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Life in the Universe: Extra-solar planets. Dr. Matt Burleigh 3677 Timetable. Today, and next Friday: MB Extrasolar planets Then Derek Raine After Xmas: Mark Sims (Life in the solar system). Contents. Methods for detection Doppler “wobble” Transits Microlensing

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life in the universe extra solar planets

Life in the Universe:Extra-solar planets

Dr. Matt Burleigh

3677 timetable
3677 Timetable
  • Today, and next Friday: MB Extrasolar planets
  • Then Derek Raine
  • After Xmas: Mark Sims (Life in the solar system)
  • Methods for detection
    • Doppler “wobble”
    • Transits
    • Microlensing
    • Direct Imaging
  • Characterisation
    • Statistics
    • Implications for formation scenarios
useful reading web sites
Useful reading / web sites
  • Nature, Vol. 419, p. 355 (26 September 2002)
  • 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
what is a planet
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 last year that Pluto should be downgraded!
a brief history of exoplanets
A brief history of exoplanets
  • 1991 Wolszczan & Frail discovered planets around a pulsar PSR1257+12
    • Variations in arrival times of pulses suggests presence of three or more planets
    • Planets probably formed from debris left after supernova explosion
  • 1995 Planets found around nearby Sun-like star 51 Peg by Doppler “wobble” method
    • Most successful detection method by far
    • 265 exoplanets found to date
radial velocity technique doppler wobble
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
measuring stellar doppler shifts
Measuring Stellar Doppler shifts
  • Method:
    • Observe star’s spectrum through a cell of iodine gas
    • Iodine superimposes many lines on star’s spectrum
    • Measure wavelength (or velocity) of star’s lines relative to the iodine
measuring stellar doppler shifts1
Measuring Stellar Doppler shifts
  • Method:
    • Measure wavelength (or velocity) of star’s lines relative to the iodine
    • Dl / le = (l0-le) / le = vr / c

lo=observed wavelength, le=emitted wavelength

doppler wobble method summary
Doppler Wobble Method: Summary
  • Precision of current surveys is now 1m/s:
    • Jupiter causes Sun’s velocity to vary by 12.5m/s
    • All nearby, bright Sun-like stars are good targets
      • Lots of lines in spectra, relatively inactive
  • Limited to gas planets and larger
    • Note recently discovered “hot Neptunes” (>14MEarth)
    • Not yet suitable for Earth-like planets
  • 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
doppler wobble method summary1
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 planet’s minimum mass
    • If i=90o (eclipsing or transiting) then know mass exactly

Orbital plane


Orbital plane


  • Planets observed at inclinations near 90o will transit their host stars
  • 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 / pR*2 = (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 planetary radius
    • Rem. Dm = mtransit – m* = 2.5 log (f* / ftransit)
      • (smaller number means brighter)

Example: first known transiting planet HD209458b

  • Dm = 0.017 mags
  • So (f* / ftransit) = 1.0158, i.e. Df=1.58%
  • From the spectral type (G0) R=1.15Rsun
  • So using Df / f* = (Rp / R*)2 and setting f*=100%
  • Find Rp=0.145Rsun
  • Since Rsun=9.73RJ then
  • Rp = 1.41RJ
  • HD209458b more:
    • From Doppler wobble method know M sin i = 0.62MJ
    • Transiting, hence assume i=90o, so M=0.62MJ
    • Density = 0.29 g/cm3
      • c.f. Saturn 0.69 g/cm3
    • HD209458b is a gas giant!
  • For an edge-on orbit, transit duration is given by:
    • Dt = (PR*) / (pa)
      • Where P=period in days, a=semi-major axis of orbit
  • Probability of transit (for random orbit)
    • Ptransit= R* / a
    • For Earth (P=1yr, a=1AU), Ptransit=0.5%
    • But for close, “hot” Jupiters, Ptransit=10%
    • Of course, relative probability of detecting Earths is lower since would have to observe for up to 1 year
  • 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
super wasp
Super WASP
  • Wide Angle Search for Planets (by transit method)
  • First telescope located in La Palma
  • Operations started May ‘04
  • Data stored at Leicester
  • Three new planets detected!
gravitational microlensing
Gravitational Microlensing
  • A consequence of general relativity
  • The grav. field of a relatively nearby star can bend the light of a more distant object as it passes in front of it, as seen from Earth
  • The star doing the lensing brightens as a result
  • We record this brightening, which can last for days
  • If the lensed star has a planetary companion, the characteristic lensing light curve is modified
  • Signals from an Earth-like planet would be strong (>5%) but brief (few hours)
  • 4 planets found so far, including one at 5.5 Earth masses!
direct detection
Direct detection
  • Imaging = spectroscopy = physics: composition & structure
  • No planet in orbit around another star has been directly imaged
  • 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”
direct detection1
Direct detection
  • Problem 1:
    • Stars bright, planets faint
  • Solution:
    • Block starlight with a coronagraph
  • Problem 2:
    • Earth’s atmosphere distorts starlight, reduces resolution
  • Solution:
    • Adaptive optics, Interferometry – difficult, expensive
    • Or look around very young and/or intrinsically faint stars (not Sun-like)
first directly imaged planet
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!
direct detection white dwarfs
Direct detection: White Dwarfs
  • White dwarfs are the end state of stars like the Sun
  • 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
  • I have a programme to search for planets around nearby WD with the Gemini 8m telescopes
  • We call it “DODO” – Degenerate Objects around Degenerate Objects or Dead Objects etc
direct detection white dwarfs1
Direct Detection: White Dwarfs

Proper motions

Two images taken one year apart

  • 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!
what we know about extra solar planets
What we know about extra-solar planets
  • 265 planets now found
  • 25 multiple systems
  • 33 transiting planets – can directly measure radii
  • Unexpected population with periods of 3-4 days: “hot Jupiters”
  • New population from transit surveys at ~2 days
  • First planet with an orbit like Jupiter discovered (55 Cancri d)
  • Is our solar system typical?
extra solar planet period distribution
Extra-solar planet period distribution
  • Notice the “pile-up” at periods of 3-4 days / 0.04-0.05AU
  • The most distant planets discovered so far are at 5-6AU
  • New discovery of transiting planets at ~2 days
extra solar planet mass distribution
Extra-solar planet mass distribution
  • Mass distribution peaks at 1-2 x mass of Jupiter
  • Lowest mass planet so far: 5.5xMEarth
  • Super-Jupiters (>few MJup) are not common
    • Implications for planet formation theories?
    • Or only exist in number at large separation?
    • Or exist around massive stars?
selection effects
Selection effects
  • Astronomical surveys tend to have built in biases
  • These “selection effects” must be understood before we can interpret results
  • The Doppler Wobble method is most sensitive to massive, close-in planets
  • It is not yet sensitive to planets as small as Earth, even close-in
  • As orbital period increases, the method becomes insensitive to planets less massive than Jupiter
  • The length of time that the surveys have been active (since 1989) sets the upper orbital period limit
    • Only now are analogues of Jupiter in our own Solar System going to be found
what we know about extra solar planets mass versus semi major axis
What we know about extra-solar planets:Mass versus semi-major axis
  • Blue – exoplanets
  • Red – solar system
  • Many of the known solar systems have ~Jupiter-mass planets in small orbits, <0.1AU
    • Selection effect of Doppler surveys
  • But almost no super-Jupiters are found in close orbits
    • Real, not a selection effect


- large distribution of e

(same as close binaries)

What we know about extra-solar planets

Eccentricity vs semi-major axis

- 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 close to the star are tidally circularized

observational bias

extra-solar planets

- planets on circular orbits do exist far

away from star

- the planets in our own system have

small eccentricities ie STABLE

solar system planets

statistics of the doppler wobble surveys summary
Statistics of the Doppler Wobble surveys: Summary
  • Of 2000 stars surveyed
    • ~5% have gas giants between 0.02AU and 5AU
      • Trends suggest ~10% of stars have planets in orbits 5-7AU
    • 0.85% have hot Jupiters
      • Real effect
    • Hot Jupiters are not massive
      • Almost all have Msini~1Mjup or less
      • No close-in, “super-Jupiters”
    • Mass distribution strongly peaks at 1Mjup and falls as dN/dM~M-0.7
      • But surveys currently biased towards hot Jupiters
      • Expect mass distribution to flatten somewhat as long periods, super-Jupiters are discovered
what about the stars themselves
What about the stars themselves?
  • Surveys began by targeting sun-like stars (spectral types F, G and K)
  • Now extended to M dwarfs
  • Incidence of planets is greatest for late F stars
    • F7-9V > GV > KV > MV
    • Few low mass M dwarfs known to have a planets despite ease of detectability
  • Stars that host planets appear to be on average more metal-rich
metallicity the abundance of elements heavier than he relative to the sun
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:
    • >20% of stars with 3x the metal content of the Sun have planets
    • ~3% of stars with 1/3rd of the Sun’s 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
  • This result also implies that chances of very old lifeforms (> few billion years) in the Universe are slim
    • With less heavy elements available terrestrial planets may be smaller and lower in mass than in our solar system
    • 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-second pulsar in globular M4 has a Jupiter size companion
    • Claim based on timing anomalies
    • If true, then planets may have been forming 12 billion years ago in a very metal-poor environment (<0.1 x solar)
    • Alternatively, planet may have formed from debris of supernova explosion that created the pulsar
    • Or planet does not exist, timing anomalies have another cause
planet formation scenarios
Planet formation scenarios
  • 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 cores
Gas 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 instability
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.
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>1000K
  • 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 >1AU
  • Hot Jupiters currently at ~0.03-0.04AU cannot have formed there
  • Migration!
planetary migration
Planetary migration
  • Planets migrate inwards and stop when disk is finally cleared
  • If migration time < disk lifetime
    • Planets fall into star
    • Excess of planets at 0.03-0.04AU is evidence of a stopping mechanism in some cases
    • Nature of stopping mechanism unclear: tides? magnetic cavities? mass transfer?
  • Large planets will migrate more slowly
    • Explanation for lack of super-Jupiters in close orbits
planetary migration terrestrial planets
Planetary migration & terrestrial planets
  • Migrating giant planets will be detrimental to terrestrial planet survivability, if they both form at same time
    • Planets interior to a migrating giant planet will be disrupted and lost
    • Of course, these small planets may also migrate into star!
  • If terrestrial planets can only survive when migration doesn’t take place through their formation zone (few AU),
    • then 3%-20% of planet forming systems will possess them
  • Alternatively, terrestrial planet formation may occur after dissipation of gas in proto-planetary disk (after 107 years)
    • Disruption by a migrating giant planet unlikely
    • Almost all planet-forming stars will have terrestrial planets
the future towards other earths
The future: towards other Earths
  • Pace of planet discoveries will increase in next few years
  • Radial velocity surveys will reveal outer giant planets with long periods like our own Solar System
  • Transit surveys will reveal planets smaller than Saturn in close orbits
  • First direct images will be obtained
  • But the greatest goal is the detection of other Earths
towards other earths habitable zones
Towards Other Earths: Habitable Zones
  • Habitable zone defined as where liquid water exists
  • Changes in extent and distance from star according to star’s spectral type (ie temperature)
  • It is possible for rocky planets to exist in stable orbits of habitable zones of known hot Jupiter systems
    • If they were not previously cleared out by migration

Left: courtesy Prof. Keith Horne, St.Andrews

Right: courtesy Prof. Barry Jones, Open

towards other earths biomarkers
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
direct detection proto planetary disks
Direct detection: proto-planetary disks
  • Dust disk around Fomalhaut
  • Sub-mm image taken from James Clarke Maxwell telescope on Hawaii
  • Disk has a hole in centre like a doughnut
  • Part of disk appears to be perturbed
direct detection proto planetary disks1
Direct detection: proto-planetary disks
  • Disk is 200 million years old
  • Like the early Solar System
  • Is a planet perturbing the disk & forming the hole?
direct detection epsilon eridani
Direct detection: Epsilon Eridani
  • Young, Sun-like star only 3pc away
  • Dust disk is clumpy
  • Clumps seen to rotate
  • Requires presence of at least one Jovian planet
  • 1.55MJup companion confirmed by HST measurements