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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 Tuesday 11am: MB Extrasolar Today and Tuesday 11am: MB Extrasolar planetsplanets
• Then Mark Sims (Life in the solar system)Then Mark Sims (Life in the solar system)
Dr. Matt Burleigh 3677: Life in the Universe
ContentsContents
• Methods for detectionMethods for detection– Doppler “wobble”Doppler “wobble”– TransitsTransits– Direct ImagingDirect Imaging
• CharacterisationCharacterisation– StatisticsStatistics– Implications for formation scenariosImplications for formation scenarios
Dr. Matt Burleigh 3677: Life in the Universe
Useful reading / web sitesUseful reading / web sites
• 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
Dr. Matt Burleigh 3677: Life in the Universe
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!
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
• N.B. MN.B. M** comes from comes from
the spectralthe spectral typetype
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
TransitsTransits
• Planets observed at inclinations near 90Planets observed at inclinations near 90o o will transit their will transit their host starshost stars
Dr. Matt Burleigh 3677: Life in the Universe
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)
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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!
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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 Palma, second in South AfricaPalma, second in South Africa
• Operations started May ‘04Operations started May ‘04• Data stored and processed at Data stored and processed at
Leicester Leicester • >20 new planets detected!>20 new planets detected!• www.superwasp.orgwww.superwasp.org• www.wasp.le.ac.www.wasp.le.ac.ukuk
Dr. Matt Burleigh 3677: Life in the Universe
Direct detectionDirect detection
• Imaging = spectroscopy = physics: Imaging = spectroscopy = physics: composition & structurecomposition & structure
• DifficultDifficult• 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”
Dr. Matt Burleigh 3677: Life in the Universe
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)
Dr. Matt Burleigh 3677: Life in the Universe
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!
Dr. Matt Burleigh 3677: Life in the Universe
First directly imaged planetsFirst directly imaged planets• 3 planets around 3 planets around
HR8799, 130 light HR8799, 130 light years away (40pc)years away (40pc)
• Young (60million years Young (60million years old)old)
• Three planets at 24, 38 Three planets at 24, 38 and 68AU separationand 68AU separation– In comparison, Jupiter is In comparison, Jupiter is
at 5AU and Neptune at at 5AU and Neptune at 30AU)30AU)
• Masses of 7Mjup, Masses of 7Mjup, 10Mjup and 10Mjup10Mjup and 10Mjup
• Also, a 3Mjup planet Also, a 3Mjup planet around Fomalhaut, at a around Fomalhaut, at a separation of 120AU separation of 120AU
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
What we know about What we know about
extra-solar planetsextra-solar planets • 328 planets now found328 planets now found• 34 multiple systems34 multiple systems• 52 transiting planets – can 52 transiting planets – can
directly measure radiidirectly measure radii• Unexpected population with Unexpected population with
periods of 2-4 days: “hot periods of 2-4 days: “hot Jupiters”Jupiters”
• Planet with orbits like Jupiter Planet with orbits like Jupiter discovered (eg 55 Cancri d)discovered (eg 55 Cancri d)
• Is our solar system typical?Is our solar system typical?
Dr. Matt Burleigh 3677: Life in the Universe
Extra-solar planet period distributionExtra-solar planet period distribution• Notice the “pile-up” Notice the “pile-up”
at periods of 2-4 at periods of 2-4 days / 0.04-0.05AUdays / 0.04-0.05AU
• The most distant The most distant planets discovered planets discovered by radial velocities by radial velocities so far are at 5-6AUso far are at 5-6AU
• Imaging surveys Imaging surveys finding very wide finding very wide orbit planetsorbit planets
Dr. Matt Burleigh 3677: Life in the Universe
Extra-solar planet mass distribution Extra-solar planet mass distribution • Mass distribution peaks at 1-Mass distribution peaks at 1-
2 x mass of Jupiter2 x mass of Jupiter• Lowest mass planet so far: Lowest mass planet so far:
5.5xM5.5xMEarthEarth
• Super-Jupiters (>few MSuper-Jupiters (>few MJupJup) )
are not commonare 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?
Dr. Matt Burleigh 3677: Life in the Universe
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 can These “selection effects” must be understood before we can
interpret resultsinterpret results– The Doppler Wobble method is most sensitive to massive, close-in The Doppler Wobble method is most sensitive to massive, close-in
planets, as is the Transit methodplanets, as is the Transit method– Imaging surveys sensitive to massive planets in very wide orbits Imaging surveys sensitive to massive planets in very wide orbits
(>10AU)(>10AU)• These methods are not yet sensitive to planets as small as These methods are not yet sensitive to planets as small as
Earth, even close-in Earth, even close-in • As orbital period increases, the Doppler Wobble method As orbital period increases, the Doppler Wobble method
becomes insensitive to planets less massive than Jupiterbecomes insensitive to planets less massive than Jupiter• The length of time that the DW surveys have been active The length of time that the DW 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 going to Only now are analogues of Jupiter in our own Solar System going to
be foundbe found– But imaging surveys can find the widest planetsBut imaging surveys can find the widest planets
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
– 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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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.
Dr. Matt Burleigh 3677: Life in the Universe
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.
Dr. Matt Burleigh 3677: Life in the Universe
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.
Dr. Matt Burleigh 3677: Life in the Universe
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!
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
Towards other EarthsTowards other Earths
TelescopeTelescope MethodMethod DateDateCorotCorot TransitsTransits 20072007
KeplerKepler TransitsTransits 20082008
GAIAGAIA AstrometryAstrometry 20122012
SIMSIM InterferometryInterferometry 2012-15 (?)2012-15 (?)
PlatoPlato TransitsTransits 20172017
Darwin/TPFDarwin/TPF InterferometryInterferometry 2020+ (?)2020+ (?)
50m ELT50m ELT ImagingImaging 2019 2019
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe
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
Dr. Matt Burleigh 3677: Life in the Universe