Life in the Universe: Extra-solar planets

DEPARTMENT OF PHYSICS AND ASTRONOMY

Life in the Universe:Life in the Universe:Extra-solar planetsExtra-solar planets

Dr. Matt BurleighDr. Matt Burleighwww.star.le.ac.uk/~mbuwww.star.le.ac.uk/~mbu

Dr. Matt Burleigh 3677: Life in the Universe

3677 Timetable3677 Timetable

• Today, and next Friday: MB Extrasolar Today, and next Friday: MB Extrasolar planetsplanets

• Then Derek Raine Then Derek Raine

• After Xmas: Mark Sims (Life in the solar After Xmas: Mark Sims (Life in the solar system)system)


ContentsContents

• Methods for detectionMethods for detection– Doppler “wobble”Doppler “wobble”– TransitsTransits– MicrolensingMicrolensing– Direct ImagingDirect Imaging

• CharacterisationCharacterisation– StatisticsStatistics– Implications for formation scenariosImplications for formation scenarios


Useful reading / web sitesUseful reading / web sites

• Nature, Vol. 419, p. 355 (26 September 2002)Nature, Vol. 419, p. 355 (26 September 2002)• Extra-solar planets encyclopaediaExtra-solar planets encyclopaedia• California & Carnegie Planets SearchCalifornia & Carnegie Planets Search• How stuff works planet-hunting pageHow stuff works planet-hunting page

– Includes lots of animations & graphicsIncludes lots of animations & graphics

• JPL planet finding pageJPL planet finding page– Look at the science & multimedia gallery pagesLook at the science & multimedia gallery pages


What is a planet?What is a planet?

• International Astronomical Union definition –International Astronomical Union definition –

– An object orbiting a star An object orbiting a star • But see later this lecture…But see later this lecture…

– Too small for dueterium fusion to occurToo small for dueterium fusion to occur• Less than 13 times the mass of JupiterLess than 13 times the mass of Jupiter

– Formation mechanism?Formation mechanism?• Forms from a circumstellar diskForms from a circumstellar disk

– Lower mass limit – IAU decided last year that Lower mass limit – IAU decided last year that Pluto should be downgraded!Pluto should be downgraded!


A brief history of exoplanetsA brief history of exoplanets

• 1991 Wolszczan & Frail discovered planets 1991 Wolszczan & Frail discovered planets around a pulsar PSR1257+12around a pulsar PSR1257+12– Variations in arrival times of pulses suggests presence Variations in arrival times of pulses suggests presence

of three or more planetsof three or more planets– Planets probably formed from debris left after Planets probably formed from debris left after

supernova explosionsupernova explosion

• 1995 Planets found around nearby Sun-like star 1995 Planets found around nearby Sun-like star 51 Peg by Doppler “wobble” method51 Peg by Doppler “wobble” method– Most successful detection method by farMost successful detection method by far– 265 exoplanets found to date265 exoplanets found to date


Radial Velocity TechniqueRadial Velocity Technique(Doppler “Wobble”)(Doppler “Wobble”)

• Star + planet Star + planet orbit common orbit common centre of gravitycentre of gravity

• As star moves As star moves towards observer, towards observer, wavelength of light wavelength of light shortens (is blue-shortens (is blue-shifted)shifted)

• Light red-shifted Light red-shifted as star moves as star moves awayaway


Measuring Stellar Doppler shiftsMeasuring Stellar Doppler shifts• Method:Method:

– Observe star’s spectrum through a cell of iodine gasObserve star’s spectrum through a cell of iodine gas– Iodine superimposes many lines on star’s spectrumIodine superimposes many lines on star’s spectrum– Measure wavelength (or velocity) of star’s lines Measure wavelength (or velocity) of star’s lines

relative to the iodinerelative to the iodine


Measuring Stellar Doppler shiftsMeasuring Stellar Doppler shifts• Method:Method:

– Measure wavelength (or velocity) of star’s lines Measure wavelength (or velocity) of star’s lines relative to the iodinerelative to the iodine

ee = ( = (ee) / ) / ee = v = vrr / c / c

observed wavelength, observed wavelength, ee=emitted wavelength=emitted wavelength


• N.B. MN.B. M** comes from comes from

the spectralthe spectral typetype


Doppler Wobble Method: SummaryDoppler Wobble Method: Summary

• Precision of current surveys is now 1m/s:Precision of current surveys is now 1m/s:– Jupiter causes Sun’s velocity to vary by 12.5m/sJupiter causes Sun’s velocity to vary by 12.5m/s– All nearby, bright Sun-like stars are good targetsAll nearby, bright Sun-like stars are good targets

• Lots of lines in spectra, relatively inactiveLots of lines in spectra, relatively inactive

• Limited to gas planets and larger Limited to gas planets and larger – Note recently discovered “hot Neptunes” (>14MNote recently discovered “hot Neptunes” (>14MEarthEarth))

– Not yet suitable for Earth-like planetsNot yet suitable for Earth-like planets

• Length of surveys limits distances planets have been Length of surveys limits distances planets have been found from starsfound from stars– Earliest surveys started 1989Earliest surveys started 1989– Jupiter (5AU from Sun) takes 12 yrs to orbit SunJupiter (5AU from Sun) takes 12 yrs to orbit Sun– Saturn takes 30 yearsSaturn takes 30 years

• Would remain undetectedWould remain undetected

• Do not see planet Do not see planet directlydirectly


Doppler Wobble Method: SummaryDoppler Wobble Method: Summary

• Since measure K (= vSince measure K (= v* * sin i), not vsin i), not v** directly, only know directly, only know

mass in terms of the orbital inclination imass in terms of the orbital inclination i• Therefore only know the planet’s Therefore only know the planet’s minimum minimum massmass

– If i=90If i=90oo (eclipsing or (eclipsing or transitingtransiting) then know mass exactly) then know mass exactly

i=90i=9000

Orbital Orbital planeplane

ii00

Orbital Orbital planeplane


TransitsTransits

• Planets observed at inclinations near 90Planets observed at inclinations near 90o o will transit their will transit their host starshost stars


TransitsTransits

• AssumingAssuming– The whole planet passes in front of the starThe whole planet passes in front of the star– And ignoring limb darkening as negligibleAnd ignoring limb darkening as negligible

• Then the depth of the eclipse is simply the ratio Then the depth of the eclipse is simply the ratio of the planetary and stellar disk areas:of the planetary and stellar disk areas:– i.e. i.e. f / ff / f** = = RRpp

22 / / RR**2 2 = (R = (Rp p / R/ R**))22

• We measure the change in magnitude We measure the change in magnitude m, and m, and obtain the stellar radius from the spectral type obtain the stellar radius from the spectral type – Hence by converting to flux we can measure the Hence by converting to flux we can measure the

planetary radiusplanetary radius– Rem. Rem. m = mm = mtransittransit – m – m* * = 2.5 log (f = 2.5 log (f* * / f/ ftransittransit))

• (smaller number means brighter)(smaller number means brighter)


TransitsTransits

Example: first known transiting planet HD209458bExample: first known transiting planet HD209458b m = 0.017 magsm = 0.017 mags

– So (fSo (f* * / f/ ftransittransit) = 1.0158, i.e. ) = 1.0158, i.e. f=1.58%f=1.58%

– From the spectral type (G0) R=1.15RFrom the spectral type (G0) R=1.15Rsunsun

– So using So using f / ff / f** = (R = (Rp p / R/ R**))2 2 and setting fand setting f**=100%=100%

– Find RFind Rpp=0.145R=0.145Rsunsun

– Since RSince Rsunsun=9.73R=9.73RJ J thenthen

– RRpp = 1.41R = 1.41RJJ


TransitsTransits

• HD209458b more:HD209458b more:– From Doppler wobble method know From Doppler wobble method know

M sin i = 0.62MM sin i = 0.62MJJ

– Transiting, hence assume i=90Transiting, hence assume i=90oo, so , so M=0.62MM=0.62MJJ

– Density = 0.29 g/cmDensity = 0.29 g/cm33

• c.f. Saturn 0.69 g/cmc.f. Saturn 0.69 g/cm33

– HD209458b is a gas giant! HD209458b is a gas giant!


TransitsTransits

• For an edge-on orbit, transit duration is given by:For an edge-on orbit, transit duration is given by: t = (PRt = (PR**) / () / (a) a)

• Where P=period in days, a=semi-major axis of orbitWhere P=period in days, a=semi-major axis of orbit

• Probability of transit (for random orbit)Probability of transit (for random orbit)– PPtransittransit= R= R** / a / a

– For Earth (P=1yr, a=1AU), For Earth (P=1yr, a=1AU), PPtransittransit=0.5%=0.5%

– But for close, “hot” Jupiters, But for close, “hot” Jupiters, PPtransittransit==10%10%

– Of course, relative probability of detecting Earths is Of course, relative probability of detecting Earths is lower since would have to observe for up to 1 yearlower since would have to observe for up to 1 year


TransitsTransits

• AdvantagesAdvantages– Easy. Can be done with small, cheap telescopesEasy. Can be done with small, cheap telescopes

• E.g. WASP, E.g. WASP,

– Possible to detect low mass planets, including “Earths”, Possible to detect low mass planets, including “Earths”, especially from space (Kepler mission, 2008)especially from space (Kepler mission, 2008)

• DisadvantagesDisadvantages– Probability of seeing a transit is lowProbability of seeing a transit is low

• Need to observe many stars simultaneouslyNeed to observe many stars simultaneously

– Easy to confuse with starspots, binary/triple systemsEasy to confuse with starspots, binary/triple systems– Needs radial velocity measurements for confirmation, Needs radial velocity measurements for confirmation,

massesmasses


Super WASPSuper WASP

• Wide Angle Search for Planets Wide Angle Search for Planets (by transit method)(by transit method)

• First telescope located in La First telescope located in La PalmaPalma

• Operations started May ‘04Operations started May ‘04• Data stored at Leicester Data stored at Leicester • Three new planets detected!Three new planets detected!• www.superwasp.orgwww.superwasp.org• www.wasp.le.ac.www.wasp.le.ac.ukuk


Gravitational MicrolensingGravitational Microlensing

• A consequence of general relativityA consequence of general relativity• The grav. field of a relatively nearby star The grav. field of a relatively nearby star

can bend the light of a more distant object can bend the light of a more distant object as it passes in front of it, as seen from as it passes in front of it, as seen from EarthEarth

• The star doing the lensing brightens as a The star doing the lensing brightens as a resultresult

• We record this brightening, which can last We record this brightening, which can last for daysfor days

• If the lensed star has a planetary If the lensed star has a planetary companion, the characteristic lensing light companion, the characteristic lensing light curve is modifiedcurve is modified

• Signals from an Earth-like planet would be Signals from an Earth-like planet would be strong (>5%) but brief (few hours)strong (>5%) but brief (few hours)

• 4 planets found so far, including one at 5.5 4 planets found so far, including one at 5.5 Earth masses!Earth masses!


Direct detectionDirect detection

• Imaging = spectroscopy = physics: Imaging = spectroscopy = physics: composition & structurecomposition & structure

• No planet in orbit around another star has No planet in orbit around another star has been directly imagedbeen directly imaged

• Why? Why? – Stars like the Sun are billions of times brighter than Stars like the Sun are billions of times brighter than

planetsplanets– Planets and stars lie very close together on the skyPlanets and stars lie very close together on the sky

• At 10pc Jupiter and the Sun are separated by 0.5”At 10pc Jupiter and the Sun are separated by 0.5”


Direct detectionDirect detection

• Problem 1:Problem 1:– Stars bright, planets faintStars bright, planets faint

• Solution:Solution:– Block starlight with a coronagraphBlock starlight with a coronagraph

• Problem 2:Problem 2:– Earth’s atmosphere distorts starlight, reduces Earth’s atmosphere distorts starlight, reduces

resolutionresolution• Solution:Solution:

– Adaptive optics, Interferometry – difficult, Adaptive optics, Interferometry – difficult, expensiveexpensive

– Or look around very young and/or intrinsically faint Or look around very young and/or intrinsically faint stars (not Sun-like)stars (not Sun-like)


First directly imaged planet?First directly imaged planet?• 2M1207 in TW Hya 2M1207 in TW Hya

associationassociation• Discovered at ESO Discovered at ESO

VLT in ChileVLT in Chile• 25M25Mjupjup Brown dwarf + Brown dwarf +

5M5Mjup jup “planet”“planet”• Distance ~55pc Distance ~55pc • Very young cluster Very young cluster

~10M years~10M years• Physical separation Physical separation

~55AU~55AU• A brown dwarf is a A brown dwarf is a

failed starfailed star– Can this really be called Can this really be called

a planet? a planet? – Formation mechanism Formation mechanism

may be crucial!may be crucial!


Direct detection: Direct detection: White DwarfsWhite Dwarfs

• White dwarfs are the end state of stars like the SunWhite dwarfs are the end state of stars like the Sun• 1,000-10,000 times fainter than Sun-like stars1,000-10,000 times fainter than Sun-like stars

– contrast problem reducedcontrast problem reduced• Outer planets should survive evolution of Sun to Outer planets should survive evolution of Sun to

white dwarf stage, and migrate outwards white dwarf stage, and migrate outwards – more easily resolvedmore easily resolved

• Over 100 WD within 20pcOver 100 WD within 20pc– At 10pc a separation of 100AU = 10” on skyAt 10pc a separation of 100AU = 10” on sky

• I have a programme to search for planets around I have a programme to search for planets around nearby WD with the Gemini 8m telescopesnearby WD with the Gemini 8m telescopes

• We call it “DODO” – Degenerate Objects around We call it “DODO” – Degenerate Objects around Degenerate Objects or Dead Objects etcDegenerate Objects or Dead Objects etc


Direct Detection: White DwarfsDirect Detection: White Dwarfs

• The faint objects in this field could be massive planets in wide orbits around this nearby white dwarf

• The white dwarf moves relatively quickly compared to background stars in the field (see movie)

• If a faint object moves with the WD, then I would get excited

• But in this case, there is nothing, but we could have detected something as small as ~5MJup!

• www.le.ac.uk/~mbu

Proper motionsTwo images taken one year apart


What we know about What we know about

extra-solar planetsextra-solar planets • 265 planets now found265 planets now found• 25 multiple systems25 multiple systems• 33 transiting planets – can 33 transiting planets – can

directly measure radiidirectly measure radii• Unexpected population Unexpected population

with periods of 3-4 days: with periods of 3-4 days: “hot Jupiters”“hot Jupiters”

• New population from New population from transit surveys at ~2 daystransit surveys at ~2 days

• First planet with an orbit First planet with an orbit like Jupiter discovered (55 like Jupiter discovered (55 Cancri d)Cancri d)

• Is our solar system Is our solar system typical?typical?


Extra-solar planet period distributionExtra-solar planet period distribution

• Notice the “pile-Notice the “pile-up” at periods of up” at periods of 3-4 days / 0.04-3-4 days / 0.04-0.05AU0.05AU

• The most distant The most distant planets planets discovered so discovered so far are at 5-6AUfar are at 5-6AU

• New discovery New discovery of transiting of transiting planets at ~2 planets at ~2 daysdays


Extra-solar planet mass distribution Extra-solar planet mass distribution

• Mass distribution peaks Mass distribution peaks at 1-2 x mass of Jupiterat 1-2 x mass of Jupiter

• Lowest mass planet so Lowest mass planet so far: 5.5xMfar: 5.5xMEarthEarth

• Super-Jupiters (>few Super-Jupiters (>few MMJupJup) are not common) are not common– Implications for planet Implications for planet

formation theories?formation theories?– Or only exist in number at Or only exist in number at

large separation?large separation?– Or exist around massive Or exist around massive

stars?stars?


Selection effectsSelection effects

• Astronomical surveys tend to have built in biasesAstronomical surveys tend to have built in biases• These “selection effects” must be understood before we These “selection effects” must be understood before we

can interpret resultscan interpret results• The Doppler Wobble method is most sensitive to The Doppler Wobble method is most sensitive to

massive, close-in planetsmassive, close-in planets• It is not yet sensitive to planets as small as Earth, even It is not yet sensitive to planets as small as Earth, even

close-inclose-in• As orbital period increases, the method becomes As orbital period increases, the method becomes

insensitive to planets less massive than Jupiterinsensitive to planets less massive than Jupiter• The length of time that the surveys have been active The length of time that the surveys have been active

(since 1989) sets the upper orbital period limit (since 1989) sets the upper orbital period limit – Only now are analogues of Jupiter in our own Solar System Only now are analogues of Jupiter in our own Solar System

going to be foundgoing to be found


What we know about extra-solar planets:What we know about extra-solar planets:Mass versus semi-major axisMass versus semi-major axis

• Blue – exoplanetsBlue – exoplanets• Red – solar systemRed – solar system• Many of the known solar Many of the known solar

systems have ~Jupiter-systems have ~Jupiter-mass planets in small mass planets in small orbits, <0.1AUorbits, <0.1AU– Selection effect of Doppler Selection effect of Doppler

surveyssurveys• But almost no super-But almost no super-

Jupiters are found in close Jupiters are found in close orbitsorbits– Real, not a selection effectReal, not a selection effect


Eccentricity vs semi-major axis

obse

rvat

iona

l bia

sextra-solar planets

solar system planets

:

- large distribution of e (same as close binaries)

- most extra-solar planets are on orbits much more eccentric than the giant planets in the solar system: bad news for survivability of terrestrial planets

- planets on circular orbits do exist far away from star

- the planets in our own system have small eccentricities ie STABLE

- planets close to the star are tidally circularized

What we know about extra-solar planetsWhat we know about extra-solar planets


Statistics of the Doppler Wobble surveys: Statistics of the Doppler Wobble surveys: SummarySummary

• Of 2000 stars surveyed Of 2000 stars surveyed – ~5% have gas giants between 0.02AU and 5AU~5% have gas giants between 0.02AU and 5AU

• Trends suggest ~10% of stars have planets in orbits 5-7AUTrends suggest ~10% of stars have planets in orbits 5-7AU– 0.85% have hot Jupiters0.85% have hot Jupiters

• Real effectReal effect– Hot Jupiters are not massiveHot Jupiters are not massive

• Almost all have Msini~1MAlmost all have Msini~1Mjupjup or less or less• No close-in, “super-Jupiters”No close-in, “super-Jupiters”

– Mass distribution strongly peaks at 1MMass distribution strongly peaks at 1Mjupjup and falls as and falls as dN/dM~MdN/dM~M-0.7-0.7

• But surveys currently biased towards hot JupitersBut surveys currently biased towards hot Jupiters• Expect mass distribution to flatten somewhat as long periods, Expect mass distribution to flatten somewhat as long periods,

super-Jupiters are discoveredsuper-Jupiters are discovered


What about the stars themselves?What about the stars themselves?

• Surveys began by targeting sun-like stars Surveys began by targeting sun-like stars (spectral types F, G and K)(spectral types F, G and K)

• Now extended to M dwarfsNow extended to M dwarfs• Incidence of planets is greatest for late F Incidence of planets is greatest for late F

starsstars– F7-9V > GV > KV > MVF7-9V > GV > KV > MV– Few low mass M dwarfs known to have a planets Few low mass M dwarfs known to have a planets

despite ease of detectabilitydespite ease of detectability• Stars that host planets appear to be on Stars that host planets appear to be on

average more metal-rich average more metal-rich


MetallicityMetallicityThe abundance of elements heavier

than He relative to the Sun

• Overall, ~5% of solar-like stars have radial velocity –detected Overall, ~5% of solar-like stars have radial velocity –detected JupitersJupiters

• But if we take metallicity into account:But if we take metallicity into account:– >20% of stars with 3x the metal content of the Sun have planets>20% of stars with 3x the metal content of the Sun have planets

– ~3% of stars with 1/3~3% of stars with 1/3rdrd of the Sun’s metallicity have planets of the Sun’s metallicity have planets


MetallicityMetallicity• Does this result imply that planets more easily form in metal-rich Does this result imply that planets more easily form in metal-rich

environments?environments?– If so, then maybe planet hunters should be targeting metal-rich starsIf so, then maybe planet hunters should be targeting metal-rich stars– Especially if we are looking for rocky planetsEspecially if we are looking for rocky planets

• This result also implies that chances of very old lifeforms (> few This result also implies that chances of very old lifeforms (> few billion years) in the Universe are slimbillion years) in the Universe are slim– With less heavy elements available terrestrial planets may be smaller With less heavy elements available terrestrial planets may be smaller

and lower in mass than in our solar systemand lower in mass than in our solar system– Is there a threshold metallicity for life to start (e.g. ½ solar)?Is there a threshold metallicity for life to start (e.g. ½ solar)?

• BUT Sigurdsson et al. (2003, Science, 301, 193) claim that a milli-BUT Sigurdsson et al. (2003, Science, 301, 193) claim that a milli-second pulsar in globular M4 has a Jupiter size companionsecond pulsar in globular M4 has a Jupiter size companion– Claim based on timing anomaliesClaim based on timing anomalies– If true, then planets may have been forming 12 billion years ago in a If true, then planets may have been forming 12 billion years ago in a

very metal-poor environment (<0.1 x solar)very metal-poor environment (<0.1 x solar)– Alternatively, planet may have formed from debris of supernova Alternatively, planet may have formed from debris of supernova

explosion that created the pulsarexplosion that created the pulsar– Or planet does not exist, timing anomalies have another causeOr planet does not exist, timing anomalies have another cause


Planet formation Planet formation scenariosscenarios

• There are two main models which have been proposed to• describe the formation of the extra-solar planets:• Planets form from dust which agglomerates into cores which then

accrete gas from a disc. • A gravitational instability in a protostellar disc creates a number of

giant planets.• Both models have trouble reproducing both the observed

distribution of extra-solar planets and the solar-system.


Gas accretion onto coresGas accretion onto cores

• Planetary cores form through the agglomeration of dust into grains, pebbles, rocks and planetesimals within a gaseous disc

• At the smallest scale (<1 cm) cohesion occurs by non-gravitational forces e.g. chemical processes.

• On the largest scale (>1 km) gravitational attraction will dominate.

• On intermediate scales the process is poorly understood. • These planetesimals coalesce to form planetary cores and for

the most massive cores these accrete gas to form the giant planets.

• Planet formation occurs over 107 yrs.


Gravitational instabilityGravitational instability

• A gravitational instability requires a sudden change in disc properties on a timescale less than the dynamical timescale of the disc.

• Planet formation occurs on a timescale of 1000 yrs.• A number of planets in eccentric orbits may be formed.• Sudden change in disc properties could be achieved by cooling

or by a dynamical interaction.• Simulations show a large number of planets form from a single

disc.• Only produces gaseous planets – rocky (terrestrial) planets are

not formed.• Is not applicable to the solar system.


Where do the hot Jupiters come from?Where do the hot Jupiters come from?

• No element will condense within ~0.1AU of a No element will condense within ~0.1AU of a star since T>1000Kstar since T>1000K

• Planets most likely form beyond the “ice-line”, Planets most likely form beyond the “ice-line”, the distance at which ice formsthe distance at which ice forms– More solids available for building planetsMore solids available for building planets– Distance depends on mass and conditions of proto-Distance depends on mass and conditions of proto-

planetary disk, but generally >1AUplanetary disk, but generally >1AU• Hot Jupiters currently at ~0.03-0.04AU cannot Hot Jupiters currently at ~0.03-0.04AU cannot

have formed therehave formed there• Migration!Migration!


Planetary migrationPlanetary migration• Planets migrate inwards and stop when disk Planets migrate inwards and stop when disk

is finally clearedis finally cleared• If migration time < disk lifetimeIf migration time < disk lifetime

– Planets fall into starPlanets fall into star– Excess of planets at 0.03-0.04AU is evidence of Excess of planets at 0.03-0.04AU is evidence of

a stopping mechanism in some casesa stopping mechanism in some cases– Nature of stopping mechanism unclear: tides? Nature of stopping mechanism unclear: tides?

magnetic cavities? mass transfer?magnetic cavities? mass transfer?

• Large planets will migrate more slowlyLarge planets will migrate more slowly– Explanation for lack of super-Jupiters in close Explanation for lack of super-Jupiters in close

orbitsorbits


Planetary migration & terrestrial planetsPlanetary migration & terrestrial planets

• Migrating giant planets will be detrimental to terrestrial Migrating giant planets will be detrimental to terrestrial planet survivability, if they both form at same timeplanet survivability, if they both form at same time– Planets interior to a migrating giant planet will be disrupted and Planets interior to a migrating giant planet will be disrupted and

lost lost – Of course, these small planets may also migrate into star!Of course, these small planets may also migrate into star!

• If terrestrial planets can only survive when migration If terrestrial planets can only survive when migration doesn’t take place through their formation zone (few AU), doesn’t take place through their formation zone (few AU), – then 3%-20% of planet forming systems will possess themthen 3%-20% of planet forming systems will possess them

• Alternatively, terrestrial planet formation may occur after Alternatively, terrestrial planet formation may occur after dissipation of gas in proto-planetary disk (after 10dissipation of gas in proto-planetary disk (after 1077 years) years)– Disruption by a migrating giant planet unlikelyDisruption by a migrating giant planet unlikely– Almost all planet-forming stars will have terrestrial planets Almost all planet-forming stars will have terrestrial planets


The future: towards other EarthsThe future: towards other Earths

• Pace of planet discoveries will increase in Pace of planet discoveries will increase in next few yearsnext few years

• Radial velocity surveys will reveal outer giant Radial velocity surveys will reveal outer giant planets with long periods like our own Solar planets with long periods like our own Solar SystemSystem

• Transit surveys will reveal planets smaller Transit surveys will reveal planets smaller than Saturn in close orbitsthan Saturn in close orbits

• First direct images will be obtainedFirst direct images will be obtained• But the greatest goal is the detection of other But the greatest goal is the detection of other

EarthsEarths


Towards other EarthsTowards other Earths

TelescopeTelescope MethodMethod DateDateCorotCorot TransitsTransits 20072007

KeplerKepler TransitsTransits 20082008

GAIAGAIA AstrometryAstrometry 20122012

SIMSIM InterferometryInterferometry 2012-15 (?)2012-15 (?)

Darwin/TPFDarwin/TPF InterferometryInterferometry 2015-20 (?)2015-20 (?)

50m ELT50m ELT ImagingImaging 2019 2019


Towards Other Earths: Habitable ZonesTowards Other Earths: Habitable Zones

• Habitable zone defined as where liquid water existsHabitable zone defined as where liquid water exists• Changes in extent and distance from star according Changes in extent and distance from star according

to star’s spectral type (ie temperature)to star’s spectral type (ie temperature)• It is possible for rocky planets to exist in stable orbits It is possible for rocky planets to exist in stable orbits

of habitable zones of known hot Jupiter systemsof habitable zones of known hot Jupiter systems– If they were not previously cleared out by migrationIf they were not previously cleared out by migration

Left: courtesy Prof. Keith Horne, St.Andrews

Right: courtesy Prof. Barry Jones, Open


Towards Other Earths: BiomarkersTowards Other Earths: Biomarkers

• So we find a planet So we find a planet with the same mass as with the same mass as Earth, and in the Earth, and in the habitable zone:habitable zone:– How can we tell it How can we tell it

harbours life?harbours life?

• Search for biomarkersSearch for biomarkers– WaterWater– OzoneOzone– AlbedoAlbedo


Direct detection: proto-planetary disksDirect detection: proto-planetary disks

• Dust disk around Dust disk around FomalhautFomalhaut

• Sub-mm image taken Sub-mm image taken from James Clarke from James Clarke Maxwell telescope on Maxwell telescope on HawaiiHawaii

• Disk has a hole in Disk has a hole in centre like a doughnutcentre like a doughnut

• Part of disk appears to Part of disk appears to be perturbedbe perturbed


Direct detection: proto-planetary disksDirect detection: proto-planetary disks

• Disk is 200 million Disk is 200 million years oldyears old

• Like the early Solar Like the early Solar SystemSystem

• Is a planet Is a planet perturbing the disk & perturbing the disk & forming the hole?forming the hole?


Direct detection: Epsilon EridaniDirect detection: Epsilon Eridani• Young, Sun-like star Young, Sun-like star

only 3pc awayonly 3pc away• Dust disk is clumpyDust disk is clumpy• Clumps seen to Clumps seen to

rotaterotate• Requires presence of Requires presence of

at least one Jovian at least one Jovian planetplanet

• 1.55M1.55MJupJup companion companion confirmed by HST confirmed by HST measurementsmeasurements

Life in the Universe: Extra-solar planets

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