Planets ● For life on a planet, so far we have three important questions: – How far is it from its Sun? – How massive is it? – What type of planet is it:

PlanetsPlanets

● For life on a planet, so far we For life on a planet, so far we have three important questions:have three important questions:

– How far is it from its Sun?How far is it from its Sun?– How massive is it?How massive is it?– What type of planet is it: is What type of planet is it: is

it rocky?it rocky?

Distance from StarDistance from Star

● Must be at distance from star Must be at distance from star that liquid water can exist with that liquid water can exist with an environmentan environment

● Not too close (Venus)Not too close (Venus)● Not too far (further than Mars)Not too far (further than Mars)

Period of orbitPeriod of orbit

● Lenth of time takes to complete Lenth of time takes to complete one orbitone orbit

● Period of orbit and distance are Period of orbit and distance are relatedrelated

● For a given star mass, period For a given star mass, period squared is proportional to squared is proportional to distance cubeddistance cubed

● Large distance – takes more Large distance – takes more time to orbittime to orbit

● Closer – orbits fasterCloser – orbits faster● If know (or can estimate) star's If know (or can estimate) star's

mass, period <-> distancemass, period <-> distance

P1year

2 1MM sun

a1AU

3

Size of PlanetSize of Planet

● Rocky planets in our system:Rocky planets in our system:

– 0.4 – 1.0 Earth diameters0.4 – 1.0 Earth diameters● Gas GiantsGas Giants

– 4.0 – 11.0 Earth diameters4.0 – 11.0 Earth diameters● Size alone gives an idea what Size alone gives an idea what

sort of planet it issort of planet it is● Size + mass of planet cinches it Size + mass of planet cinches it

(why?)(why?)

We can see proto-planetary disks...We can see proto-planetary disks...

● Observed around very young Observed around very young starsstars

● Obscures new star in visible Obscures new star in visible lightlight

● Glows in infrared (why?)Glows in infrared (why?)● Can we see planets?Can we see planets?

– Much harderMuch harder– Condensed objectsCondensed objects– Lost in glare of starLost in glare of star– Until 10 years ago, answer: Until 10 years ago, answer:

NO.NO.

Can we see planets?Can we see planets?

● Answer today: yes!Answer today: yes!● >110 extra solar planets discovered>110 extra solar planets discovered● More almost every monthMore almost every month


● Easier to make Easier to make measurements of measurements of nearby starsnearby stars

● Many of the stars with Many of the stars with known planets are known planets are easily visible to the easily visible to the eye, even in Chicagoeye, even in Chicago

● Gamma Cephei is near Gamma Cephei is near the north pole the north pole (Polaris)(Polaris)


● Easier to make Easier to make measurements of measurements of nearby starsnearby stars

● Many of the stars with Many of the stars with known planets are known planets are easily visible to the easily visible to the eye, even in Chicagoeye, even in Chicago

● 47 Ursae Majoris is 47 Ursae Majoris is below the big dipperbelow the big dipper

Finding Extra-solar PlanetsFinding Extra-solar Planets● TechniquesTechniques

– Direct(ish) measuring of planetDirect(ish) measuring of planet– Indirect measurement – effect on starIndirect measurement – effect on star

● Results of search so farResults of search so far

– `Hot Jupiters'`Hot Jupiters'● ImplicationsImplications

– Can life be found in these systems?Can life be found in these systems?– Are most systems like this?Are most systems like this?– Migration vs. Direct FormationMigration vs. Direct Formation

Finding Extra-solar PlanetsFinding Extra-solar Planets● Direct(ish) MethodsDirect(ish) Methods

– Light from planetLight from planet● Visible, InfraredVisible, Infrared

– Dark from planetDark from planet● Planet transitsPlanet transits

– Bending light from other Bending light from other objectobject

● Indirect methods – gravitational Indirect methods – gravitational effect on stareffect on star

– Pulsar TimingPulsar Timing– AstrometryAstrometry– Doppler ShiftDoppler Shift

Direct MethodsDirect Methods

● 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

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



● Infrared is between visible light Infrared is between visible light and radioand radio

● `Near' infrared most easily `Near' infrared most easily detected with telescopesdetected with telescopes

● Very far infrared can be observed Very far infrared can be observed with radio telescopes with radio telescopes


● InterferometryInterferometry● Allows (with some computation) Allows (with some computation)

using several radio telescopes as if using several radio telescopes as if it were one large telescopeit were one large telescope

● Easier to do with radio than with Easier to do with radio than with visible lightvisible light

● Amount of signal proportional to Amount of signal proportional to total area total area

● Resolution increases with size of Resolution increases with size of arrayarray

● Infrared interferometry has some Infrared interferometry has some promise for observing planets promise for observing planets directly directly


Dark from the planetDark from the planet

● 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

Planetary Transits/OccultationsPlanetary Transits/Occultations




Time

Brightness





Time

Brightness





Time

Brightness





Time

Brightness





Time

Brightness





Time

Brightness


● What sort of planets/systems does What sort of planets/systems does this work best for?this work best for?


● What information can we get?What information can we get?

– If can watch until repeats, can If can watch until repeats, can find period of planets orbitfind period of planets orbit

– Length of dip: amount of time Length of dip: amount of time planet in front of starplanet in front of star

● Speed of PlanetSpeed of Planet● Size of StarSize of Star

– Amount of dip: Size of Amount of dip: Size of planet / size of starplanet / size of star

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

?


● How are these observed?How are these observed?

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



● Similar requirements to transit Similar requirements to transit searchessearches

● Lots of careful images of large Lots of careful images of large amount of skyamount of sky

● Comparison to see any changesComparison to see any changes● Lensing searches get transit data Lensing searches get transit data

`for free'`for free'● Both transit search, lensing data Both transit search, lensing data

here from same operation (OGLE)here from same operation (OGLE)


● 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?


● 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

Indirect MethodsIndirect Methods

● Gravitational Effect on StarGravitational Effect on Star

– Pulsar TimingPulsar Timing– AstrometryAstrometry– Doppler ShiftDoppler Shift

Center of MassCenter of Mass

● `For every action there is an equal `For every action there is an equal and opposite reaction’ and opposite reaction’

● Gravitational force Earth exerts on Gravitational force Earth exerts on Sun the same as the force the Sun Sun the same as the force the Sun exerts on the Earthexerts on the Earth

● So why does the Earth orbit the Sun, So why does the Earth orbit the Sun, and not vice-versa?and not vice-versa?


● Same force, but Sun is much heavier Same force, but Sun is much heavier than earththan earth

● Same force moves Sun very littleSame force moves Sun very little● But Earth (say) a LotBut Earth (say) a Lot● Relative amount of motion = Relative amount of motion =

relative masses of objectsrelative masses of objects


● Same force, but Sun is much heavier Same force, but Sun is much heavier than earththan earth

● Same force moves Sun very littleSame force moves Sun very little● But Earth (say) a LotBut Earth (say) a Lot● Relative amount of motion = Relative amount of motion =

relative masses of objectsrelative masses of objects


● Sun is 300,000 times more massive Sun is 300,000 times more massive than Earththan Earth

● So Sun moves 1/300,000 as much as So Sun moves 1/300,000 as much as EarthEarth

● Both orbit a Center of Mass which is Both orbit a Center of Mass which is 300,000x closer to center of Sun 300,000x closer to center of Sun than Earththan Earth

– 1/10% of Sun’s radius1/10% of Sun’s radius


● Sun is 1,000 times more massive Sun is 1,000 times more massive than Jupiterthan Jupiter

● So Sun moves 1/1,000 as much as So Sun moves 1/1,000 as much as JupiterJupiter

● Both orbit a Center of Mass which is Both orbit a Center of Mass which is 1,000x closer to center of Sun than 1,000x closer to center of Sun than JupiterJupiter

– Sun’s radiusSun’s radius

PulsarsPulsars

● `Cosmic Lighthouses'`Cosmic Lighthouses'● Send out beam of high-energy Send out beam of high-energy

radiationradiation● RotatesRotates● If we're along line of sight, see very If we're along line of sight, see very

regular bursts of light/energyregular bursts of light/energy● Easy visibility + regularity -> very Easy visibility + regularity -> very

easy to detect changeseasy to detect changes

PulsarsPulsars

● Two planets have been so far Two planets have been so far discovered around pulsarsdiscovered around pulsars

● Significance for life? Probably Significance for life? Probably small.small.

– Pulsar likely the result of a Pulsar likely the result of a supernovasupernova

– Neutron star doesn't emit Neutron star doesn't emit much energymuch energy

– Column of high-energy Column of high-energy radiation every few seconds radiation every few seconds probably not helpfulprobably not helpful

PulsarsPulsars

● What sort of systems would this What sort of systems would this work well for?work well for?

PulsarsPulsars

● What sort of systems would this What sort of systems would this work well for?work well for?

– Need a pulsarNeed a pulsar– Massive planet (large Massive planet (large

gravitational effect)gravitational effect)– Near the pulsar (large Near the pulsar (large

gravitational effect)gravitational effect)

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 noticable changesnoticable 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 succesfully used to detect white-dwarf Has been succesfully 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 strsNearby strs– 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 succesful succesful

● 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)


● Sun rotates around a circle 1/10% of Sun rotates around a circle 1/10% of Sun’s radius in size every yearSun’s radius in size every year

● Maximum velocity: 3 inches/secMaximum velocity: 3 inches/sec


● Sun rotates about a circle its radius Sun rotates about a circle its radius in size every 11 yearsin size every 11 years

– 10 yards/sec10 yards/sec

Hot JupitersHot Jupiters

● How did these planets get so close to How did these planets get so close to their sun?their sun?

● Normal planetary formation theory: Normal planetary formation theory: < 1 AU is too close< 1 AU is too close

● Gasses would have evaporatedGasses would have evaporated● No Juptiers fromNo Juptiers from● Migration?Migration?

– High eccentricitiesHigh eccentricities– Gas could stay, just not form…Gas could stay, just not form…


● Jupiters are large enough to disrupt Jupiters are large enough to disrupt other planetsother planets

● Asteroid beltAsteroid belt● Jupiters less than 4 or 5 AU away Jupiters less than 4 or 5 AU away

from sun would probably prevent from sun would probably prevent any Earth-like planets forming any Earth-like planets forming within habitable zone.within habitable zone.


● Unlikely place for lifeUnlikely place for life● BUTBUT● If some have ~ Earth sized moons:If some have ~ Earth sized moons:

– Rocky “planets” in habitable Rocky “planets” in habitable zone?zone?

Reading for Next Class (Apr 30)Reading for Next Class (Apr 30)

● Chapter 19, 20: Interstellar spaceflight, communicationsChapter 19, 20: Interstellar spaceflight, communications● Chapter 19: Interstellar spaceflightChapter 19: Interstellar spaceflight

– Energy, fuel requirementsEnergy, fuel requirements– Time requirementsTime requirements– Time dilation – Special RelativityTime dilation – Special Relativity– Manned vs. Unmanned probesManned vs. Unmanned probes

● Chapter 20: Interstellar communicationsChapter 20: Interstellar communications

– Spectrum, and choice of frequencySpectrum, and choice of frequency– Choice of messageChoice of message– Listening vs. SpeakingListening vs. Speaking– SETI@homeSETI@home

Documents

Planets ● For life on a planet, so far we have three important questions: – How far is it from its Sun? – How massive is it? – What type of planet is it: