170 160 150 ● 140 130 120 110 100 0 2000 5000 Alt it ude (ft ) 7500 10000 Effect of atmospheric pressure happens on our own planet Reason for `high-altitude cooking instructions' on some boxes Higher altitude -> lower air pressure -> evaporation is easier > lower boiling point Evaporation ● Martian atmospheric pressure < 1% of Earth's – ● ● (Earth's atmosphere at 15 miles / 80,000 ft) Water boiling point is so low that any liquid water evaporates immediately No free water possible on surface Evaporation ● But water ice DOES exist on Mars: – Polar ice caps ● ● – Mostly (on top) dry ice (frozen CO2) Underneath, visible when CO2 has sublimated, water ice Quite likely some trapped under surface: `permafrost' The Giants ● ● The Giants are sometimes all called `Jovian' planets after Jupiter After more exploration showed their diversity, this term lost favour The Giants ● ● ● ● The giant planets can be weighed very accurately by measuring the speed of their moons. Much heavier than Earth, but not so heavy considering their size Densities 600 – 1600 kg/m3, compared with Earth's 5700 kg/m3 Mostly made of gas/liquids? The Birth of Giants ● ● ● ● In outer solar system, cooler Less evaporative stripping of volatile gases If sufficiently massive cores form, can keep even volatile gases These gases will be representative of the very early solar system The Birth of Giants ● ● ● ● Since early solar system is largely composed of Hydrogen, so will gas giants Rocky or Icy or Slushy core High-hydrogen atmosphere has some similarities to atmosphere in Miller-Urey experiment Can form lots of organics The Birth of Giants ● ● ● ● ● Jupiter in Infra-red Large mass -> high pressure, temperature at centre Temperature at centre of Jupiter ~ 4 times surface of Sun! Collapse from origin of planet still slowly continuing Releases heat energy These planets have a source of heat The Birth of Giants ● ● ● ● ● Jupiter in Infra-red Gas giants emit more heat than they absorb from Sun At earlier times, would have been much hotter Moons, which are nearby, heated by their nearby planet Many of these moons are large (planet-sized) Moons might be interesting for life? The Moons of Giants ● ● ● ● Planets large enough that many moons were also formed Many of them planet sized in their own right Get heat from planet Some (Io/Jupiter) effected by planets magnetic field ● Atmosphere? (Titan, Saturn) ● Water? (Europa, Jupiter) The Moons of Giants ● Formation: like planets around sun ● Rotating body, disk forms ● Moons generally along plane of rotation of planet Gas Giants ● Convection is a fundamental process – ● Happens everywhere Fluid heated at bottom rises, cools, falls back down ● Gas giants have hot centres ● Large-scale motions ● Mix material Gas Giants ● ● ● ● Makes it difficult to imagine life forming No real surface to live on Chemicals constantly being mixed around No originally contained environment (`protocell') Moons ● ● ● Gas giants have planet-sized moons At least one (Titan) has a significant atmosphere Another (Europa) very likely has liquid salty water under a layer of ice Europa ● ● Very suggestive it has a liquid underneath – No cratering – Many fractures, ridges on surface What would this mean for life? – If some source of energy on inside (geothermal, chemical), very real possibility of some sort of life Titan ● ● Very Cold Massive, Cold enough to have an atmosphere (1.5 x as dense as ours!) ● No oxygen ● No liquid water ● Hydrogen rich ● Interesting organic chemistry ● Lakes of hydrocarbons? ● Huygens probe 2005 How Unique is Earth? ● What is special about Earth? ● How important/rare are those things? ● How many such planets are there likely to be? Earth ● Atmosphere – Large surface gravity ● Reasonable temperature ● Rocky surface ● Large moon ● Lots of heavy elements How Important/Rare are these? ● Heavy elements; – Likely ubiquitous in planets around Pop I stars How Important/Rare are these? ● Rocky Surface – Can happen if there is heavy elements (see above) – Probably true of all planets close enough to have liquid water – (But planet migration) How Important/Rare are these? ● Atmosphere – Requires not too close to sun – Requires massive enough planet How Important/Rare are these? ● Reasonable Temperature – `Goldilocks zone’ – Needs to be right distance to star How Important/Rare are these? ● So we require – Rocky Planet – Of the right mass – At the right distance from the star Habitable Zone ● ● ● Corresponds to further than Venus to about Mars distance for our Sun Using inverse-square law, could calculate for other stars Main requirement: liquid water in the presence of an atmosphere. Habitable Zone: Binary Stars ● ● ● ● ● About half of all stars are in binary systems Stars orbit a common centre of mass (more on that next week) Can planets have reasonable orbits in such systems? Yes, but must orbit one star or be far away from both; `Figure 8’ orbits aren’t stable Finding Other Planets ● ● Light from planet – Reflected visible light – Reflected+generated infrared Dark from planet – ● Light bent by planet – ● Transits (shadows from planets) Gravitational Lensing Star's Motion from planet – Proper Motions – Doppler Shift Light from the planet ● ● ● Small brown dwarf (not planet) companion to a star directly imaged Stars observed by emitting their own light Planets don't emit light, but do reflect sunlight Problem: reflect a billionth or less of the light from the companion star Light from the planet ● ● Has yet to be observed What sort of planets/systems does this work best for? Light from the planet ● Small brown dwarf (not planet) companion to a star directly imaged Would work best for: – Large planets (more reflecting surface) – Reflective planets (ammonia clouds?) – Near enough star to reflect lots of light – Far enough not to be overwhelmed by light from star Light from the planet ● ● Small brown dwarf (not planet) companion to a star directly imaged Large planets near star: `Hot Jupiters' Gas giants (presumably) very near star Light from the planet ● ● ● ● ● How observed? Very careful imaging of nearby stars Probably with telescopes above atmosphere (Hubble) As long as planet isn't in front of/behind star, will be reflecting light towards Earth Just a question of being able to observe it Light from the planet ● ● ● Small brown dwarf (not planet) companion to a star directly imaged This is actually an infrared image Jupiter-type planets may emit their own infrared light Terrestrial planets reflect a lot of infrared ● Star emits most of its light in visible ● Better chance in IR Planetary Transits/Occultations ● ● Brightness ● Time Light from planet can be blocked by orbiting planet Careful measurement of total light from star can show this Can't see directly; the star is just a point Planetary Transits/Occultations ● Brightness ? Time If period is measured (multiple transits) and mass estimate for star exists, have: – Planet's distance – Planet's size – Planet's orbital period – Star's size Planetary Transits/Occultations ● How are these observed? ● Fairly rare events: – Has to be exactly along line of sight ● ● ● Only planetary systems aligned along line of sight Planet directly in front of star only very briefly (Jupiter: ~1 day / 11 yrs) Fairly careful measurements must be made – Jupiter: 1% decrease in Sun's brightness Planetary Transits/Occultations ● Large survey – Dedicated telescope – Look at large fraction of sky every night (or nearly) Planetary Transits/Occultations ● ● Works best for: – Large planets (blocks more of star) – Planets near star (shorter period – easier to observe) – Hot Jupiters Has been used to find planets Gravitational lensing ● ● ● A very powerful technique to measure dim objects Used in searches for brown dwarfs or other large clumps of `dark matter' Requires – distant, bright, source star, – very accurate measurements of the brightness of the source star over time Gravitational lensing ● ● ● At least one planet has been `seen' this way Results: – Mass of planet, star – Distance to star – Distance planet <-> star Difficult, because only get one chance at measuring system Gravitational lensing ● Works best for what systems? – Dim Stars – Massive planets – (relatively) insensitive to distance between star and planet – Jupiters at any radii / temperature Astrometry: Proper Motions ● Stars motion towards/away from us can be measured very accurately – ● Doppler Shift Motions `side-to-side' on the sky take VERY long time to make noticeable changes Astrometry: Proper Motions ● If star has a large enough proper motion – ● (probably means very near us) Wobble in the star's motion could indicate that the star is being tugged on by a nearby planet Astrometry: Proper Motions ● Has been successfully used to detect white-dwarf companions ● Shown below: Sirius ● No successful measurement of planets however Astrometry: Proper Motions ● Would work best for? Astrometry: Proper Motions ● Would work best for? – Nearby stars – Large mass companion – Distant from planet: can pull further distance – Near planet: faster orbit, more visible wobble Doppler Shifting ● ● ● Star has slight motion in orbit If that motion is largely towards/away from us, might be detected by Doppler shift Motions towards/away can be very accurately measured (few meters/sec) Doppler Shifting ● ● ● ● ● Has so far been extremely successful If can watch for several periods, can get very accurate period measurements Sine wave: circular orbit `Tilted' sine wave: elliptical orbit Get: period, total velocity induced by planet Doppler Shifting ● Works best for: Doppler Shifting ● Works best for: – Large planets – Close in: ● Faster period (easier to detect) Interstellar Travel, Interstellar Communication ● ● Interstellar Travel – Rockets – Fuel – Speeds – Time Dilation Interstellar Communication – What frequencies do we use? – Meaningful signals – SETI@home Rockets ● Net Force -> acceleration Gravitational Force Force exerted by exhaust ● ● ● Have to exert force to overcome that of gravity Reactions from some sort of fuel – Chemical – Electrical... Propel exhaust downwards By Newton's 3rd law, propel rocket upwards ● ● ● Easy to accelerate upwards Hard to keep from falling back down! Can either: – Net Accelerate very quickly to escape vel (25,000 mph) and coast up ● Grav Force exhaust – ● Gravity will keep decelerating you but never quite pull you back Or accelerate slowly through ascent Luckily, further up you get, weaker force from Earth's gravity becomes Rockets: Fuel ● ● ● ● Takes a lot of fuel to move something into Earth's orbit or further Would take about as much fuel to launch me into orbit as it takes to heat a Chicago home through an entire winter Unlike a car trip, fuel starts weighing a lot, even compared to rocket Shuttle launch: – Empty Shuttle: 230,000 lb – Fuel : 2,700,000 lb Fuel along the way? ● ● ● ● ● Interstellar medium VERY tenuous Sprinkled with hydrogen Could it be collected and then burned (nuclear fusion?) Hard to see how – Drag on ship – Power to magnetic fields But would solve enormous fuel problem Special Relativity ● Einstein: – Physics is the same in all inertial frames of reference – Speed of light in a vacuum is a fundamental physical constant of the Universe Special Relativity ● ● ● ● But for higher velocities, can be significant! Astronaut goes to Alpha Centauri and back at 95% of speed of light Astronaut ages 3 years, people back home 9 At closer and closer to speed of light, effect gets bigger and bigger. Special Relativity ● ● ● ● Speed of light becomes moving target Astronaut can put more and more energy into traveling faster But because can never pass light (light must always travel at same velocity!) can never pass speed of light Takes infinite amount of energy to even get to speed of light Automated Probes? ● High-tech Voyagers or Pioneers ● Aim towards nearby stars ● Enough fuel to accelerate ● Enough smarts to navigate toward system ● Get solar power once near star ● Send message – To nearby planets – To us Travel Difficult ● Communication much simpler than Transportation. Messages ● ● Its a lot easier sending signals than things Messages – Have no mass – Don't require fuel – Don't require food/provisions for long journey – Cheap to produce – Travel at speed of light What frequencies to use? ● ● ● Two choices for long-distance forces: – Gravity (difficult) – Electromagnetic But there's an essentially infinite range of frequencies to examine Radio waves: – Easy/cheap to generate, focus SETI@home ● ● ● ● ● Several different SETI listening experiment One is called `Project SERENDIP' `Listen in' on other astronomical uses of the Arecibo radio telescope in Puerto Rico Can't choose where the observers are looking, but can listen (nearly) 24x7 Receiver installed which listens to 168 million narrow channels near 21cm Hydrogen line SETI@home ● Done as part of screen saver on thousands of volunteer's computers Results ● ● ● ● Several candidate signals discovered 2500 persistent gaussians (longish spikes seen at least twice) Need to be checked to make sure not interference/noise Also searching data for persistent spikes, pulses, triplets... Has the Search Happened Already? ● UFO sightings ● What Evidence is Necessary? ● If no UFOs yet, why not? UFO Sightings ● ● No shortage of UFO observation stories, photos A moment spent with google provides thousands of ernest, probably mostly honest web pages describing – UFO sightings – Abductions What Evidence is Required? ● ● Large amount of documentary evidence that the Universe has apparently searched for life here Why not accept this as truth? Extrordinary Claims require Extrordinary Evidence ● ● Let me make two claims – This morning, violence broke out in an up-til-now quiet region of Iraq, in the southern town of Rajaf. Four US soldiers were killed. – With great effort, I can fly short distances (10-20 ft) using the power of my mind. Which (if either) do you believe? Extrordinary Claims require Extrordinary Evidence ● ● ● You have exactly the same evidence for both claims: my say-so. Clearly, the Iraq claim has more serious immediate consequences (death, future violence) Why is the same evidence more likely to be sufficient in one case (the more serious, even) than in the other? What Evidence is Required? ● Photographs are easily misinterpreted ● Photographs also easily faked ● These: Robert Schaefer What Evidence is Required? ● ● ● Eyewitness evidence notoriously unreliable Human brain very good at seeing patterns, filling in blanks Too good, in fact, to be good at mundanely reciting uninterpreted observations Observation Test ● ● ● Quantitative test Count basketball passes by one team (dressed in white) in a complicated, dynamic scene http://viscog.beckman.uiuc.edu/grafs/demos/15.html Same lab: `change blindness' ● http://viscog.beckman.uiuc.edu/grafs/demos/10.html Post-event Suggestibility ● Elizabeth Loftus: – Film shown of car accident – Questionaire after film – Followup questionaire afterwards – Leading questions, misinformation in questions could cause people to misremember event afterwards ● ● ● Wrong color of car `Remembering' stop signs, buildings that weren't there ... What Evidence is Required? ● ● ● This doesn't mean that all the evidence is proven wrong/mistaken Not enough evidence to be convincing What would be convincing evidence? What Evidence is Required? ● ● ● This doesn't mean that all the evidence is proven wrong/mistaken Not enough evidence to be convincing What would be convincing evidence? – Chunk of spacecraft material/technology – Cheek swab from alien – ... Fermi's Paradox ● No signals from aliens yet. ● No visitors yet either, perhaps. ● Why not? Fermi's Paradox ● ● ● ● ● Even if 1,000,000 civilizations in our galaxy today, that's one per ~300,000 stars Would have to explore by chance to find Earth Radio signals identifying Earth are very new: 1960s or so Even if travel speed of light, on has been time for 20ly round trip: Only a handful of stars that close Next week ● Assignment covering Weeks 8-13 due ● Projects also due next week ● Class summary ● Any student presentations