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DEPARTMENT OF PHYSICS AND ASTRONOMY
Extra-solar planets
Dr. Matt Burleigh
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
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
– 405 exoplanets found to date
Dr. Matt Burleigh
Planet Hunting: The Radial Velocity Technique
(“Doppler Wobble”)
• Star + planet orbit
common centre of
gravity
377 planets
detected by
Doppler Wobble
inc. 38 multiple
systems
Dr. Matt Burleigh
• As star moves
towards observer,
wavelength of light
shortens (is blueshifted)
• Light red-shifted
as star moves
away
Planet Hunting 2: Transits
Dr. Matt Burleigh
Transits
• The depth of the transit is simply the ratio of the
planetary and stellar disk areas:
– pRp2 / pR*2 = (Rp / R*)2
– so we can measure the planet’s radius
• But the probability of a transit is very low: only 1 in
>10,000 stars has a transiting planet
Dr. Matt Burleigh
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
• >20 new planets detected
• www.superwasp.org
• www.wasp.le.ac.uk
Dr. Matt Burleigh
Super WASP
• SuperWASP monitors about 1/4
of the sky from each site
• That means millions of stars,
every night!
Dr. Matt Burleigh
“Hot Jupiter” planets
• Doppler Wobble and transit surveys
find many gas giants in orbits of 2-4
days
– cf Mercury’s orbit is 80 days
• Surveys are biased towards finding
them
– Larger Doppler Wobble signal
– Greater probability of transit
• These planets are heated to >1000oF
on “day” side
– And are “tidally locked” like the Moon
– Causes extreme weather conditions
• Must have formed much farther away
from star and migrated inwards
– Environment too hot for dust and ice to
form
Dr. Matt Burleigh
Planet Hunting 3: Direct Imaging
• 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
Dr. Matt Burleigh
Fomalhaut (alpha Piscis Austrini)
• One of the brightest stars in the
sky (southern hemisphere)
• There is a 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
• Is there a giant planet orbiting
Fomalhaut?
Dr. Matt Burleigh
Fomalhaut (alpha Piscis Austrini)
• 2Mjup planet
imaged inside disk
• 200Myr old
• Like early solar
system
Dr. Matt Burleigh
Dr. Matt Burleigh
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”
• Planet with orbits like
Jupiter discovered (eg
55 Cancri d)
• Smallest planet:
CoRoT-7b - 1.7Rearth
Extra-solar planet masses
• Mass distribution
peaks at 1-2 x mass
of Jupiter
• Lowest mass planet
so far: 5.5xMEarth
Dr. Matt Burleigh
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
– <1% have Hot Jupiters
• How many have
Earths…..?
Dr. Matt Burleigh
Hunting for Earth-like planets
Dr. Matt Burleigh
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
Dr. Matt Burleigh
Kepler
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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…
Dr. Matt Burleigh
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)
Dr. Matt Burleigh
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
Dr. Matt Burleigh
Recommended reading
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Wikipedia Extra-Solar Planets
NASA JPL PlanetQuest
University of California Planet Search
Extra-Solar Planets Encyclopaedia
SuperWASP
Kepler
Dr. Matt Burleigh