Today ● Today: – Review of Parts 4 & 5 of the text (Weeks 8-12) – Cover the last of the...

TodayToday

● Today:Today:

– Review of Parts 4 & 5 of the text (Weeks 8-12)Review of Parts 4 & 5 of the text (Weeks 8-12)– Cover the last of the materialCover the last of the material

● Next weekNext week

– Assignment covering Weeks 8-13 due Assignment covering Weeks 8-13 due – Projects also due next weekProjects also due next week– Class summary Class summary – Any student presentationsAny student presentations

Review: Weeks 8-12Review: Weeks 8-12

● History of our Planetary systemHistory of our Planetary system● Planets Other than Our Own in our Solar SystemPlanets Other than Our Own in our Solar System● Habitability of places other than EarthHabitability of places other than Earth● Finding Planets outside of the Solar SystemFinding Planets outside of the Solar System● Visiting or Communicating?Visiting or Communicating?

Our Solar SystemOur Solar System

● Orbits and GravityOrbits and Gravity● Planetary System FormationPlanetary System Formation

OrbitsOrbits

● Planets are falling towards Sun Planets are falling towards Sun due to gravitational accelerationdue to gravitational acceleration

● Moving toward the side fast Moving toward the side fast enough that they missenough that they miss

● Moving too fast – escape entirely, Moving too fast – escape entirely, leave Sunleave Sun

● Move too slowly – fall into SunMove too slowly – fall into Sun● Same with satellites circling Earth, Same with satellites circling Earth,

or Sun orbiting in our galaxy, or...or Sun orbiting in our galaxy, or...

GravityGravity

● Gravity acts between all massive Gravity acts between all massive objectsobjects

● Gravitational force is equal on Gravitational force is equal on both objectsboth objects

● If orbiting, both objects move, not If orbiting, both objects move, not just one, since both are being just one, since both are being acted on by gravityacted on by gravity

● Both orbit the center of mass of Both orbit the center of mass of the systemthe system

● Equal mass objects; center of mass Equal mass objects; center of mass is at the center of the two objectsis at the center of the two objects

GravityGravity

● If one body is more massive, If one body is more massive, then gravitational force is then gravitational force is increasedincreased

● Center of mass tilts towards Center of mass tilts towards more massive bodymore massive body

● Forces still equalForces still equal● Equal force on lighter body Equal force on lighter body

moves it more than the same moves it more than the same force on the heavier bodyforce on the heavier body

● Lighter object moves larger Lighter object moves larger distance than heavier objectdistance than heavier object

GravityGravity

● Force of gravity also increases Force of gravity also increases as objects get neareras objects get nearer

● Inverse Square Law (same as Inverse Square Law (same as light)light)

OrbitsOrbits

● Kepler's Laws: (EMPERICAL)Kepler's Laws: (EMPERICAL)

– Planets travel in ellipses, with Planets travel in ellipses, with sun at one focus of ellipsesun at one focus of ellipse

– Area swept out by radius is Area swept out by radius is equal over any equal amount equal over any equal amount of timeof time

– Square of the planet's period Square of the planet's period (the `year' for that planet) (the `year' for that planet) proportional to the distance to proportional to the distance to the sun cubed.the sun cubed.

– PP22 ~ a ~ a33

PlanetsPlanets● Almost all planets are in same Almost all planets are in same

planeplane● All planets (except Uranus) All planets (except Uranus)

rotate more or less in the same rotate more or less in the same plane, as does Sunplane, as does Sun

● Very suggestive of the idea that Very suggestive of the idea that planets, Sun formed from a planets, Sun formed from a disk, as we discussed beforedisk, as we discussed before

● Suggested by Laplace in 1600s.Suggested by Laplace in 1600s.● Disk near star is depleted in Disk near star is depleted in

Hydrogen, Helium by Hydrogen, Helium by evaporationevaporation

Planet FormationPlanet Formation● As disk cools, gas/dust disk can begin As disk cools, gas/dust disk can begin

condensingcondensing● Grains form, which themselves Grains form, which themselves

agglomerate to larger particlesagglomerate to larger particles● Regions where disk is originally dense Regions where disk is originally dense

condense faster, gravitationally attract condense faster, gravitationally attract more materialmore material

● Process of continued agglomeration can Process of continued agglomeration can form planetsform planets

InstabilityInstability● Some processes are naturally stableSome processes are naturally stable

– Burning in main sequence starsBurning in main sequence stars– Core heats up – outer layers puff Core heats up – outer layers puff

up – core cools downup – core cools down– Automatically stabilizes itselfAutomatically stabilizes itself– Ball in a right-side-up bowlBall in a right-side-up bowl

● Once there's a region of high density in Once there's a region of high density in a gas cloud or disk, increase in a gas cloud or disk, increase in gravitational attraction to that region...gravitational attraction to that region...

● UnstableUnstable

– Ball on an up-side-down bowlBall on an up-side-down bowl

Planet FormationPlanet Formation

● Proto-planetary-core starts Proto-planetary-core starts sweeping out material and sweeping out material and planetesimals at its radiusplanetesimals at its radius

● Accrete material streams in from Accrete material streams in from just outside or inside its radiusjust outside or inside its radius

● There is a limit to this process; if There is a limit to this process; if there are planets forming on either there are planets forming on either side, eventually the gaps collide – side, eventually the gaps collide – no more new materialno more new material

● This process of slowly sweeping This process of slowly sweeping up and accreting material can take up and accreting material can take millions of yearsmillions of years

Mystery: `Hot Jupiters'Mystery: `Hot Jupiters'

● A Jupiter couldn't form at 1AU; A Jupiter couldn't form at 1AU; evaporation would prevent such a evaporation would prevent such a gas giant from forminggas giant from forming

● Many of the extra-solar planets Many of the extra-solar planets observed are gas giants at observed are gas giants at distances ~ 1AUdistances ~ 1AU

● What happened?What happened?● Two possibilities:Two possibilities:

– MigrationMigration– Different formation Different formation

mechanismmechanism


● Migration is possibleMigration is possible● As planets form and As planets form and

accrete material, they accrete material, they experience a drag forceexperience a drag force

● Drag takes energy from Drag takes energy from planets motion and they planets motion and they fall inwardsfall inwards


● Fast formation is also Fast formation is also possiblepossible

● In sufficiently massive disk, In sufficiently massive disk, instabilities can occur much instabilities can occur much faster, and on larger scalesfaster, and on larger scales

● Can happen quickly enough Can happen quickly enough that perhaps giants can form that perhaps giants can form near starnear star

Our Solar SystemOur Solar System

● Other BodiesOther Bodies

– MercuryMercury– The MoonThe Moon– VenusVenus– MarsMars– Gas GiantsGas Giants– Gas Giant MoonsGas Giant Moons

The MoonThe Moon

● No atmosphereNo atmosphere● No geological activityNo geological activity● No waterNo water● -> no erosion-> no erosion● Can provide information Can provide information

about formation of solar about formation of solar system that is absent from system that is absent from EarthEarth

MercuryMercury

● Similar to moonSimilar to moon● Similar sizeSimilar size● Small, empty, simpleSmall, empty, simple● Very close to SunVery close to Sun● No atmosphere to mediate No atmosphere to mediate

temperature swings:temperature swings:

– +750+750oo F in sun F in sun– -230-230o o F in shadeF in shade

Moon's CrateringMoon's Cratering

● Nothing to alter surfaceNothing to alter surface● Complete history of Complete history of

cratering in Moon's historycratering in Moon's history● From predicted cratering From predicted cratering

rate, one expects that crust of rate, one expects that crust of moon formed very quickly moon formed very quickly in solar system historyin solar system history

Possible Moon Formation ScenarioPossible Moon Formation Scenario

Possible Moon Formation ScenarioPossible Moon Formation Scenario

● Explains similar Oxygen abundancesExplains similar Oxygen abundances

– Very different from meteoritesVery different from meteorites● Explains fewer volatilesExplains fewer volatiles

– If Earth's iron core had already settled, If Earth's iron core had already settled, impact would have dislodged crust materialimpact would have dislodged crust material

– Heat of impact would have vaporized Heat of impact would have vaporized volatiles volatiles

VenusVenus

● Closest to EarthClosest to Earth● ¾ as far away from Sun as ¾ as far away from Sun as

Earth isEarth is● Very similar to Earth's size, Very similar to Earth's size,

densitydensity● Covered by thick, opaque Covered by thick, opaque

cloudsclouds

VenusVenus

● Runaway greenhouse effectRunaway greenhouse effect● Hot: very near sunHot: very near sun● Water begins to evaporateWater begins to evaporate● Water vapor is a greenhouse Water vapor is a greenhouse

gas!gas!● Surface gets hotter, more water Surface gets hotter, more water

evaporationevaporation● Surface is hundreds of degreesSurface is hundreds of degrees● No liquid waterNo liquid water

MarsMars

● Red planet between Earth and Red planet between Earth and Asteroid BeltAsteroid Belt

● Half again as far away from Sun Half again as far away from Sun as the Earth isas the Earth is

– Expect it to be ~100Expect it to be ~100oo F F colder than Earth on averagecolder than Earth on average

– Average too cool for waterAverage too cool for water– Peak temps ~ 70Peak temps ~ 70o o F (but -130 F (but -130

at night!)at night!)

MarsMars

● Near asteroid beltNear asteroid belt

– Likely more collisions than Likely more collisions than EarthEarth

● Large impacts can blow off Large impacts can blow off significant rocky material significant rocky material

– MeteoritesMeteorites● As well as gases (atmosphere)As well as gases (atmosphere)

MarsMars

● ~1/2 radius of Earth~1/2 radius of Earth● ~1/10 mass~1/10 mass● ~40% surface gravity~40% surface gravity

– Force of a 1 lb weight less Force of a 1 lb weight less than ½ lb on Marsthan ½ lb on Mars

– Less gravity holding the Less gravity holding the atmosphere in placeatmosphere in place

MarsMars

● Too little gravity to be able to hold Too little gravity to be able to hold onto a significant atmosphereonto a significant atmosphere

● Atmospheric pressure less than Atmospheric pressure less than 1% of Earth's1% of Earth's

EvaporationEvaporation

● What causes evaporation of liquid, What causes evaporation of liquid, and what prevents it?and what prevents it?


● What causes evaporation of liquid, What causes evaporation of liquid, and what prevents it?and what prevents it?

● Fastest moving water (say) Fastest moving water (say) molecules can escape into molecules can escape into atmosphereatmosphere

● Water molecules in atmosphere Water molecules in atmosphere can collide into water and become can collide into water and become part of the liquidpart of the liquid

● Balance is reached when Balance is reached when evaporating water = condensing evaporating water = condensing waterwater


● Can change balance:Can change balance:

– Little water in atmosphere, Little water in atmosphere, evaporation happens fasterevaporation happens faster

● (Why feel so sticky on a (Why feel so sticky on a humid day)humid day)

– If air pressure is very low, If air pressure is very low, evaporated water molecules evaporated water molecules can move very far away from can move very far away from pool of waterpool of water

● Fewer around to condenseFewer around to condense● Faster evaporationFaster evaporation


● Effect of atmospheric pressure Effect of atmospheric pressure happens on our own planethappens on our own planet

● Reason for `high-altitude cooking Reason for `high-altitude cooking instructions' on some boxesinstructions' on some boxes

● Higher altitude -> lower air Higher altitude -> lower air pressure -> evaporation is easier -pressure -> evaporation is easier -> lower boiling point> lower boiling point

0 2000 5000 7500 10000100

120

140

160

180

200

220

Boiling Point at Altitude

Altitude (ft)

Boilin

g P

t of

Wate

r (F

)


● Martian atmospheric pressure < Martian atmospheric pressure < 1% of Earth's1% of Earth's

– (Earth's atmosphere at 15 (Earth's atmosphere at 15 miles / 80,000 ft)miles / 80,000 ft)

● Water boiling point is so low that Water boiling point is so low that any liquid water evaporates any liquid water evaporates immediatelyimmediately

● No free water possible on surfaceNo free water possible on surface


● But water ice DOES exist on But water ice DOES exist on Mars:Mars:

– Polar ice capsPolar ice caps● Mostly (on top) dry ice Mostly (on top) dry ice

(frozen CO2)(frozen CO2)● Underneath, visible when Underneath, visible when

CO2 has sublimated, water iceCO2 has sublimated, water ice

– Quite likely some trapped Quite likely some trapped under surface: `permafrost'under surface: `permafrost'

The GiantsThe Giants

● The Giants are sometimes all The Giants are sometimes all called `Jovian' planets after called `Jovian' planets after JupiterJupiter

● After more exploration After more exploration showed their diversity, this showed their diversity, this term lost favourterm lost favour

The GiantsThe Giants

● The giant planets can be The giant planets can be weighed very accurately by weighed very accurately by measuring the speed of their measuring the speed of their moons.moons.

● Much heavier than Earth, but Much heavier than Earth, but not so heavy considering their not so heavy considering their sizesize

● Densities 600 – 1600 kg/mDensities 600 – 1600 kg/m33, , compared with Earth's 5700 compared with Earth's 5700 kg/mkg/m33

● Mostly made of gas/liquids?Mostly made of gas/liquids?

The Birth of GiantsThe Birth of Giants

● In outer solar system, coolerIn outer solar system, cooler● Less evaporative stripping of Less evaporative stripping of

volatile gasesvolatile gases● If sufficiently massive cores If sufficiently massive cores

form, can keep even volatile form, can keep even volatile gases gases

● These gases will be These gases will be representative of the very representative of the very early solar systemearly solar system


● Since early solar system is Since early solar system is largely composed of largely composed of Hydrogen, so will gas giantsHydrogen, so will gas giants

● Rocky or Icy or Slushy coreRocky or Icy or Slushy core● High-hydrogen atmosphere High-hydrogen atmosphere

has some similarities to has some similarities to atmosphere in Miller-Urey atmosphere in Miller-Urey experimentexperiment

● Can form lots of organicsCan form lots of organics


● Large mass -> high pressure, Large mass -> high pressure, temperature at centretemperature at centre

● Temperature at centre of Temperature at centre of Jupiter ~ 4 times surface of Jupiter ~ 4 times surface of Sun!Sun!

● Collapse from origin of planet Collapse from origin of planet still slowly continuingstill slowly continuing

● Releases heat energyReleases heat energy● These planets have a source of These planets have a source of

heatheatJupiter in Infra-red


● Gas giants emit more heat Gas giants emit more heat than they absorb from Sunthan they absorb from Sun

● At earlier times, would have At earlier times, would have been much hotterbeen much hotter

● Moons, which are nearby, Moons, which are nearby, heated by their nearby planetheated by their nearby planet

● Many of these moons are Many of these moons are large (planet-sized)large (planet-sized)

● Moons might be interesting Moons might be interesting for life?for life?

Jupiter in Infra-red

The Moons of GiantsThe Moons of Giants

● Planets large enough that many Planets large enough that many moons were also formedmoons were also formed

● Many of them planet sized in their Many of them planet sized in their own rightown right

● Get heat from planetGet heat from planet● Some (Io/Jupiter) effected by planets Some (Io/Jupiter) effected by planets

magnetic fieldmagnetic field● Atmosphere? (Titan, Saturn)Atmosphere? (Titan, Saturn)● Water? (Europa, Jupiter) Water? (Europa, Jupiter)

The Moons of GiantsThe Moons of Giants

● Formation: like planets around sunFormation: like planets around sun● Rotating body, disk formsRotating body, disk forms● Moons generally along plane of Moons generally along plane of

rotation of planet rotation of planet

Gas GiantsGas Giants

● Convection is a fundamental Convection is a fundamental processprocess

– Happens everywhereHappens everywhere● Fluid heated at bottom rises, Fluid heated at bottom rises,

cools, falls back downcools, falls back down● Gas giants have hot centresGas giants have hot centres● Large-scale motionsLarge-scale motions● Mix materialMix material

Gas GiantsGas Giants

● Makes it difficult to imagine Makes it difficult to imagine life forminglife forming

● No real surface to live onNo real surface to live on● Chemicals constantly being Chemicals constantly being

mixed aroundmixed around● No originally contained No originally contained

environment (`protocell')environment (`protocell')

MoonsMoons

● Gas giants have planet-sized Gas giants have planet-sized moonsmoons

● At least one (Titan) has a At least one (Titan) has a significant atmospheresignificant atmosphere

● Another (Europa) very likely Another (Europa) very likely has liquid salty water under a has liquid salty water under a layer of icelayer of ice

EuropaEuropa

● Very suggestive it has a liquid Very suggestive it has a liquid underneathunderneath

– No crateringNo cratering– Many fractures, ridges on Many fractures, ridges on

surfacesurface● What would this mean for life?What would this mean for life?

– If some source of energy If some source of energy on inside (geothermal, on inside (geothermal, chemical), very real chemical), very real possibility of some sort of possibility of some sort of lifelife

TitanTitan

● Very ColdVery Cold● Massive, Cold enough to have Massive, Cold enough to have

an atmosphere (1.5 x as dense an atmosphere (1.5 x as dense as ours!)as ours!)

● No oxygenNo oxygen● No liquid waterNo liquid water● Hydrogen richHydrogen rich● Interesting organic chemistryInteresting organic chemistry● Lakes of hydrocarbons?Lakes of hydrocarbons?● Huygens probe 2005Huygens probe 2005

How Unique is Earth?How Unique is Earth?

● What is special about Earth?What is special about Earth?● How important/rare are those things?How important/rare are those things?● How many such planets are there likely How many such planets are there likely

to be?to be?

EarthEarth

● AtmosphereAtmosphere

– Large surface gravityLarge surface gravity● Reasonable temperatureReasonable temperature● Rocky surfaceRocky surface● Large moonLarge moon● Lots of heavy elementsLots of heavy elements

How Important/Rare are these?How Important/Rare are these?

● Heavy elements;Heavy elements;

– Likely ubiquitous in Likely ubiquitous in planets around Pop I starsplanets around Pop I stars


● Rocky SurfaceRocky Surface

– Can happen if there is Can happen if there is heavy elements (see heavy elements (see above)above)

– Probably true of all Probably true of all planets close enough to planets close enough to have liquid waterhave liquid water

– (But planet migration)(But planet migration)


● AtmosphereAtmosphere

– Requires not too close to Requires not too close to sun sun

– Requires massive enough Requires massive enough planetplanet


● Reasonable TemperatureReasonable Temperature

– `Goldilocks zone’`Goldilocks zone’– Needs to be right distance Needs to be right distance

to starto star


● So we requireSo we require

– Rocky PlanetRocky Planet– Of the right massOf the right mass– At the right distance from At the right distance from

the starthe star

Habitable ZoneHabitable Zone

● Corresponds to further than Corresponds to further than Venus to about Mars distance Venus to about Mars distance for our Sunfor our Sun

● Using inverse-square law, could Using inverse-square law, could calculate for other starscalculate for other stars

● Main requirement: liquid water Main requirement: liquid water in the presence of an in the presence of an atmosphere.atmosphere.

Habitable Zone: Binary StarsHabitable Zone: Binary Stars

● About half of all stars are in About half of all stars are in binary systemsbinary systems

● Stars orbit a common centre of Stars orbit a common centre of mass (more on that next week)mass (more on that next week)

● Can planets have reasonable Can planets have reasonable orbits in such systems?orbits in such systems?

● Yes, but must orbit one star or Yes, but must orbit one star or be far away from both;be far away from both;

● `Figure 8’ orbits aren’t stable`Figure 8’ orbits aren’t stable

Finding Other PlanetsFinding Other Planets

● Light from planetLight from planet

– Reflected visible lightReflected visible light– Reflected+generated infraredReflected+generated infrared

● Dark from planetDark from planet

– Transits (shadows from planets)Transits (shadows from planets)● Light bent by planetLight bent by planet

– Gravitational LensingGravitational Lensing● Star's Motion from planetStar's Motion from planet

– Proper MotionsProper Motions– Doppler ShiftDoppler Shift

Light from the planetLight from the planet

● Stars observed by emitting Stars observed by emitting their own light their own light

● Planets don't emit light, but do Planets don't emit light, but do reflect sunlightreflect sunlight

● Problem: reflect a billionth or Problem: reflect a billionth or less of the light from the less of the light from the companion starcompanion star

Small brown dwarf (not planet) companion to a star directly imaged

● Has yet to be observedHas yet to be observed● What sort of planets/systems does What sort of planets/systems does

this work best for?this work best for?


● Would work best for:Would work best for:

– Large planets (more Large planets (more reflecting surface)reflecting surface)

– Reflective planets Reflective planets (ammonia clouds?)(ammonia clouds?)

– Near enough star to Near enough star to reflect lots of light reflect lots of light

– Far enough not to be Far enough not to be overwhelmed by light overwhelmed by light from starfrom star



● Large planets near star: `Hot Large planets near star: `Hot Jupiters'Jupiters'

● Gas giants (presumably) very Gas giants (presumably) very near starnear star



● How observed?How observed?● Very careful imaging of nearby Very careful imaging of nearby

stars stars ● Probably with telescopes above Probably with telescopes above

atmosphere (Hubble)atmosphere (Hubble)● As long as planet isn't in front As long as planet isn't in front

of/behind star, will be reflecting of/behind star, will be reflecting light towards Earthlight towards Earth

● Just a question of being able to Just a question of being able to observe itobserve it


● This is actually an infrared imageThis is actually an infrared image● Jupiter-type planets may emit their Jupiter-type planets may emit their

own infrared lightown infrared light● Terrestrial planets reflect a lot of Terrestrial planets reflect a lot of

infraredinfrared● Star emits most of its light in visibleStar emits most of its light in visible● Better chance in IRBetter chance in IR



Planetary Transits/OccultationsPlanetary Transits/Occultations

● Light from planet can be blocked Light from planet can be blocked by orbiting planetby orbiting planet

● Careful measurement of total Careful measurement of total light from star can show thislight from star can show this

● Can't see directly; the star is just a Can't see directly; the star is just a pointpoint

Time

Brightness


● If period is measured (multiple If period is measured (multiple transits) and mass estimate for star transits) and mass estimate for star exists, have:exists, have:

– Planet's distancePlanet's distance– Planet's sizePlanet's size– Planet's orbital periodPlanet's orbital period– Star's sizeStar's size

Time

Brightness

?

Planetary Transits/OccultationsPlanetary Transits/Occultations● How are these observed?How are these observed?● Fairly rare events:Fairly rare events:

– Has to be exactly along line of Has to be exactly along line of sightsight

● Only planetary systems aligned Only planetary systems aligned along line of sightalong line of sight

● Planet directly in front of star Planet directly in front of star only very briefly (Jupiter: ~1 only very briefly (Jupiter: ~1 day / 11 yrs)day / 11 yrs)

● Fairly careful measurements must Fairly careful measurements must be madebe made

– Jupiter: 1% decrease in Sun's Jupiter: 1% decrease in Sun's brightnessbrightness


● Large surveyLarge survey

– Dedicated telescopeDedicated telescope– Look at large fraction of sky Look at large fraction of sky

every night (or nearly)every night (or nearly)


● Works best for:Works best for:

– Large planets (blocks more of Large planets (blocks more of star)star)

– Planets near star (shorter Planets near star (shorter period – easier to observe)period – easier to observe)

– Hot JupitersHot Jupiters● Has been used to find planetsHas been used to find planets

Gravitational lensingGravitational lensing

● A very powerful technique to A very powerful technique to measure dim objectsmeasure dim objects

● Used in searches for brown dwarfs Used in searches for brown dwarfs or other large clumps of `dark or other large clumps of `dark matter'matter'

● RequiresRequires

– distant, bright, source star,distant, bright, source star,– very accurate measurements very accurate measurements

of the brightness of the of the brightness of the source star over timesource star over time


● At least one planet has been `seen' At least one planet has been `seen' this waythis way

● Results:Results:

– Mass of planet, starMass of planet, star– Distance to starDistance to star– Distance planet <-> starDistance planet <-> star

● Difficult, because only get one Difficult, because only get one chance at measuring systemchance at measuring system


● Works best for what systems?Works best for what systems?

– Dim StarsDim Stars– Massive planetsMassive planets– (relatively) insensitive to (relatively) insensitive to

distance between star and distance between star and planetplanet

– Jupiters at any radii / Jupiters at any radii / temperaturetemperature

Astrometry: Proper MotionsAstrometry: Proper Motions

● Stars motion towards/away from us Stars motion towards/away from us can be measured very accuratelycan be measured very accurately

– Doppler ShiftDoppler Shift● Motions `side-to-side' on the sky Motions `side-to-side' on the sky

take VERY long time to make take VERY long time to make noticeable changesnoticeable changes


● If star has a large enough proper If star has a large enough proper motion motion

– (probably means very near us)(probably means very near us)● Wobble in the star's motion could Wobble in the star's motion could

indicate that the star is being indicate that the star is being tugged on by a nearby planettugged on by a nearby planet


● Has been successfully used to detect white-dwarf Has been successfully used to detect white-dwarf companionscompanions

● Shown below: SiriusShown below: Sirius● No successful measurement of planets howeverNo successful measurement of planets however


● Would work best for?Would work best for?


● Would work best for?Would work best for?

– Nearby starsNearby stars– Large mass companionLarge mass companion– Distant from planet: can pull further distanceDistant from planet: can pull further distance– Near planet: faster orbit, more visible wobbleNear planet: faster orbit, more visible wobble

Doppler ShiftingDoppler Shifting

● Star has slight motion in Star has slight motion in orbit orbit

● If that motion is largely If that motion is largely towards/away from us, towards/away from us, might be detected by might be detected by Doppler shiftDoppler shift

● Motions towards/away can Motions towards/away can be very accurately measured be very accurately measured (few meters/sec)(few meters/sec)


● Has so far been extremely Has so far been extremely successful successful

● If can watch for several If can watch for several periods, can get very periods, can get very accurate period accurate period measurementsmeasurements

● Sine wave: circular orbitSine wave: circular orbit● `Tilted' sine wave: elliptical `Tilted' sine wave: elliptical

orbitorbit● Get: period, total velocity Get: period, total velocity

induced by planetinduced by planet



– Large planets Large planets – Close in:Close in:

● Faster period (easier to Faster period (easier to detect)detect)

Interstellar Travel, Interstellar Interstellar Travel, Interstellar CommunicationCommunication

● Interstellar TravelInterstellar Travel

– RocketsRockets– FuelFuel– SpeedsSpeeds– Time DilationTime Dilation

● Interstellar CommunicationInterstellar Communication

– What frequencies do we use?What frequencies do we use?– Meaningful signalsMeaningful signals– SETI@homeSETI@home

RocketsRockets

● Have to exert force to overcome Have to exert force to overcome that of gravitythat of gravity

● Reactions from some sort of fuelReactions from some sort of fuel

– Chemical Chemical – Electrical...Electrical...

● Propel exhaust downwardsPropel exhaust downwards● By Newton's 3By Newton's 3rdrd law, propel law, propel

rocket upwardsrocket upwards

GravitationalForce Force exerted

by exhaust

Net Force-> acceleration

● Easy to accelerate upwardsEasy to accelerate upwards● Hard to keep from falling back Hard to keep from falling back

down!down!● Can either:Can either:

– Accelerate very quickly to Accelerate very quickly to escape vel (25,000 mph) escape vel (25,000 mph) and coast upand coast up

● Gravity will keep Gravity will keep decelerating you but never decelerating you but never quite pull you backquite pull you back

– Or accelerate slowly Or accelerate slowly through ascentthrough ascent

● Luckily, further up you get, Luckily, further up you get, weaker force from Earth's weaker force from Earth's gravity becomesgravity becomes

GravForce exhaust

Net

Rockets: FuelRockets: Fuel

● Takes a lot of fuel to move something Takes a lot of fuel to move something into Earth's orbit or furtherinto Earth's orbit or further

● Would take about as much fuel to Would take about as much fuel to launch me into orbit as it takes to heat launch me into orbit as it takes to heat a Chicago home through an entire a Chicago home through an entire winterwinter

● Unlike a car trip, fuel starts weighing a Unlike a car trip, fuel starts weighing a lot, even compared to rocketlot, even compared to rocket

● Shuttle launch:Shuttle launch:

– Empty Shuttle: 230,000 lbEmpty Shuttle: 230,000 lb– Fuel : 2,700,000 lbFuel : 2,700,000 lb

● Interstellar medium VERY Interstellar medium VERY tenuoustenuous

● Sprinkled with hydrogenSprinkled with hydrogen● Could it be collected and then Could it be collected and then

burned (nuclear fusion?)burned (nuclear fusion?)● Hard to see howHard to see how

– Drag on shipDrag on ship– Power to magnetic fieldsPower to magnetic fields

● But would solve enormous fuel But would solve enormous fuel problemproblem

Fuel along the way?Fuel along the way?

Special RelativitySpecial Relativity

● Einstein:Einstein:

– Physics is the same in all Physics is the same in all inertial frames of referenceinertial frames of reference

– Speed of light in a vacuum is Speed of light in a vacuum is a fundamental physical a fundamental physical constant of the Universeconstant of the Universe

Special RelativitySpecial Relativity● But for higher velocities, can be But for higher velocities, can be

significant!significant!● Astronaut goes to Alpha Centauri and Astronaut goes to Alpha Centauri and

back at 95% of speed of lightback at 95% of speed of light● Astronaut ages 3 years, people back Astronaut ages 3 years, people back

home 9home 9● At closer and closer to speed of light, At closer and closer to speed of light,

effect gets bigger and bigger.effect gets bigger and bigger.

Special RelativitySpecial Relativity● Speed of light becomes moving targetSpeed of light becomes moving target● Astronaut can put more and more Astronaut can put more and more

energy into traveling fasterenergy into traveling faster● But because can never pass light (light But because can never pass light (light

must always travel at same velocity!) must always travel at same velocity!) can never pass speed of lightcan never pass speed of light

● Takes infinite amount of energy to Takes infinite amount of energy to even get to speed of lighteven get to speed of light

Automated Probes?Automated Probes?

● High-tech Voyagers or PioneersHigh-tech Voyagers or Pioneers● Aim towards nearby starsAim towards nearby stars● Enough fuel to accelerateEnough fuel to accelerate● Enough smarts to navigate toward Enough smarts to navigate toward

systemsystem● Get solar power once near starGet solar power once near star● Send messageSend message

– To nearby planetsTo nearby planets– To us To us

Travel DifficultTravel Difficult

● Communication much simpler than Communication much simpler than Transportation.Transportation.

MessagesMessages

● Its a lot easier sending signals than Its a lot easier sending signals than thingsthings

● MessagesMessages

– Have no mass Have no mass – Don't require fuelDon't require fuel– Don't require food/provisions Don't require food/provisions

for long journeyfor long journey– Cheap to produceCheap to produce– Travel at speed of lightTravel at speed of light

What frequencies to use?What frequencies to use?

● Two choices for long-distance Two choices for long-distance forces:forces:

– Gravity (difficult)Gravity (difficult)– ElectromagneticElectromagnetic

● But there's an essentially infinite But there's an essentially infinite range of frequencies to examinerange of frequencies to examine

● Radio waves:Radio waves:

– Easy/cheap to generate, Easy/cheap to generate, focusfocus

SETI@homeSETI@home

● Several different SETI listening Several different SETI listening experimentexperiment

● One is called `Project SERENDIP'One is called `Project SERENDIP'● `Listen in' on other astronomical `Listen in' on other astronomical

uses of the Arecibo radio telescope uses of the Arecibo radio telescope in Puerto Ricoin Puerto Rico

● Can't choose where the observers Can't choose where the observers are looking, but can listen (nearly) are looking, but can listen (nearly) 24x724x7

● Receiver installed which listens to Receiver installed which listens to 168 million narrow channels near 168 million narrow channels near 21cm Hydrogen line21cm Hydrogen line

SETI@homeSETI@home● Done as part of screen saver on thousands of volunteer's Done as part of screen saver on thousands of volunteer's

computerscomputers

ResultsResults

● Several candidate signals Several candidate signals discovereddiscovered

● 2500 persistent gaussians (longish 2500 persistent gaussians (longish spikes seen at least twice)spikes seen at least twice)

● Need to be checked to make sure Need to be checked to make sure not interference/noisenot interference/noise

● Also searching data for persistent Also searching data for persistent spikes, pulses, triplets...spikes, pulses, triplets...

Has the Search Happened Already?Has the Search Happened Already?

● UFO sightingsUFO sightings● What Evidence is Necessary?What Evidence is Necessary?● If no UFOs yet, why not?If no UFOs yet, why not?

UFO SightingsUFO Sightings

● No shortage of UFO observation No shortage of UFO observation stories, photosstories, photos

● A moment spent with google A moment spent with google provides thousands of ernest, provides thousands of ernest, probably mostly honest web probably mostly honest web pages describingpages describing

– UFO sightingsUFO sightings– AbductionsAbductions

What Evidence is Required?What Evidence is Required?

● Large amount of documentary Large amount of documentary evidence that the Universe has evidence that the Universe has apparently searched for life hereapparently searched for life here

● Why not accept this as truth?Why not accept this as truth?

Extrordinary Claims require Extrordinary Claims require Extrordinary EvidenceExtrordinary Evidence

● Let me make two claimsLet me make two claims

– This morning, violence broke out in an This morning, violence broke out in an up-til-now quiet region of Iraq, in the up-til-now quiet region of Iraq, in the southern town of Rajaf. Four US southern town of Rajaf. Four US soldiers were killed.soldiers were killed.

– With great effort, I can fly short With great effort, I can fly short distances (10-20 ft) using the power of distances (10-20 ft) using the power of my mind.my mind.

● Which (if either) do you believe?Which (if either) do you believe?

Extrordinary Claims require Extrordinary Claims require Extrordinary EvidenceExtrordinary Evidence

● You have exactly the same evidence for both You have exactly the same evidence for both claims: my say-so.claims: my say-so.

● Clearly, the Iraq claim has more serious Clearly, the Iraq claim has more serious immediate consequences (death, future immediate consequences (death, future violence)violence)

● Why is the same evidence more likely to be Why is the same evidence more likely to be sufficient in one case (the more serious, sufficient in one case (the more serious, even) than in the other?even) than in the other?


● Photographs are easily Photographs are easily misinterpretedmisinterpreted

● Photographs also easily fakedPhotographs also easily faked● These: Robert SchaeferThese: Robert Schaefer


● Eyewitness evidence notoriously Eyewitness evidence notoriously unreliableunreliable

● Human brain very good at seeing Human brain very good at seeing patterns, filling in blankspatterns, filling in blanks

● Too good, in fact, to be good at Too good, in fact, to be good at mundanely reciting uninterpreted mundanely reciting uninterpreted observationsobservations

Observation TestObservation Test

● Quantitative testQuantitative test● Count basketball passes by one team (dressed in Count basketball passes by one team (dressed in

white) in a complicated, dynamic scenewhite) in a complicated, dynamic scene● http://viscog.beckman.uiuc.edu/grafs/demos/15.htmlhttp://viscog.beckman.uiuc.edu/grafs/demos/15.html

Same lab: `change blindness'Same lab: `change blindness'

● http://viscog.beckman.uiuc.edu/grafs/demos/10.htmlhttp://viscog.beckman.uiuc.edu/grafs/demos/10.html

Post-event SuggestibilityPost-event Suggestibility

● Elizabeth Loftus:Elizabeth Loftus:

– Film shown of car accidentFilm shown of car accident– Questionaire after filmQuestionaire after film– Followup questionaire afterwardsFollowup questionaire afterwards– Leading questions, Leading questions,

misinformation in questions could misinformation in questions could cause people to misremember cause people to misremember event afterwardsevent afterwards

● Wrong color of carWrong color of car● `Remembering' stop signs, `Remembering' stop signs,

buildings that weren't therebuildings that weren't there● ......


● This doesn't mean that all the This doesn't mean that all the evidence is proven evidence is proven wrong/mistakenwrong/mistaken

● Not enough evidence to be Not enough evidence to be convincingconvincing

● What would be convincing What would be convincing evidence?evidence?


● This doesn't mean that all the This doesn't mean that all the evidence is proven evidence is proven wrong/mistakenwrong/mistaken

● Not enough evidence to be Not enough evidence to be convincingconvincing

● What would be convincing What would be convincing evidence?evidence?

– Chunk of spacecraft Chunk of spacecraft material/technologymaterial/technology

– Cheek swab from alienCheek swab from alien– ......

Fermi's ParadoxFermi's Paradox

● No signals from aliens yet.No signals from aliens yet.● No visitors yet either, perhaps.No visitors yet either, perhaps.● Why not?Why not?

Fermi's ParadoxFermi's Paradox

● Even if 1,000,000 civilizations in our Even if 1,000,000 civilizations in our galaxy today, that's one per ~300,000 galaxy today, that's one per ~300,000 starsstars

● Would have to explore by chance to Would have to explore by chance to find Earthfind Earth

● Radio signals identifying Earth are very Radio signals identifying Earth are very new: 1960s or sonew: 1960s or so

● Even if travel speed of light, on has Even if travel speed of light, on has been time for 20ly round trip:been time for 20ly round trip:

● Only a handful of stars that closeOnly a handful of stars that close

Next weekNext week

● Assignment covering Weeks 8-13 due Assignment covering Weeks 8-13 due ● Projects also due next weekProjects also due next week● Class summary Class summary ● Any student presentationsAny student presentations

Today ● Today: – Review of Parts 4 & 5 of the text (Weeks 8-12) – Cover the last of the...

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Transcript of Today ● Today: – Review of Parts 4 & 5 of the text (Weeks 8-12) – Cover the last of the...