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Andrew Collier Cameron
University of St Andrews
Are we alone in the Universe?
The plurality of worlds
• In some worlds there is no Sun and
Moon, in others they are larger than in
our world, and in others more
numerous. In some parts there are
more worlds, in others fewer (...); in
some parts they are arising, in others
failing. There are some worlds devoid
of living creatures or plants or any
moisture.
– Democritus (ca. 460-370 B.C.), after Hyppolytus
(3rd cent. A.D.)
• There cannot be more worlds than one.
– Aristotle [ De Caelo ]
How do
galaxies
, stars and planets form
and evolve?
• The worlds come into being
as follows: many bodies of
all sorts and shapes move
from the infinite into a great
void; they come together
there and produce a single
whirl, in which, colliding with
one another and revolving in
all manner of ways, they
begin to separate like to like.
– Leucippus (480-420(?) B.C.), after
Diogenes Laertios (3rd cent. A.D.)
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Dusty discs around young stars
• Roughly half of all
new-born Sun-like
stars are surrounded
by solar system-sized
dusty discs.
• Could this mean that
half of all Sun-like
stars have planetary
systems?
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Proto-planetary discs in the Orion Nebula (NASA/STScI)
Searching for extra-solar Jupiters
• A planet and its parent
star orbit round their
common centre of
gravity.
• The star is much more
massive than the planet,
so the reflex orbital
speed is small.
• A massive planet in a
close orbit gives its star a
reflex velocity of a few
tens of ms–1.
• This gives a small but
measurable Doppler shift.
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51 Pegasi: The first wobbling star
Discovered by
Michel Mayor
& Didier Queloz
in mid-1995.
Today’s state
of play
• 237 planets in 203
systems, Oct 1995 20-Aug-2007 from
“Doppler wobble”
searches.
• 25 multiple systems
• 24 transiting
systems, 19 from
transit searches
• 4 microlensing
planets (more
distant!)
Recipe for building Jupiters
• Ingredients:
– 10 Earth masses of ice-coated dust
particles
– Lots of gas (mostly hydrogen)
• Method:
– Allow dust & ice to coagulate
– Allow solid core to sweep up gas
– Leave to cool for 5 billion years
• Common problems:
– Tidal gaps starve planet of gas.
– Gas accretion takes tens of millions
of years, longer than lifetime of disc.
– Migrating planets spiral into star.
Numerical simulation by Pawel Artymowicz,
Stockholm.
Tip of the iceberg?
• Left panel: Core
accretion+migration
simulation by Ida & Lin
(2004), showing gas giants,
ice giants, rocky planets.
• Right panel: Radial-velocity
discoveries so far.
Iron abundance and planet formation
Eccentric
Orbits
Unclear why.
Planet-planet interactions
Eccentricity pumping
Small planets ejected
Tidal circularisation
Other planet-building recipes
• If disk cools efficiently by
infrared radiation,
fragments can collapse
spontaneously to form
“instant planets”.
• Several dozen planets
form and interact.
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Numerical simulation by Ken Rice,
University of St Andrews.
Other planet-building recipes
• If disk cools efficiently by
infrared radiation,
fragments can collapse
spontaneously to form
“instant planets”.
• Several dozen planets
form and interact.
• Smaller planets get
ejected from system.
• One “big fish” survives
in an eccentric orbit.
• Problems:
– Hard to get multiple, smaller
planets to survive in near
circular orbits.
Only one planetary object remains after
N-body evolution for 21 Myr.
• Mp = 7.4 Jupiter masses
• a = 1.66 au
• e = 0.63
Lessons from Doppler Wobbles
• > 5% of Sun-like stars host a Jupiter
• Metallicity matters
• Orbits differ from Solar System
–
wide range of orbit radii ( P > 2d )
–
wide range of eccentricities
• New processes
–
Migration -- spiral-in
–
eccentricity pumping
–
ejection
• What sort of planets are the hot Jupiters ?
Transit
Lightcurves
rJup  0.1 RSun
Depth :
2
2




r
f
R
p
 1% 





f
rJup  RSun 
Duration :
 M 2 / 3  P 1/ 3
t  3h 
  
M
 Sun  4d 
Probability :
 R  M 1/ 3  P 2 / 3
Pt  10% 

  
R
M
 Sun  Sun  4d 

SuperWASP hardware
• Pollacco et al 2006, PASP 118,
1407
• Lenses
– Canon 200mm f/1.8
– Aperture 11.1 cm
• CCD Detector
– 2048 x 2048 thinned e2v (Andor,
Belfast)
– 13.5x13.5 micron pixels
• Field of View
– 7.8 x 7.8 degrees
– 13.7 arcsec/pixel
• Mount
– OMI/Torus robotic mount
• Operating Temperature
– –50 ºC
– 3-stage Peltier Cooling
WASP data reduction pipeline
Flatfield
Bias
Pre-processed
Raw
Dark
Current
Exposure
Map
16h43+31d26 field 2004 May-Aug
1
Flux-RMS
RMS scatter (mag)
0.1
Series1
0.01
12
0.001
9
10
11
12
WASP V magnitude
13
14
Field
recognition,
astrometry,
aperture
photometry,
calibration/detrending
Current Observing fields
Data processed so far
(stellar density plot)
Substellar mass-radius relation
Mass-radius relation for hot
Jupiters
• WASP-1b,-2b: Cameron et al 2007, MNRAS
(+ XO-2b, HAT-P-2b, HAT-P-3b, TrES-3, TrES-4, CoRoT-EXO1b, Gl 436b since 2007 May 1)
Why we need many more …
• How does planet radius scale with
–
–
–
–
–
–
–
Planet mass? (Fortney et al 2007)
Planet age? (Many!)
Metallicity/opacity? (Burrows et al 2007, Guillot et al 2006)
Existence/size of core? (Guillot et al 2006)
Proximity to host star? (Fortney et al 2007)
Migration history?
?
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Core mass and
formation
mechanism
• Recent example:
–
–
–
–
HD 149026b
Transiting hot Saturn
High density => massive core
Sato et al, ApJ, in press
• Test formation models:
– Core accretion+migration
– Gravitational instability
GJ 436 b
• Gillon et al 2007 May 17, astroph/0705.2219
• Neptune-mass planet
• Neptune-like radius
• Radius depends strongly on
composition (cf. Fortney et al
2007, astro-ph/0612671)
• Ice-giant structure.
WASP-1b
WASP-2b
Exoplanet “Discovery Space”
~100 Doppler wobble planets
Hot Planets
Cool Planets
Microlensing by a star
Background
Lensing star
Observer
• Light from background stars is gravitationally bent
around a foreground star.
• Light is amplified near the “Einstein Ring”.
• Misaligned objects produce 2 images, one inside and
one outside the Einstein Ring
Now if I had a REALLY big
telescope...
• ...this is what a Sunlike star would do to
the view of dust
clouds in a nearby
galaxy, 150,000 ly
away.
• The Einstein ring of
the star is about the
size of Jupiter’s
orbit round the Sun.
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First definitive planetary lens event!
OGLE-2003-BLG-235/MOA-2003-BLG-53
• OGLE/PLANET/MOA
collaboration
– 45 microlensing events
monitored intensively over
the last 5 years.
– No convincing Jupiter-like
secondary peaks found…
until last week!
– Conclusion: less than 30%
of lensing stars have
Jupiters.
• First definitive planet
detection announced in
NASA press release by
D. Bennett’s team, 2004
April 15.
Courtesy Dave Bennett and
OGLE/PLANET/MOA team members
Planetary Parameters
• Best-fitting model:
• Planet mass:
1.5 Jupiter masses
• Star mass:
0.36 solar masses
• Planet-star distance:
3 times Earth-Sun distance
• Distance from Earth:
16000 light-years!
Space-based transit detection
HST
MOST
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CoRoT
KEPLER
Line photons blocked
high in atmosphere
Continuum photons
blocked by clouds
Opaque silicate cloud deck
Extended atmosphere
With gaseous Na, K, H2O, CH4, ...
What’s in their
atmospheres?
1.5%
Brown (2001)
1.6%
Na I
wavelength (microns)
Na I absorption in
HD 209458b
• Charbonneau et al (2002:
ApJ 568, 377)
– Hubble Space telescope / STIS
– Weak detection of Na!
– f/f ~ 2x10–4
The amazing evaporating planet
• Vidal-Madjar et al (2003) Nature 422, 123
Star occults planet
0.2 %
Spitzer/IRAC 4.5, 8.0
micron
Direct detection of infrared
light from planet
TrES-1: Charbonneau et al. 2005
HD 209458: Deming et al. 2005
-> effective temperature
Water in HD 189733b
• Planet silhouette size measured in SPITZER/IRAC
3.6, 5.8, 8.0 mm bands during primary transit.
• Wavelength dependence matches water
transmission spectrum, mixing ratio ~ 5x10–4 .
Tinetti et al 2007, Nature 448, 163
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The continuously habitable zone
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The Darwin Mission: 2018?
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• Aim: to discover
Earth-like planets
orbiting nearby stars
and seek atmospheric
biosignatures.
• Four interlinked
collector mirrors
flying ~100m apart.
• Light waves from
collectors interfere to
cancel out glare of
central star.
You’d look pretty simple from 30
light-years away too
• Nulling interferometry
with infrared light from
Darwin’s four collectors
eliminates light from star.
• Simulated image of our
own solar system seen
from 30 light-years away
detects all inner planets
except Mercury:
Earth
(Sun)
Venus
Mars
Mid-IR spectra of terrestrial planets
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History of the Earth’s atmosphere
Methane,
Ammonia
Nitrogen
Water
Carbon Dioxide
Oxygen
Time before present (billions of years)
Summary
•
Transiting hot Jupiters probe:
– Interior structure
– Formation history
– Atmospheric composition
– Albedo and energy budget
•
Wide but shallow surveys (WASP, HAT, TrES, XO) yielding several
planets/year bright enough for transit spectroscopy, Spitzer /JWST
secondary-eclipse studies.
•
Space-based transit studies capable of detecting hot (CoRoT) and warm
(KEPLER) super-Earths and determing bulk composition.
•
Efficient spectroscopic confirmation essential to eliminate impostors and
determine planet masses.
•
Long-term, high-precision transit timings may reveal lower-mass planets.
•
DARWIN/TPF: nulling interferometry will permit 10-micron spectroscopy of
terrestrial planet atmospheres.
Postcards from Titan
Image Credit:
NASA/JPL/University of Arizona
Transiting extrasolar giant planets
• 19 examples known.
• Stellar mass and period yield
orbital separation a.
• Transit shape yields
– impact parameter
– stellar radius
• Transit depth yields ratio of radii
• Hence get direct measure of
planetary density.