3677 life in the universe extra solar planets
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3677 Life in the Universe: Extra-solar planets. Dr. Matt Burleigh www.star.le.ac.uk/mrb1/lectures.html. Course outline. Lecture 1 Definition of a planet A little history Pulsar planets Doppler “ wobble ” (radial velocity) technique Lecture 2 Transiting planets Transit search projects

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3677 life in the universe extra solar planets

3677 Life in the Universe:Extra-solar planets

Dr. Matt Burleigh


course outline
Course outline
  • Lecture 1
    • Definition of a planet
    • A little history
    • Pulsar planets
    • Doppler “wobble” (radial velocity) technique
  • Lecture 2
    • Transiting planets
    • Transit search projects
    • Detecting the atmospheres of transiting planets: secondary eclipses & transmission spectroscopy
    • Transit timing variations
course outline1
Course outline
  • Lecture 3
    • Microlensing
    • Direct Imaging
    • Other methods: astrometry, eclipse timing
    • Planets around evolved stars
  • Lecture 4
    • Statistics: mass and orbital distributions, incidence of solar systems, etc.
    • Hot Jupiters
    • Super-Earths
    • Planetary formation
    • Planetary atmospheres
    • The host stars
course outline2
Course outline
  • Lecture 5
    • The quest for an Earth-like planet
    • Habitable zones
    • Results from the Kepler mission
      • How common are rocky planets?
      • Amazing solar systems
    • Biomarkers
    • Future telescopes and space missions
useful web sites
Useful web sites
  • Extra-solar planets encyclopaedia: exoplanets.eu
  • Exoplanet Data Explorer (California Planet Survey): exoplanets.org
  • NASA exoplanet archive: exoplanetarchive.ipac.caltech.edu
  • Planet hunters (Zooniverse): www.planethunters.org
  • Kepler mission: kepler.nasa.gov
  • Next Generation Transit Survey: www.ngtransits.org
useful books
Useful books
  • Extrasolar planets & Astrobiology: Caleb A. Scharf
  • Extrasolar planets: the search for new worlds: Stuart Clark
  • Transiting Exoplanets: Carole A. Haswell
  • The Exoplanet Handbook: Michael Perryman
  • An Introduction to Astrobiology: Iain Gilmore & Mark Sephton
  • Life in the Universe: Bennett & Shostak
useful numbers
Useful numbers
  • RSun = 6.995x108m
  • Rjup= 6.9961x107m ~ 0.1RSun
  • Rnep= 2.4622x107m ~ 4Rearth
  • Rearth= 6.371x106m ~ 0.1Rjup ~ 0.01RSun
  • MSun= 1.989x1030kg
  • Mjup= 1.898x1027kg ~ 0.001MSun = 317.8Mearth
  • Mnep= 1.02x1026kg ~ 5x10-5MSun~ 0.05Mjup = 17.15Mearth
  • Mearth= 5.97x1024kg = 3x10-6MSun = 3.14x10-3Mjup
  • 1AU = 1.496x1011m
  • 1 day = 86400s
what is a planet
What is a planet?
  • International Astronomical Union definition –
    • An object orbiting a star
    • Too small for dueterium fusion to occur
      • Less than 13 times the mass of Jupiter
    • Formation mechanism?
      • Forms from a circumstellar disk of dust and gas around a young star
    • Lower mass limit – IAU decided that Pluto should be downgraded!
what is a planet1
What is a planet?

Above, left to right: limb of Sun, late M (red) dwarf, L brown dwarf, T brown dwarf, Jupiter.

The coolest stars, old brown dwarfs and gas giant planets have the same radii!

a brief history of extra solar planets
A brief history of extra-solar planets
  • 16th century: the Italian philosopher Giordano Bruno said that the fixed stars are really suns like our own, with planets going round them
  • 19th Century: astronomers believed orbital anomalies in the binary star 70 Oph could be explained by an unseen planet, but later disproved
  • 1950s & 60s: Peter van de Kamp concluded that irregularities in the high proper motion of nearby Barnard’s Star were caused by a planet. Sadly, this too turned out to be erroneous.
  • late 1980s: Canadian Gordon Walker found tentative evidence for exoplanets using radial velocity method: but not confirmed until 2000s!
  • 1991: Andrew Lyne & Setnam Shemar at Jodrell Bank claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. They withdrew the claim later that year due to an error in their calculations.

New York Times 16th April 1963

a brief history of extra solar planets1
A brief history of extra-solar planets
  • 1991 Radio astronomers Alex Wolszczan & Dale 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 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
  • >1000 exoplanets confirmed to date by all methods
    • >100 found since I gave this lecture last year
    • Kepler has several thousand more candidates
Blue: radial velocity, Green: transiting, Red: microlensing, Orange: direct imaging, Yellow: pulsar timing
pulsar planets
Pulsar planets
  • Pulsars are neutron stars that emit radio pulses every ~second as they spin
    • More stable and accurate than an atomic clock
  • If a planet accompanies the pulsar, then the pulsar will orbit the centre of mass of the system
    • The pulses will then arrive earlier or later than expected
  • Radio observations have found a dozen or so such “pulsar planets”
    • Wolszczan & Frail’s discovery of PSR1257+12’s planets in 1991 at Arecibo in Puerto Rico were the first confirmed exoplanets
    • PSR1257+12’s planets are all ~Earth mass or smaller
pulsar planets1
Pulsar planets
  • Pulsars are created when a massive star (>8Msun) explodes as a supernova
    • Their original planetary systems will not survive
    • Radio-detected planets thought to have formed from supernova debris
    • Planets will be bathed in high energy radiation from pulsar – no chance of life!
planet hunting the radial velocity technique doppler wobble
Planet Hunting: The Radial Velocity Technique(“Doppler Wobble”)
  • Star + planet orbit common centre of gravity
  • As star moves towards observer, wavelength of light shortens (blue-shifted)
  • Light red-shifted as star moves away

873 planets detected by Doppler Wobble inc. 142 multiple systems

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
  • Measure:
    • Dl / le = (l0-le) / le = vr / c lo=observed wavelength, le=emitted wavelength
doppler wobble method
Doppler Wobble Method
  • 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, M sin i
    • If i=90o (eclipsing or transiting) then know mass exactly

Orbital plane


Orbital plane


example 51 peg
Example: 51 Peg
  • P= 4.15days = 4.15x86400s = 3.5856x105s
  • G5V star, M*=1.11Msun = 1.11x1.989x1030kg = 2.21x1030kg
  • Find r = 0.052AU, vpl=1.37x105ms-1Mplsin i = 0.45Mjup
Above: eccentric orbit (e=0.93)

Top right: 55 Cancri multiple-planet system (4, maybe 5 planets)

Bottom right: 3 planet HD37124 system

harps radial velocity spectrograph
HARPS radial velocity spectrograph
  • Built by Geneva Observatory
  • First installed on ESO 3.6m at La Silla, Chile in 2002/3
  • Has found over 130 planets
  • Precision 30cm/s – 1m/s
  • Simultaneously observes star and a reference Thorium lamp through two separate fibres
  • Highly stable optical bench, housed in sealed, thermally stable room
  • Second HARPS installed on Italain Galileo telescope on la Palma 2012
doppler wobble method1
Doppler Wobble Method
  • Precision of current surveys routinely <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
    • Smallest planet found by this method is ~1Mearth: Alpha Cen Bb – nearest star system to us!
    • Most are Neptune size and larger
  • Length of surveys limits distances planets have been found from stars
    • Earliest surveys started 1988
    • Jupiter (5AU from Sun) takes 12 yrs to orbit Sun
    • Saturn takes 30 years
      • Would be strongly hinted at but not yet completed one orbit since surveys began
    • Do not see planet directly
alpha cen bb
Alpha Cen Bb
  • Alpha Cen system is the nearest star system to us
  • Alpha Cen B has been monitored by radial velocity method
  • Very recent discovery of a rocky planet:
    • Minimum mass 1.1xEarth
    • Period 3.2 days
    • Dumusque et al. 2012, Nature
  • Thought: if Alpha Cen B has a rocky planet, do most stars have rocky planets?