planets around other...

Planets Aroundd Other Stars

How to makHow to mak

• Large, cool cloud of gas– Gas makes the star, dus,formation

– Dust is usually made ofDust is usually made of(silicates) and ices (soliMostly H and He (these– Mostly H and He (theseabout 98% of our Solar

• Cloud begins to collapsgravity

ke a planetke a planet

s and dustst is necessary for planet y p

f metals (Fe Ni Al) rocksf metals (Fe, Ni, Al), rocks d H2O, CH4, NH3)e two elements make upe two elements make up r System)

se under its own self‐


ll• Collapse causes– Increase in temperatur– Increase in rate of rotatangular momentum)

• Result is a rapidly rotat– Again, dust means rockAgain, dust means rock– at sufficient distance frhydrogen compounds (hydrogen compounds (important in formationare far from the Sun))


re (conservation of energy)tion (conservation of

ting disk of gas and dustks, metals and icesks, metals and icesrom the parent star, (H2O, CH4 NH3) are(H2O, CH4 NH3) are n of giants (Jovian planets


• Accretion– Dust grains collide to fog– Collide small particles tCollide large “boulders– Collide large boulders(asteroids, comets, Kuill d l l– collide planetesimals to


orm larger particles ‐‐>g pto form still larger particles ‐‐> ” to form planetesimals to form planetesimals per Belt Objects)‐‐> f lo form planets


b lNebular H• Solar Nebula• Contraction into rotat• Dust grains accrete to• Dust grains accrete to

particlesL i l• Large particles sweepform planetesimals

• Planetesimals eventuaplanetsp


h iHypothesis

ting disk w/ hot centero form larger and largero form larger and larger

i l out more material to

ally collide to form

Basic prA good scientific theory

we only have one Soycannot go back in tim

We needed to finWe needed to finSo we d

roblem:y has to be tested, but olar System and we yme.nd other planets!nd other planets!id and …

ExoplExopl

• A planet that orbits a s• Also can be called an “eAlso can be called an e• There are 1890 exoplan

– Many more than in our

• First planet (similar to tp (System) discovered in ~

Detection is very challe– Detection is very challe

anetsanets

star other then the Sunextrasolar planet”extrasolar planetnets as of February 2015r Solar System

those in our Solar ~1995enging!enging!

ExoplExopl

• Known Exoplanets cha– Largest ~50 Jupiter masg p– Smallest ~1 Earth massClosest to parent star ~– Closest to parent star ~

• Period less than 1 day

h f– Furthest from parent st

• Very wide range!y g

anetsanets

racteristicsssess~0 01 AU~0.01 AU

tar ~1000 AU

Examples of StExamples of St

Gli 8 6• Gliese 876a) Parent star:

M3 5 V T 3480 K L 0 01– M3.5 V, T=3480 K, L=0.01

b) Gas Giant– M=2MJ, a=0.208 AUJ,

c) Gas Giant– M=0.62 MJ, within orbit o

d) Super Earth– Inside orbits of both Glie

tellar Systemstellar Systems

124 L124 Lsun

of Gliese 876 b

se 876 b and c

Examples of StExamples of St

Gli 581• Gliese 581a) Parent star:

– M3 V T=3480 K L=0 013M3 V, T 3480 K, L 0.013 b) Gas Giant (Neptune‐si

– M=16 ME, a = 0.04 AU) R k Pl tc) Rocky Planet

– M=5 ME, a = 0.07 AU, witbe as low as ‐3 oC or as hgreenhouse akin to Venugreenhouse akin to Venu

d) Super Earth– M=7 ME, a = 0.22 AU, wit

e) Super Earth– M=1.9 ME, a=0.03 AU

tellar Systemstellar Systems

LLsunized)

thin habitable zone (Temperature could igh as 500 oC, due to runaway s)s)

thin habitable zone

Upsilon Anp

Two planets are several times moreThe third planet mass 75% that of JThe third planet, mass 75% that of J

it completes a full orbit every 4.6

dromedae

e massive than JupiterJupiter is so close to the star thatJupiter, is so close to the star that 6 Earth days

Why is it so hard tod tharound other

• Faint planet glimmer is lostFaint planet glimmer is lost – Planets are small, close to their par

• Thought experiment:Thought experiment:– Imagine grain of rice an inch from a

at end of a long dark hall would see

• Consider the case of Jupiter– As seen from the nearest star, Alph

billionth as bright as the Sun.– Jupiter would also be extremely clo

Si ll th t f t• Since all other stars are fartJupiter would be even harde

find planets t ?r stars?

in glare from parent starin glare from parent starrent star, and shine by reflected starlight

a 100 Watt light bulb. Someone standing e only the light bulb, not the grain of rice.

r and the Sun:a Centauri, Jupiter would appear a

ose to the Sun, only 4 arc sec away.

h th Al h C t iher than Alpha Centauri, er to detect from other stars

Planets are very hard todetecting theidetecting thei

• Planets are too faint compap– This brown dwarf star is bare

• Planets shine by reflected li– Planets close to parent stars a

o observe directly (by r own light)r own light)

ared with their starly visible ‐ and its star is faintightare brightest, but hardest to see

Gravitational Motions

• The Sun and Jupiter orbit around their common center of mass.

• The Sun therefore wobbles around that center of mass with same period as Jupiter.

Gravitational Motions

Th S ’ ti• The Sun’s motion around the solar system’s center ofsystem s center of mass depends on tugs from all the l tplanets.

• Astronomers around other stars thatother stars that measured this motion could determine the masses and orbits of all the planetsall the planets.

Astrometric Motion

W d• We can detect planets by measuring the change in a star’sthe change in a star s position on sky.

• These tiny motions• These tiny motions are very difficult to measure (~ 0 001measure ( 0.001 arcsecond).

Doppler Tepp Techniqueq

M i ’• Measuring a star’s Doppler shift can tell us its motion towardus its motion toward and away from us.

• Current techniques can measure motionscan measure motions as small as 1‐2 m/s (walking speed!)(walking speed!).

First Extrasolar P

Insert TCP 6e Figure 13.4a unannotated

Planet: 51 Pegasi• Doppler shifts of the

star 51 Pegasistar 51 Pegasiindirectly revealed a planet with 4-day orbital period.

• This short period means that the planetmeans that the planet has a small orbital distance – well within orbit of Mercuryorbit of Mercury.

• This was the first extrasolar planet discovered around a normal stars (1995).

51 Peg b51 Peg b

Half the mass of Jupiter, ythe Sun tha

b (1995)b (1995)

yet orbiting much closer to an Mercury!

Other Extras

• Doppler shift data tell usDoppler shift data tell us the shape of its orbit.

olar Planets

about a planet’s mass andabout a planet s mass and

PlanetaryPlanetarySize depe

yy

mass of

Low mass, high mass

Periodicityon period

Small period, large period

Changes in RV depend onChanges in RV depend oneccentricity of planeteccentricity of planet

y signaturesy signaturesends on

y gy g

planet

Circular, eccentric

y depends of planet

Shape depends on eccentricity of planet

mass, period and mass, period and

Dopplepp

• Look for periodic shift in st• Look for periodic shift in st– Does not depend on distan

d l– Need massive planet near • the closer the planet, thb h l d )both planet and star)

– Need very good spectrum

• Many planet detections• Incredibly hard measuremIncredibly hard measuremstandard

er shift

tar’s spectrumtar s spectrumnce of starstarhe faster the orbital speed (of

ments have now becomements have now become

Limitations of DoLimitations of DoLimitations of DoLimitations of Do

From ground‐based gobservatories, detect shifts > 1‐2 m/sec

The nearer to the star andThe nearer to the star and the more massive the planet, the easier toplanet, the easier to detect

oppler techniqueoppler techniqueoppler techniqueoppler technique

Radial velocity metholl th bit l i fall the orbital info

• Doppler shift only detects• Doppler shift only detectsvelocity along line of sight– Can’t distinguish massive gplanet (or brown dwarf!) in tilted orbit from less massive planet in edge onmassive planet in edge‐onorbit

– They both have the same yline‐of‐sight velocity

• The only way to resolve hi bi i ithis ambiguity is to observe using another methodmethod

d doesn’t give tiormation

ss t

nn

TranTran

• As planet moves across tiny bit of starlighty g

• Watch for periodic dimm

nsitsnsits

face of star, it blocks a

ming of star

Planet detected around thmetmet

e star HD209458 by transit thodthod

• Planet is 70% mass of JupiterPlanet is 70% mass of Jupiter, but orbits in just 3.5 days

• So it is very close to its parent star

• Many planets have been found this wayfound this way

• Amateur astronomers have organized to watch for gfluctuations in star brightness– http://www.transitsearch.org/

Transits andTransits and

• A transit is when a planet cl h l• Eclipse is when a planet goe

• From both, can learn about(b d d f(beginning and end of trans

EclipsesEclipses

crosses in front of a star.b h des behind a star

t atmosphere of planet l )sit, eclipse)

TransitinTransitin

Orbital inclination must beOrbital inclination must beof‐sight

This makes many additionThis makes many additionproperties possible to mea

size, mass, temperature and sp

Can test the atmospheric modepplanets in our solar system

Some of these planets are subjethe ones in the Solar System:

Small distance from sta

Extreme eccentricities

ng planetsng planetsg pg p

e closely aligned to our line‐e closely aligned to our line

al interesting planetaryal interesting planetary asure

pectra

els that have been developed for p

ect to more exciting conditions than

r

NASA’s Kepler Spactransiting

March 2009: NASA mission Ke

Repeated images ~100,000 stCygnus for transiting planet si

Has found thousands of possi

some have been confirsome have been confir

not called a planet unt

e Mission to detect g planets

epler was launched

ars in the constellation ignals

ble planets

rmedrmed

il follow‐up is executed

NASA’s KepDetermining the frequency of E

in the habitable zon

pler MissionEarth-size and larger planets

ne of Sun-like stars

Kepler 10‐b

• Earth‐sized planet• Very close to its parent star, so veryparent star, so very hot

Transit andMeasurements

10b Size

Mass

Den

Volume

d Dopplerpps Yield Density

++

nsity

= 8.8 g/cm3

Composition oComposition oof Kepler 10bof Kepler-10b

Gravitational

• Background: Microlensing• Background: Microlensing

Microlensing g

around a star (or black hole)around a star (or black hole)

N d l tNeeds almost perfect alignment between source and lens.

Planet detection: fmicrolensingg

• A few planets have been pdiscovered this way

• Potentially very useful:Potentially very useful: can detect planets at large distances from uslarge distances from us– Even farther away than transit method can

– Much farther than radial velocity or astrometry can

• Samples a different population of stars than p pother techniques

fine structure on light curveg

Direct IDirect I

• Use planet’s own lightUse planet s own light• Take image of it

– Can reconstruct full orbit– Can reconstruct full orbit by watching it go around

• Can also obtain spectrap– Learn about physical conditions, atmosphere,

b fmaybe even presence of life

• Jupiter is a billion times• Jupiter is a billion times fainter than the Sun, in visible light!visible light!

magingmaging

First Images oFirst Images oHR 8799 SHR 8799 SHR 8799 SoHR 8799 So

Marois et al. 2008, Science Magazine

of of ExoplanetsExoplanets: : l S tl S tolar Systemolar System

First images of exoplaFirst images of exoplag pg p

Neptune-sizedorbitorbit

http://dps.aas.org/education/dpsdisc/

nets: Fomalhautnets: Fomalhaut

Hubble Space TelescopeHubble Space Telescope visible image of the star Fomalhaut (whose light was blocked) with a

Star location

was blocked), with a dust belt similar to the Kuiper belt.

Inset: Images taken ~2 years apart show a planet moving around the star.

Old list of other planetary systems

Characteristics of

• Much higher eccentriciorbits than in our Solar

• Much higher fraction otheir parent starstheir parent stars

• Many planets are “suptimes more massive th

• Microlensing suggestsMicrolensing suggests may be around ~1/3 of

extra‐solar planetsp

ity in most of their r Systemyof planets very close to

er‐Jupiters” (up to 10 an Jupiter)systems like our ownsystems like our own f all stars

Eccentric OOrbits

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Many extrasolar pvery close to parevery close to pare

• This is a selection effect• This is a selection effect(caused by detection method) )

• But it does show that such l t i t!planets exist!

• Our Solar System is veryOur Solar System is very different (green points) -Mercury is farther away fromSun than many of the extra-solar planets are from their starsstars

lanets are ent starsent stars

m -

Patterns in eccePatterns in ecce

• Most new planets are in elliptical orbits

• Short period planets:Short period planets: – Very close to parent stars very lowstars, very low eccentricityS h– Same process that moved planets close to

i l i d h istar circularized their orbits

entricityentricity

Parent stars of exParent stars of ex

Hi h i l t h i• High in elements heavier than hydrogen and h li ( ll > S )helium (usually > Sun)– reasonable: planetsform from dustform from dust

• Probability of finding a l t i hplanet increases as heavy

element content of t t iparent star increases

xtrasolar planetsxtrasolar planets

PPplanetplanet ~ ~ ((NNFeFe/ N/ NHH))1.61.6

What are theWhat are the• Our Solar System has small large gas giants further awa– no experience of large massivS l S tSolar System

• Theory of giant planet formform outside “frost line”form outside frost line

ese planets?ese planets?rocky planets close to star, ayve planets close to sun in our

mation says they have to

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Hot Jupitersp

How are “Hot JuHow are Hot JuTheory for our Solar System:Theory for our Solar System:

Stellar wind from young Sun blew volatiles outwards

Ice condensation at 5 AU where water-ice solidified

Fast accretion of large icy planetFast accretion of large icy planet (~10 MEarth) which then collected H/He atmosphere

Gas giants Jupiter, Saturn just outside “frost line”

Small rocky planets insideSmall rocky planets insideSlowly accreting icy planets

in outer system (Uranus, Nept ne)Neptune)

upiters” formed?upiters formed?Extrasolar giant planets:Extrasolar giant planets:Do they form in situ?

looks impossible: too hot for i t littl t i l f kices, too little material for rock

Do they form outside frost line and migrate inwards?gplanet forms in gas/dust disc around stardrag from remaining gas/dustdrag from remaining gas/dust causes it to spiral inwards

Does scattering from other giant planets causes migrationwhy does it stop?

InterestingInteresting

O i i l h i f l• Original theories of solar sywhen our own Solar System

M tl i l bit– Mostly circular orbits– Giant planets in outer solar s

• Many new Solar Systems a• Many new Solar Systems a• Needs new ideas• How to arrive at a new par

– Use computer simulations tomake predictionsmake predictions

– Test predictions against obse

questions!questions!

f i d l dystem formation developed m was the only one

system, terrestrial planets insidere (in general) not like oursre (in general) not like ours

radigm?o develop ideas, test hypotheses,

erved young solar systems, disks

Page

Theories for how glclose to t

1. Interactions between indigaseous disk. “Migration”

2. After gas disk cleared awaouter parts of solar system– Three‐body gravitational int– One giant planet got slung o

inwards and got “captured”– But why isn’t the close orbit

• Why didn’t Jupiter migrat

iant planets got so hheir starsvidual new planets and ”ay, several giant planets in m were leftteractions between themoutwards, a second was slung ” by the star in a close orbitt very elliptical?

e inwards close to Sun?

What have weWhat have we

h l• Most stars have planets • Many systems not similay y• A large fraction do have thoughthough– Roughly 1/3M k i d d t• More work is needed to and to better refine howi il tsimilar to ours– Also need more work to

learned so far?learned so far?

fof some sortar to ourssystems similar to ours

d t d tunderstand new systems w many stars have systems

find Earth analogs

What is a HabitaWhat is a Habita

• N• N• N

A good planet is:• N

able Planet?able Planet?

Not too biggNot too smallNot too hot or too coldNot too hot or too cold

What does “habit• Right temperature

• Liquid water

• Free oxygen to breath• Free oxygen to breath

• Light for warmth and perception

• Radiation shieldRadiation shield

• Some form of asteroid/comet protectiasteroid/comet protecti

table” mean

onon

Things That Affect T

Want temperature so you can haWant temperature so you can hawater on the surface of the p

Temperature of star

Distance from the star

Shape of planet’s orbit: circular op pelliptical

Planet’s atmosphere: greenhousep ggases

Temperature

ave liquidave liquid planet

240°

280°

260°

Water boils ->

160°

200°

180°

220°

120°

160

100°

140°

or

Water freezes ->

80°

40°

60°

20°e

-40°

0°

20

-20°

The Habitable Zone for

The Habitable Zone (HZ) in grwhere liquid water is expectedsurface.

Stars of Various Masses

reen is the distance from a star d to exist on the planets

SummSumm

• The ~ 2000 planets we haveThe 2000 planets we havea sub‐set of potential plane

• These new solar systems ha• These new solar systems haabout how planets formed F t h th d h• Future search methods havmore (and more varied) pla

• It’s hard to find Earth‐like p– But prospect of finding Earth‐

marymary

e detected to date are onlye detected to date are only ets out thereave raised big questionsave raised big questions in generalhi h b bilit f fi dive high probability of finding

anetsplanets‐like planets is exciting!

Page

f dSuppose you found a stas the Sun moving back

of 16 months. What

a. The star has a planet orb. The star has a planet orc The star has a planet orc. The star has a planet ord. The star has a planet, b

information to know its oinformation to know its o

What do you think of the obmay exist in the habitable zomay exist in the habitable zo

h htar with the same mass and forth with a period could you conclude?

rbiting at less than 1 AUrbiting at greater than 1 AUrbiting at exactly 1 AUrbiting at exactly 1 AUut we do not have enough

orbital distanceorbital distance

servation that other planets ones of other stars?ones of other stars?

planets around other...

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