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DEPARTMENT OF PHYSICS AND ASTRONOMY
3677 Life in the Universe:
Extra-solar planets
Dr. Matt Burleigh
www.star.le.ac.uk/mrb1/lectures.html
Course outline
• Lecture 1
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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
Dr. Matt Burleigh
3677: Life in the Universe
Course outline
• Lecture 3
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–
–
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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
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
Useful numbers
•
•
•
•
RSun = 6.995x108m
Rjup = 6.9961x107m ~ 0.1RSun
Rnep = 2.4622x107m ~ 4Rearth
Rearth = 6.371x106m ~ 0.1Rjup ~ 0.01RSun
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•
•
•
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
Dr. Matt Burleigh
3677: Life in the Universe
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!
Dr. Matt Burleigh
3677: Life in the Universe
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!
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
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
• >1700 exoplanets confirmed to date by all
methods
– Kepler has several thousand more candidates
Dr. Matt Burleigh
3677: Life in the Universe
Blue: radial velocity, Green: transiting, Red: microlensing,
Orange: direct imaging, Yellow: pulsar timing
Dr. Matt Burleigh
3677: Life in the Universe
Dr. Matt Burleigh
3677: Life in the Universe
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
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!
Dr. Matt Burleigh
3677: Life in the Universe
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
517 planets detected by Doppler
Wobble and many more transiting
planets confirmed by this method
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
lo=observed wavelength, le=emitted wavelength
3677: Life in the Universe
M* from spectral type
Dr. Matt Burleigh
3677: Life in the Universe
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
i=900
Orbital
plane
i0
Dr. Matt Burleigh
3677: Life in the Universe
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-1
Mpl sin i = 0.45Mjup
Dr. Matt Burleigh
3677: Life in the Universe
Above: eccentric orbit
(e=0.93)
Top right: 55 Cancri
multiple-planet system (4,
maybe 5 planets)
Bottom right: 3 planet
HD37124 system
Dr. Matt Burleigh
3677: Life in the Universe
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 Italian
Galileo telescope on la Palma 2012
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
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?
Dr. Matt Burleigh
3677: Life in the Universe