Life in the Universe: Extra-solar planets

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Life in the UniverseExtra-solar planets

• Dr. Matt Burleigh
• www.star.le.ac.uk/mbu

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3677 Timetable

• Today and Tuesday 11am MB Extrasolar planets
• Then Mark Sims (Life in the solar system)

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Contents

• Methods for detection
• Doppler wobble
• Transits
• Direct Imaging
• Characterisation
• Statistics
• Implications for formation scenarios

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Useful reading / web sites

• Extra-solar planets encyclopaedia
• California Carnegie Planets Search
• How stuff works planet-hunting page
• Includes lots of animations graphics
• JPL planet finding page
• Look at the science multimedia gallery pages

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What is a planet?

• International Astronomical Union definition
• An object orbiting a star
• But see later this lecture
• Too small for dueterium fusion to occur
• Less than 13 times the mass of Jupiter
• Formation mechanism?
• Forms from a circumstellar disk
• Lower mass limit IAU decided that Pluto should
  be downgraded!

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A brief history of extra-solar planets

• In the 16th century the Italian philosopher
  Giordano Bruno said that the fixed stars are
  really suns like our own, with planets going
  round them
• 1991 Radio astronomers Alex Wolszczan Dale
  Frail discovered planets around a pulsar
  PSR125712
• Variations in arrival times of pulses suggests
  presence of three or more planets
• Planets probably formed from debris left after
  supernova explosion
• 1995 Planet found around nearby Sun-like star 51
  Peg by Swiss astronomers Michel Mayor Didier
  Queloz using the Doppler Wobble method
• Most successful detection method by far, but
  other methods like transits are now very
  successful
• 405 exoplanets in total found to date by all
  methods

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Planet Hunting The Radial Velocity
Technique(Doppler Wobble)

• Star planet orbit common centre of gravity
• As star moves towards observer, wavelength of
  light shortens (is blue-shifted)
• Light red-shifted as star moves away
• Measure
• Dl / le (l0-le) / le vr / c
• loobserved wavelength, leemitted wavelength

377 planets detected by Doppler Wobble inc. 38
multiple systems
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• M from spectral type

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Doppler Wobble Method Summary

• Precision of current surveys is now 1m/s
• Jupiter causes Suns velocity to vary by 12.5m/s
• All nearby, bright Sun-like stars are good
  targets
• Lots of lines in spectra, relatively inactive
• Smallest planet found by this method is 2Mearth
• Length of surveys limits distances planets have
  been found from stars
• Earliest surveys started 1989
• Jupiter (5AU from Sun) takes 12 yrs to orbit Sun
• Saturn takes 30 years
• Would remain undetected
• Do not see planet directly

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Doppler Wobble Method Summary

• Since measure K ( v sin i), not v directly,
  only know mass in terms of the orbital
  inclination i
• Therefore only know the planets minimum mass, M
  sin i
• If i90o (eclipsing or transiting) then know mass
  exactly

Orbital plane
i900
Orbital plane
i0
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Transits

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

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Transits

• Assuming
• The whole planet passes in front of the star
• And ignoring limb darkening as negligible
• Then the depth of the eclipse is simply the ratio
  of the planetary and stellar disk areas
• i.e. Df / f pRp2 / pR2 (Rp / R)2
• We measure the change in magnitude Dm, and obtain
  the stellar radius from the spectral type
• Hence by converting to flux we can measure the
  planets radius
• Rem. Dm mtransit m 2.5 log (f /
  ftransit)
• (smaller number means brighter)

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Transits

• Example first known transiting planet HD209458b
• Dm 0.017 mags
• So (f / ftransit) 1.0158, i.e. Df1.58
• From the spectral type (G0) R1.15Rsun
• So using Df / f (Rp / R)2 and setting f100
• Find Rp0.145Rsun
• Since Rsun9.73RJ then
• Rp 1.41RJ

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Transits

• HD209458b more
• From Doppler wobble method know M sin i 0.62MJ
• Transiting, hence assume i90o, so M0.62MJ
• Density 0.29 g/cm3
• c.f. Saturn 0.69 g/cm3
• HD209458b is a gas giant!

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Transits

• For an edge-on orbit, transit duration is given
  by
• Dt (PR) / (pa)
• Where Pperiod in days, asemi-major axis of
  orbit
• Probability of transit (for random orbit)
• Ptransit R / a
• For Earth (P1yr, a1AU), Ptransit0.5
• But for close, hot Jupiters, Ptransit10
• Of course, relative probability of detecting
  Earths is lower since would have to observe for
  up to 1 year

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Transits

• Advantages
• Easy. Can be done with small, cheap telescopes
• E.g. WASP
• Possible to detect low mass planets, including
  Earths, especially from space (Kepler mission,
  2008)
• Disadvantages
• Probability of seeing a transit is low
• Need to observe many stars simultaneously
• Easy to confuse with starspots, binary/triple
  systems
• Needs radial velocity measurements for
  confirmation, masses

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Super WASP

• Wide Angle Search for Planets (by transit method)
• First telescope located in La Palma, second in
  South Africa
• Operations started May 04
• Data stored and processed at Leicester
• gt30 new planets detected!
• www.superwasp.org
• www.wasp.le.ac.uk

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Super WASP

• SuperWASP monitors about 1/4 of the sky from each
  site
• That means millions of stars, every night!

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Direct detection

• Imaging spectroscopy physics composition
  structure
• Difficult
• Why?
• Stars like the Sun are billions of times brighter
  than planets
• Planets and stars lie very close together on the
  sky
• At 10pc Jupiter and the Sun are separated by 0.5

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Direct detection

• Problem 1
• Stars bright, planets faint
• Solution
• Block starlight with a coronagraph
• Problem 2
• Earths atmosphere distorts starlight, reduces
  resolution
• Solution
• Adaptive optics, Interferometry difficult,
  expensive
• Or look around very young and/or intrinsically
  faint stars (not Sun-like)

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First directly imaged planet?

• 2M1207 in TW Hya association
• Discovered at ESO VLT in Chile
• 25Mjup Brown dwarf 5Mjup planet
• Distance 55pc
• Very young cluster 10M years
• Physical separation 55AU
• A brown dwarf is a failed star
• Can this really be called a planet?
• Formation mechanism may be crucial!

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First directly imaged planetary system

• Last year 3 planets imaged around the star HR8799
• 130 light years away (40pc)
• Three planets at 24, 38 and 68AU separation
• In comparison, Jupiter is at 5AU and Neptune at
  30AU
• Masses of 7Mjup, 10Mjup and 10Mjup
• Young 60Myr
• Earth is 4.5Gyr

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Fomalhaut (alpha Piscis Austrini)

• One of the brightest stars in the southern sky
• Long known to have a dusty debris disk
• Shape of disk suggested presence of planet
• 2Mjup planet imaged by HST inside disk
• 200Myr old
• Like early solar system

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Direct detection White Dwarfs

• White dwarfs are the end state of stars like the
  Sun
• What will happen to the solar system in the
  future?
• WDs are 1,000-10,000 times fainter than Sun-like
  stars
• contrast problem reduced
• Outer planets should survive evolution of Sun to
  white dwarf stage, and migrate outwards
• more easily resolved
• Over 100 WD within 20pc
• At 10pc a separation of 100AU 10 on sky
• At Leicester we are searching for planets
  around nearby WD with 8m telescopes and the
  Spitzer space telescope

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Direct detection White Dwarfs

• No planets yet, just brown dwarfs
• Currently limited to finding planets gt5Mjup
• Not very common
• May have to wait for JWST
• But have found some WDs are surrounded by dust
  and gas disks
• Remains of small rocky planets and asteroids that
  strayed too close to WD
• Ripped apart by tidal forces
• Can study composition of extra-solar terrestrial
  bodies!

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What we know about extra-solar planets

• 405 planets now found
• 38 multiple systems
• 62 transiting planets
• Unexpected population with periods of 2-4 days
  hot Jupiters
• Planets with orbits like Jupiter discovered (eg
  55 Cancri d)
• Smallest planet CoRoT-7b - 1.7Rearth

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Extra-solar planet period distribution

• Notice the pile-up at periods of 2-4 days /
  0.04-0.05AU
• The most distant planets discovered by radial
  velocities so far are at 5-6AU
• Imaging surveys finding very wide orbit planets

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Hot Jupiter planets

• Doppler Wobble and transit surveys find many gas
  giants in orbits of 2-4 days
• cf Mercurys orbit is 80 days
• Surveys are biased towards finding them
• Larger Doppler Wobble signal
• Greater probability of transit
• These planets are heated to gt1000oF on day side
• And are tidally locked like the Moon
• Causes extreme weather conditions

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Extra-solar planet mass distribution

• Mass distribution peaks at 1-2 x mass of Jupiter
• Lowest mass planet so far 5.5xMEarth
• Super-Jupiters (gtfew MJup) are not common
• Implications for planet formation theories?
• Or only exist in number at large separation?
• Or exist around massive stars?

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What we know about extra-solar planets
Eccentricity vs semi-major axis
most extra-solar planets are in much more
eccentric orbits than the giant planets in the
solar system
-
- planets close to the star are tidally
circularized
observational bias
extra-solar planets
- but some planets in circular orbits do exist
far away from star - the planets in our own
system have small eccentricities ie STABLE

solar system planets
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Results of the Planet Hunting surveys

• Of 2000 stars surveyed
• 5 have gas giants between 0.02AU and 5AU
• 10 may have gas giants in wider orbits
• lt1 have Hot Jupiters
• How many have Earths..?

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What about the stars themselves?

• Surveys began by targeting sun-like stars
  (spectral types F, G and K)
• Now extended to M dwarfs (lt1 Msun) and subgiants
  (gt1.5Msun)
• Subgiants are the descendants of A stars
• Incidence of planets is greatest for late F stars
• F7-9V gt GV gt KV gt MV
• Stars that host planets appear to be on average
  more metal-rich
• More massive stars tend to have more massive
  planets

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MetallicityThe abundance of elements heavier
than He relative to the Sun

• Overall, 5 of solar-like stars have radial
  velocity detected Jupiters
• But if we take metallicity into account
• gt20 of stars with 3x the metal content of the
  Sun have planets
• 3 of stars with 1/3rd of the Suns metallicity
  have planets
• Does this result imply that planets more easily
  form in metal-rich environments?
• If so, then maybe planet hunters should be
  targeting metal-rich stars
• Especially if we are looking for rocky planets

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Planet formation scenarios

• 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.

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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 (lt1 cm) cohesion occurs by
  non-gravitational forces e.g. chemical processes.
• On the largest scale (gt1 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.

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Gravitational 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.

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Where do the hot Jupiters come from?

• No element will condense within 0.1AU of a star
  since Tgt1000K
• Planets most likely form beyond the ice-line,
  the distance at which ice forms
• More solids available for building planets
• Distance depends on mass and conditions of
  proto-planetary disk, but generally gt1AU
• Hot Jupiters currently at 0.03-0.04AU cannot
  have formed there
• Migration Planets migrate inwards and stop when
  disk is finally cleared
• If migration time lt disk lifetime
• Planets fall into star
• Excess of planets at 0.03-0.04AU is evidence of a
  stopping mechanism
• tides? magnetic cavities? mass transfer?
• Large planets will migrate more slowly
• Explanation for lack of super-Jupiters in close
  orbits

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Hunting for Earth-like planets

• Pace of planet discoveries will continue to
  increase in next few years
• Radial velocity and direct imaging surveys will
  reveal outer giant planets with long periods like
  our own Solar System
• Transit surveys will reveal small planets in
  close orbits to their suns
• But the greatest goal is the detection of other
  Earths

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Towards other Earths
Telescope Method Date
Corot (Fr) Transits 2007
Kepler (NASA) Transits 2008
GAIA (ESA) Astrometry 2012
SIM (NASA) Astrometry 2015 (?)
Plato (ESA) Transits 2017
Darwin (ESA) Imaging 2025 (?)
42m E-ELT Imaging 2018
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Kepler

• Searching for Earths by transit method
• Launched last year by NASA
• Aims to find an Earth around a Sun-like star in a
  one year orbit
• Need three transits to confirm
• So mission lasts at least three years

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Towards Other Earths Habitable Zones

• Habitable zone defined as where liquid water
  exists
• Changes in extent and distance from star
  according to stars spectral type (ie temperature)

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Towards Other Earths Biomarkers

• So we find a planet with the same mass as Earth,
  and in the habitable zone
• How can we tell it harbours life?
• Search for biomarkers
• Water
• Ozone
• Albedo