3677 life in the universe: extra-solar planets

DEPARTMENT OF PHYSICS AND ASTRONOMY

3677 Life in the Universe:3677 Life in the Universe:Extra-solar planetsExtra-solar planets

Dr. Matt BurleighDr. Matt Burleighwww.star.le.ac.uk/mrb1/lectures.htmlwww.star.le.ac.uk/mrb1/lectures.html

Dr. Matt Burleigh 3677: Life in the Universe


Course outlineCourse outline

• Lecture 1Lecture 1– Definition of a planetDefinition of a planet– A little historyA little history– Pulsar planetsPulsar planets– Doppler “wobble” (radial velocity) techniqueDoppler “wobble” (radial velocity) technique

• Lecture 2Lecture 2– Transiting planetsTransiting planets– Transit search projects Transit search projects – Detecting the atmospheres of transiting planets: Detecting the atmospheres of transiting planets:

secondary eclipses & transmission spectroscopysecondary eclipses & transmission spectroscopy– Transit timing variationsTransit timing variations



• Lecture 3Lecture 3– MicrolensingMicrolensing– Direct ImagingDirect Imaging– Other methods: astrometry, eclipse timingOther methods: astrometry, eclipse timing– Planets around evolved starsPlanets around evolved stars

• Lecture 4Lecture 4– Statistics: mass and orbital distributions, incidence of solar Statistics: mass and orbital distributions, incidence of solar

systems, etc.systems, etc.– Hot JupitersHot Jupiters– Super-EarthsSuper-Earths– Planetary formationPlanetary formation– Planetary atmospheresPlanetary atmospheres– The host starsThe host stars



• Lecture 5Lecture 5– The quest for an Earth-like planetThe quest for an Earth-like planet– Habitable zonesHabitable zones– Results from the Kepler missionResults from the Kepler mission

• How common are rocky planets?How common are rocky planets?• Amazing solar systemsAmazing solar systems

– BiomarkersBiomarkers– Future telescopes and space missionsFuture telescopes and space missions


Useful numbersUseful numbers• RRSunSun = 6.995x10 = 6.995x1088m m

• RRjupjup = 6.9961x10 = 6.9961x1077m ~ 0.1Rm ~ 0.1RSunSun

• RRnep nep = 2.4622x10= 2.4622x1077m ~ 4Rm ~ 4Rearthearth

• RRearth earth = 6.371x10= 6.371x1066m ~ 0.1Rm ~ 0.1Rjup jup ~ 0.01R~ 0.01RSunSun

• MMSunSun= 1.989x10= 1.989x103030kgkg

• MMjupjup= 1.898x10= 1.898x102727kg ~ 0.001Mkg ~ 0.001MSunSun = 317.8M = 317.8Mearthearth

• MMnepnep= 1.02x10= 1.02x102626kg ~ 5x10kg ~ 5x10-5-5MMSunSun ~ 0.05M ~ 0.05Mjupjup = 17.15M = 17.15Mearthearth

• MMearthearth= 5.97x10= 5.97x102424kg = 3x10kg = 3x10-6-6MMSunSun = 3.14x10 = 3.14x10-3-3MMjupjup

• 1AU = 1.496x101AU = 1.496x101111mm• 1 day = 86400s1 day = 86400s


Exoplanet count 10/11/13 Exoplanet count 10/11/13 (exoplanet.eu)(exoplanet.eu)

• 1039 confirmed planets1039 confirmed planets– In 787 planetary systems In 787 planetary systems – 173 multi-planet systems173 multi-planet systems– 873 radial velocity detected planets873 radial velocity detected planets– 425 transiting planets425 transiting planets– 41 directly imaged41 directly imaged– ““Confirmed” = have “measured” massesConfirmed” = have “measured” masses

• Unexpected population with periods Unexpected population with periods of <1 to ~4 days: of <1 to ~4 days: ““hot Jupitershot Jupiters””

• Planets with orbits like Jupiter Planets with orbits like Jupiter discovered (eg 55 Cancri d)discovered (eg 55 Cancri d)

• Smallest planets: Smallest planets: – Kepler-20e: 0.87RKepler-20e: 0.87Rearth earth , , – Alpha Cen Bb M sin i > 1.1MAlpha Cen Bb M sin i > 1.1Mearthearth


Hit 1000 Hit 1000 exoplanet markexoplanet mark

Transiting planets in blue


Eccentricity of exoplanet orbitsEccentricity of exoplanet orbits

Solar systems with highly eccentric planets may be bad news for life


Extra-solar planet period distributionExtra-solar planet period distribution• Notice the Notice the ““pile-uppile-up”” at at

periods of <1 to ~4 periods of <1 to ~4 days / 0.04-0.05AUdays / 0.04-0.05AU

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

• Imaging surveys Imaging surveys finding very wide finding very wide (>10AU) orbit planets(>10AU) orbit planets

• Orange are “hot Orange are “hot Jupiters”Jupiters”

• Yellow is Jupiter-Yellow is Jupiter-mass in Jupiter-like mass in Jupiter-like orbitsorbits


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

can interpret resultscan interpret results– The Doppler Wobble method is most sensitive to massive, The Doppler Wobble method is most sensitive to massive,

close-in planets, as is the Transit methodclose-in planets, 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 These methods are not yet sensitive to planets as small

as Earth, even close-in as 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 becomes insensitive to planets less massive than JupiterJupiter

• The length of time that the DW surveys have been The length of time that the DW surveys have been active (since 1989) sets the upper orbital period limit active (since 1989) sets the upper orbital period limit – But imaging surveys can find the widest planetsBut imaging surveys can find the widest planets


““Hot Jupiter” planetsHot Jupiter” planets

• Doppler Wobble and transit surveys Doppler Wobble and transit surveys find many gas giants in orbits of <1 to find many gas giants in orbits of <1 to ~4 days~4 days– cf Mercury’s orbit is 80 dayscf Mercury’s orbit is 80 days

• These survey methods are These survey methods are biasedbiased towards finding themtowards finding them– Larger Doppler Wobble signalLarger Doppler Wobble signal– Greater probability of transitGreater probability of transit

• These planets are heated to >1000These planets are heated to >1000ooF F on “day” sideon “day” side– And are “tidally locked” like the MoonAnd are “tidally locked” like the Moon– Causes extreme weather conditionsCauses extreme weather conditions


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 confirmed Lowest mass confirmed

planet so far: Alpha Cen Bb planet so far: Alpha Cen Bb M sin i M sin i =1.1xM=1.1xMEarthEarth

• 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?


A continuum of planet massA continuum of planet mass

1’000’000

Red box indicates “Super-Earths”


Transiting planets in blueRed box indicates “Super-Earths”


Super-EarthsSuper-Earths

• In the solar system, there is no planet with a mass and radius between that of Earth and Neptune/Uranus

• But we see many such exoplanets

• What are they? Gas giants, terrestrial, or something else?


What are exoplanetsWhat are exoplanetsmade of?made of?

?? ??

ice mantle/volatile envelope

thin atmosphere

hydrogen/helium envelope

solid core (rocks+metals)

?? ??


??



thin atmosphere



telluric super-Earths?

ocean planets?

mini Neptunes?

gas dwarfs?


??



thin atmosphere



telluric super-Earths?

ocean planets?

mini Neptunes?

gas dwarfs?

HD HD 149026b149026b


How common are gas giants?How common are gas giants?

• Radial velocity surveysRadial velocity surveys– ~10% of FGK stars have gas ~10% of FGK stars have gas

giants between 0.02AU and giants between 0.02AU and 5AU5AU

– At least 20% have gas giants At least 20% have gas giants in wider orbitsin wider orbits

• Known population will grow as Known population will grow as radial velocity surveys cover radial velocity surveys cover longer periods, & direct imaging longer periods, & direct imaging improvesimproves

– <0.1% have Hot Jupiters<0.1% have Hot Jupiters• Hot Jupiters are easy to discover, Hot Jupiters are easy to discover,

but in fact are rarebut in fact are rare

• How many have How many have Earths…..?Earths…..?


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

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

• Now extended to M dwarfs (<1 MNow extended to M dwarfs (<1 Msunsun) and ) and subgiants (>1.5Msubgiants (>1.5Msunsun))– Subgiants are the descendants of A starsSubgiants are the descendants of A stars

• Incidence of planets is greatest for late Incidence of planets is greatest for late F starsF stars– F7-9V > GV > KV > MVF7-9V > GV > KV > MV

• More massive stars tend to have more More massive stars tend to have more massive planetsmassive planets


MetallicityMetallicityThe abundance of elements

heavier than He relative to the Sun

• Overall, ~10% of solar-like stars have radial velocity –detected JupitersOverall, ~10% of solar-like stars have radial velocity –detected Jupiters• 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 gas giants>20% of stars with 3x the metal content of the Sun have gas giants– ~3% of stars with 1/3~3% of stars with 1/3rdrd of the Sun of the Sun’’s metallicity have gas giantss metallicity have gas giants

• Does this result imply that planets more easily form in metal-rich environments?Does this result imply that planets more easily form in metal-rich environments?– Possibly true for gas giantsPossibly true for gas giants– But Kepler results suggest super-Earths & terrestrial planets equally common around lower But Kepler results suggest super-Earths & terrestrial planets equally common around lower

metallicity stars!metallicity stars!


Planet formation Planet formation scenariosscenarios

• There are two main models which have been proposed to• describe the formation of the extra-solar planets:

– (I) Planets form from dust which agglomerates into cores which then accrete gas from a disc.

– (II) 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.


Accretion onto coresAccretion 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

• The most massive cores 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.• Could explain the directly imaged HR8799

system


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

• No element will condense within ~0.1AU of a star since T>1000KNo element will condense within ~0.1AU of a star since T>1000K• Planets most likely form beyond the Planets most likely form beyond the ““ice-lineice-line””, the distance at which , the distance at which

ice formsice forms– More solids available for building planetsMore solids available for building planets– Distance depends on mass and conditions of proto-planetary disk, but Distance depends on mass and conditions of proto-planetary disk, but

generally >1AUgenerally >1AU• Hot Jupiters currently at ~0.03-0.04AU cannot have formed thereHot Jupiters currently at ~0.03-0.04AU cannot have formed there

– MigrationMigration: Planets migrate inwards and stop when disk is finally cleared: Planets migrate inwards and stop when disk is 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 a stopping mechanismExcess of planets at 0.03-0.04AU is evidence of a stopping mechanism

• tides? magnetic cavities? mass transfer?tides? magnetic cavities? mass transfer?

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

3677 life in the universe: extra-solar planets

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