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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
– 573 exoplanets found to date
Dr. Matt Burleigh
Planet Hunting: The Radial Velocity Technique
(“Doppler Wobble”)
• Star + planet orbit
common centre of
gravity
523 planets
detected by
Doppler Wobble
inc. 52 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 2004
• Data stored and processed at
Leicester
• >50 new planets detected by
WASP
– 144 by transit experiments in total
• 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
• In 2008 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
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
• 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
Dr. Matt Burleigh
The end of our solar system
Dr. Matt Burleigh
Spitzer 4.5micron image
GJ3483 (LTT3059 / WD0808-661)
130” /
2500AU
I maybe a planet…
or a brown dwarf
Dr. Matt Burleigh
I am the
white
dwarf
Dr. Matt Burleigh
What we know about
extra-solar planets
• 573 planets now
found
• 69 multiple systems
• 144 transiting planets
• Unexpected
population with
periods of 1-4 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
Jupiter
Dr. Matt Burleigh
Earth
Planet population predictions
?
Observations
Population
Synthesis
Small planets expected to be very common
Dr. Matt Burleigh
Towards other Earths
Telescope
Method
Date
Corot (Fr)
Transits
2007
Kepler (NASA)
Transits
2008
GAIA (ESA)
Astrometry
2013
SIM (NASA)
Astrometry
2015 (?)
Plato (ESA)
Transits
2018
Darwin (ESA)
Imaging
2025+ (?)
39m E-ELT
Imaging
2019
Dr. Matt Burleigh
Next Generation
Transit Survey
Queen’s University Belfast, University of
Leicester, University of Warwick, Observatoire
de Genève
Dr. Matt Burleigh
NGTS Prototype La Palma 2010
Dr. Matt Burleigh
Early NGTS v SuperWASP
Dr. Matt Burleigh
NGTS – Site
• Requires an excellent photometric
site
• ESO STC recently approved
construction at Paranal
– Near to VISTA
– Status as official ESO project
Dr. Matt Burleigh
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…
Dr. Matt Burleigh
Small planets from the Kepler faint
star sample
Frequency of different sized planets
as a function of orbital period.
Borucki et al 2010
Kepler size distribution compared
to that from the exoplanet
encyclopedia.
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