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Andrew Collier Cameron University of St Andrews. Are we alone in the Universe? The plurality of worlds.

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Andrew Collier Cameron

University of St Andrews

are we alone in the universe the plurality of worlds
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
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.)
dusty discs around young stars
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?

Proto-planetary discs in the Orion Nebula (NASA/STScI)

searching for extra solar jupiters
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.
51 pegasi the first wobbling star
51 Pegasi: The first wobbling star

Discovered by

Michel Mayor

& Didier Queloz

in mid-1995.

today s state of play
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
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,


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.

Tip of the iceberg?
eccentric orbits
Eccentric Orbits

Unclear why.

Planet-planet interactions

Eccentricity pumping

Small planets ejected

Tidal circularisation

other planet building recipes
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.

Numerical simulation by Ken Rice,

University of St Andrews.

other planet building recipes1

Only one planetary object remains after N-body evolution for 21 Myr.

  • Mp = 7.4 Jupiter masses
  • a = 1.66 au
  • e = 0.63
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.
lessons from doppler wobbles
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 ?

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

Dark Current

Exposure Map

WASP data reduction pipeline





Field recognition,


aperture photometry, calibration/de-trending




Current Observing fields

Data processed so far

(stellar density plot)

mass radius relation for hot jupiters
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-EXO-1b, Gl 436b since 2007 May 1)

why we need many more
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?
    • ?
core mass and formation mechanism
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
GJ 436 b


  • Gillon et al 2007 May 17, astro-ph/0705.2219
  • Neptune-mass planet
  • Neptune-like radius
  • Radius depends strongly on composition (cf. Fortney et al 2007, astro-ph/0612671)
  • Ice-giant structure.


exoplanet discovery space
Exoplanet “Discovery Space”

~100 Doppler wobble planets

Hot Planets

Cool Planets

microlensing by a star



Lensing star

Microlensing by a star
  • 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
Now if I had a REALLY big telescope...
  • ...this is what a Sun-like 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.
first definitive planetary lens event
First definitive planetary lens event!


  • 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
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!
what s in their atmospheres
What’s in their atmospheres?


Brown (2001)

transit depth


Na I

wavelength (microns)

na i absorption in hd 209458b
Na I absorption in HD 209458b
  • Charbonneau et al (2002: ApJ 568, 377)
    • Hubble Space telescope / STIS
    • Weak detection of Na!
    • Df/f ~ 2x10–4
the amazing evaporating planet
The amazing evaporating planet
  • Vidal-Madjar et al (2003) Nature 422, 123
star occults planet
Star occults planet

Spitzer/IRAC 4.5, 8.0 micron

0.2 %

Direct detection of infrared light from planet

-> effective temperature

TrES-1: Charbonneau et al. 2005

HD 209458: Deming et al. 2005

water in hd 189733b
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

the darwin mission 2018
The Darwin Mission: 2018?
  • 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
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:





history of the earth s atmosphere
History of the Earth’s atmosphere

Methane, Ammonia



Carbon Dioxide


Time before present (billions of years)

  • 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
Postcards from Titan

Image Credit:

NASA/JPL/University of Arizona

transiting extrasolar giant planets
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.