Extrasolar planets

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Extrasolar planets
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In this lecture

• Planets, Stars and Brown dwarves
• Recap planetary formation stages
• A missing population?
• Planet detection methods
• Radial velocity
• Transits
• Microlensing
• Direct Imaging
• Pulsar timing
• Astrometry
• Detection limits
• weird systems detected
• Giant branch stars
• Hot Jupiters
• Typical systems?
• Prevalence of planets
• Brown dwarf desert
• Orbital behavior
• The Future

www.exoplanet.eu
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Planets and Brown Dwarves

• Star
• Interstellar cloud collapse
• Burns hydrogen
• Mass gt 75 MJ
• Brown dwarf
• Interstellar cloud collapse
• Burns deuterium runs out fast
• Mass gt 13 MJ
• Jeans mass
• Determines the mass of the smallest
• object a collapsing cloud can form
• At T10K
• Densities not high enough to form

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Planetary Formation Stages Revisited

• Collapse of molecular cloud
• In lt 106 year
• Development of accretion disk
• Create central gap at stars co-rotation distance
• Form planetesimals
• Bodies grow by pair-wise accretion
• Self-gravity becomes important (r1km)
• Oligarchic growth of largest objects
• Gas Giant planets
• Cores of 10 ME capture large gas envelopes
• Opens gap in disk slowly migrates inward
• Gaseous component dissipates lt 10 Myr

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• Disk
• Gaseous component dissipates lt 10 Myr
• Debris disks remain 200 Myr
• Planets can migrate further by scattering small
  bodies
• Large collisions still frequent

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• Inner planets are composed of rock and iron
• Heated by radioactive decay

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• Gas giant planets Jupiter and Saturn
• Similar rock/ice cores of about 10 earth masses
• Large hydrogen envelopes molecular and metallic
• Ice Giant Planets Uranus and Neptune
• Rocky cores
• Water and Ammonia interiors
• Large hydrogen molecular envelopes

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• Another type of planet?
• We have a gap in our solar system
• An entire class of planet is unrepresented

?

• More than 10 Earth Masses results in a gas giant
• We know what planets in the 0-1 Earth Mass range
  look like
• What about the 1-10 Earth Mass range -
  Super-Earths?

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Planetary Detection Radial Velocity

• Star and planet orbit systems center of mass
• Stars motion small compared to planets as M gtgt
  MP
• Star appears to approach and recede from us
  periodically
• Causes Doppler blue- and red-shifting of spectral
  lines
• Very precise spectral measurements needed
  iodine cell
• Current technology 1-3 ms-1
• Invert wavelength shift for velocity
• e.g. max velocity of the sun due to Jupiter is
  12.5 ms-1
• In reality fit signal for period, MP sin(i) and
  eccentricity
• Cannot solve for inclination so mass is a lower
  limit

• Star mass
• Stars bigger (hotter) than the sun not enough
  absorption features
• Stars smaller than the sun dont have a strong
  signal
• Technique sensitive to big planets with small
  periods

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Planetary Detection Transits

• Planet passes in front of the star as seen from
  Earth
• Inclination must be close to 90º
• cos(i) lt (RRP)/a
• If a1AU, R1 solar radius then probability
  0.4
• If a0.05AU, R1 solar radius then probability
  8
• Limits on transit technique
• Brightness measurements precise to 0.1
• (RP/R)2 gt 10-3 or Rp gt 0.032 R
• e.g. for our solar system RPgt 22,000 km
• Gas/ice giants, not terrestrial planets
• Benefits of the transit technique
• Gives you a size
• All discoveries are good radial velocity
  candidates
• Gets you a mass (and mean density)
• Inclination is constrained

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Planetary Detection Direct Imaging

• 1AU separation at 1 parsec distance is 1
  arc-second
• Definition of a par-sec (3.086x1016 m)
• Angular separation is easy in ground-based
  telescopes
• Problem is the glare from the main star
• Use coronagraph
• Speckle imagery
• Longer wavelengths are better for contrast
• But worse for resolution

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• Other methods
• Microlensing
• Pulsar timing
• Supernova remnant
• Very precise but rarely used
• Astrometry

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Whats Detectable?

• Closer objects are easier
• Radial velocity
• Transits
• Pulsar timing
• Further objects are easier
• Astrometry
• Direct imaging
• Microlensing not affected by distance
• Heavy planets are always easier
• Greater tug on star
• Bigger signal easier to detect
• Current sensitivity limits up to gt MSATURN
• At reasonable distances from the star
• Current observational record of 10-15 years is a
  big limitation
• gt97 of these planets are within Jupiters
  distance

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• Relative performance of various methods

www.exoplanet.eu
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• Were on the verge of an explosion in exoplanet
  discovery rate

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Prevalence of Planets

• Population
• Total of 450 extrasolar planets
• Discovery rate 2 week-1
• Radial velocity searches show planets around 10
  of stars
• Stars are generally solar type
• Selection effect best signal / spectral feature
  trade off
• Planets are generally close in
• Selection effect most easily detected, short
  observational record

• Most planets orbit metal rich(er) stars
• Metals for astronomers means Z gt 2
• The sun is also metal rich

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Geoff Marcys internet page.
Detection of weird systems

• Planets around supergiants
• Outward migration due to stellar mass loss
• Some planets do survive
• Many hot Jupiters
• Massive planets found close to star
• Pile up at periods of 4 days (0.05 AU)
• Can only be the result of orbital migration
• Large cores and gas envelopes require cool Ts

www.exoplanet.eu
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• What about these super-Earths
• Only two to talk about so far but there will soon
  be many more

Uranus
Earth
www.exoplanet.eu
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Properties of Super-Earths

• Size vs Mass
• Size from transits
• Mass from radial velocities
• Equation of state needed to get size/mass
  relation
• Usually a non-unique solution
• This is a very simplistic approach
• With some knowledge of planetary science we can
  eliminate a few possibilities

Swift et al., AstroPH, 2010
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• Super-Earths have a minimum radii
• Collision stripping can build super-Mercurys
• But material cant escape easily from such a
  large planet
• Theres no process that actually get you to a
  pure iron planet
• i.e. super-Earths are not like asteroids

Green 1/10 projectile/target Red1/4
projectile/target Blue 1/1 projectile/target
Earth
Pure iron
Marcus et al., ApJ 2010
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Properties of Super-Earths

• Plate tectonics?
• Earth yes
• Venus No
• What about bigger planets?
• Competing factors
• Higher gravity compresses and strengthens the
  lithosphere
• More vigorous convection more shear stress to
  move and deform plates
• Also, Faster convection means plates are younger
  at subduction zones
• Cooled enough (dense enough) to subduct?

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• Valencia et al. 2010
• Argues lithosphere will be thinner on larger
  planets
• Counters the strengthening due to gravity
• Super-Earths have plate tectonics
• Earth is borderline for plate tectonics in this
  model
• Requires wet conditions

Increasing shear stress
Dry
Wet
Fault strength vs planetary mass
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• Alternate viewpoint
• ONeill and Lenardic 2007

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• Two case examples
• CaRoT 7b
• GJ 1214b
• Both masses and sizes known
• A few other super-Earth planets where we only
  have the mass are known

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• GJ1214b
• Mass 6.6 Earth Masses
• Radius 2.7 Earth Radii
• Density 1900 kg/m3
• Composition mostly water
• H/He envelope probably required
• Temperatures 450K
• Probably a very deep ocean

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• CaRoT7b
• Mass 4.8 Earth Masses
• Radius 1.7 Earth Radii
• Density Earth
• Composition is rocky
• Temperatures 2000K
• Plate tectonics here?

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The Story
4.5 Billion years

• Weve seen 400 years worth of this story 1st
  hand
• One ten millionth of solar system history
• Equivalent to watching 1 milli-second of a three
  hour movie
• Which we use to figure out the plot, the actors,
  and what plays out at each location

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Dramaticus Personae
Volcanoes
Tectonics
Impact cratering
Eolian
Fluvial
Glacial
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On location
Mercury
Enceladus
Titan
Mars
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The Story
4.5 Billion years

• Weve seen 400 years worth of this story 1st
  hand
• One ten millionth of solar system history
• Equivalent to watching 1 milli-second of a three
  hour movie
• Which we use to figure out the plot, the actors,
  and what plays out at each location

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Masses and the Brown Dwarf Desert

• Binary stars are common 50
• Initially controversial as to whether these
  orbiting bodies were
• Low mass binary companions (brown dwarves)
• Planets formed from an accretion disk
• Mass distribution is heavily skewed to lower
  masses
• Not (for once) a selection effect
• Low mass bodies are harder to detect yet we still
  find more of them
• Follows a power law
• Indicates large population of smaller objects
• The brown dwarf desert
• Few bodies between 10 and 80 MJ
• This low-mass population probably had a separate
  formation mechanism

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Orbital Behavior

• 26 multiple planet systems
• E.g. Upsilon Andromedea
• Some with mean motion-resonances
• Hot-Jupiters/Neptunes
• Pile up at 4 day periods
• Eccentricities very small
• Tidal damping has been very effective
• Other systems
• Unusually high eccentricities
• Unexplained passing stars, disk interactions
• Eccentric gas giants are a problem for
  terrestrial planets

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Physical Properties

• Not much to go on
• Future missions will provide spectra
• Planets that both transit and have radial
  velocity measurements indicate gas giant
  densities
• Hot Neptunes now also known
• Overall understanding hasnt improved much in the
  past 10 years
• Modeling of expected atmospheric behavior yields
  some exotic results

Gillon et al., 2007
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Future Missions

• Terrestrial Planet Finder (TPF)
• Direct imaging and spectra
• 1. Coronagraph
• 2. Interferometer
• KEPLER
• Transit observatory 1m mirror
• Launch 2009
• Monitor 105 stars for 4 years
• Space Interferometry Mission (SIM)
• Astrometric observatory
• 4 µ arcsec
• Will discover a lot of Jupiters
• Probably not terrestrial planets
• DARWIN
• ESA mission
• Direct imaging and spectra

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• Hopes for TPF and DARWIN
• Non-equilibrium component in atmospheres may be
  signs of life

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Planetary Detection Microlensing

• Planet and star pass in front of a background
  star
• Masses act as gravitational lenses
• Background star amplified
• Spread into Einstein ring (unresolved)

• Limitations
• Once off observation
• Lensing from planets lasts less than a day
• Retrieves mass, but not period
• Need dense background of stars
• E.g. galactic bulge
• Low mass planet detected
• 5.5 Earth mass around a cool red dwarf (a
  super-Pluto)

Beaulieu et al., 2006
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Planetary Detection Pulsar Timing

• Neutron star with magnetic and rotational axes
  unaligned
• Rotation causes energetic beams to sweep through
  space
• We measure very regular radio pulses
• Pulsars are thought to be the best time-keepers
  in the universe, but
• some have pulses speeding up and slowing down
• Pulses take longer to travel here when the star
  is further away
• Rotation around a center of mass causes distance
  changes
• Pulse frequency oscillates with orbit
• Analogous to Doppler effect
• Technique also applied to pulsating white dwarves
• Very precise technique
• Can detect planets down to below 0.1 Earth Masses
• Smallest planet to date PSR 125712 b (2 Earth
  Mass)
• Drawbacks
• Pulsars are relatively rare
• Planets probably affected by supernovae blast
  wave

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Planetary Detection Astrometry

• Look directly for positional shifting
• Remove effects of proper motion

• Angular motion given by
• E.g. Jupiter around the sun at 10pc 0.5
  milli-arcsec
• Doable with interferometers
• Most sensitive to large orbits
• but you have to wait a long time
• Good mass estimates but poor eccentricity
  control
• Still no planet detected this way
• but it can pin down inclinations of known planets

Benedict et al., 2002
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• Type I migration spiral density waves (fast)
• Torque exerted at Lindblad resonances
• Torque on outer disk is stronger than inner disk
• So planet migrates inwards on timescale
• Not very long!
• Type II migration gap opens (slower)
• Planet moves inward with the viscous gas
  timescales of 106 years
• Independent of planet mass
• Disk is truncated at the inner edge.
• Planets can stall there

Phil Armitages internet page.

• Close proximity high mass
• Ideal candidates for radial velocity and transit
  obs
• Transit gives sin(i) and size
• Radial velocity gives mass, eccentricity and
  period
• \We can get density values
• Objects are definitely gas giants
• Hot Neptunes now also discovered