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Planets Beyond the Solar System:Is our Solar System Unusual or Normal?

Eric JensenSwarthmore CollegeAlumni Weekend, 06/08/2007

The past

~ 4000 BCE to 1995

How many planets are there?

• ~ 4000 BCE: 5 (Mercury, Venus, Mars, Jupiter, Saturn)

• ~ 1610: 6 (Earth)• 1781: 7 (Uranus)• 1846: 8 (Neptune)• 1930: 9 (Pluto)• Mid-1995: Still 9!• June 8, 2007: 237

How many planets are there?• June 8, 2007: 237

– 229 extrasolar planets around 195 stars– 8 planets in our solar system

This and similar graphs are created from data in the Extrasolar Planets Encyclopedia, http://exoplanet.eu/

Known extrasolar planets, Dec. 1997

Currently known extrasolar planets

Image from the California and Carnegie Planet Search, http://exoplanets.org/

The present1995 to now

The era of Doppler-shift detections

The giant-planet era

Techniques for planet discovery

• Detecting stellar “wobbles” caused by planets:

– Doppler shift: measuring velocity changes

– Astrometry: measuring position changes

• Detecting eclipses of starlight by orbiting planets

• Direct imaging of planets

Techniques for planet discovery

• Detecting stellar “wobbles” caused by planets:

– Doppler shift: measuring velocity changes

– Astrometry: measuring position changes

• Detecting eclipses of starlight by orbiting planets

• Direct imaging of planets

Successful search strategies to date

The Sun moves as the planets orbit it

Figure from Planet Quest, © 1997 by Ken Croswell

Sun’s diameter

Detecting the wobble

• The Sun moves at about 12 meters/second (25 mph), mostly due to Jupiter.

• The size of this path is about 0.001 arcsec (less than a millionth of a degree) as seen from 10 parsecs (33 light years) away.

Doppler Shift:

• Motion toward or away from us shifts the wavelength of light received.

• Side-to-side motion has no effect.

Image from http://threadless.com/

Example: velocity vs. time for 51 Peg

Doppler shift surveys

• Current surveys are monitoring ~ 2000 nearby stars.• Results to date show that about 6% of Sun-like stars

surveyed have Jupiter-like planets closer than 5 AU.• These surveys cannot detect Earth-like planets;

Saturns (at Saturn’s distance from the star) are marginal.

Photometry of planet eclipses, a.k.a. ‘transits’

Image © Hans Deeg

Artist’s conception of a transit

© 1999 Lynette Cook; used by permission of the artist

Transits measure a planet’s radius:

We know Rstar, so measuring the dimming gives us Rplanet!€

Planet's areaStar's area

=πRPlanet

2

πRStar2

The amount of light blocked by the planet is

1999: Transits detected!

Charbonneau et al. 1999

Hubble Space Telescope data from Brown et al. 2001

Transits across the star HD 209458• Transits tell us that Rplanet = 1.5 RJupiter

• Doppler shift tells us that Mplanet = 0.7 MJupiter.• Combined, we find that the density is 0.23 g/cm3.

(Earth is 5.5 g/cm3, Jupiter is 1.3 g/cm3)• This is clearly a gaseous, Jupiter-like planet!

Charbonneau et al. 2000

The era of transiting planet detections is just starting

Secondary eclipses can reveal planet surface structure

Planet getting brighter as it orbits

Knutson et al., Nature, May 2007

Model map of planet surface brightness

What have we learned so far?

Not all stars are created equalAbundance of ‘heavy’ elements matters

Log fraction of elements > helium, relative to Sun

Sun’s abundance

25% of stars observedhave detected planets

<3% of stars observedhave detected planets

The more ‘heavy’ elements the parent star has, the more likely

it is to host a planet

Plot courtesy of Greg Laughlin

Planet mass (Jupiter masses)

Low-mass planets are more common

Planets found so far are Jupiter-like, not rocky

JupiterSaturn

Planets

Stars

Bakos et al. 2007

Planets have weather!Simulations of HD 185269 b

Temperature Range: 900K to 1760K

Simulations by Jonathan Langton

Limitations of current techniques for finding planets

They are largely sensitive only to close-in, Jupiter-like planets.

Limitations of Doppler shift

• Convection on stars’ surfaces, and stellar pulsations, cause radial velocity variations of ~ 1 meter/second.

• Motion of Sun due to Earth is 0.1 meter/second.

Convection on Sun’s surface

Limitations of Doppler shiftRadial velocity variations from stellar pulsations

Bouchy et al. 2005

Limitations of photometry• Even the “quietest” stars

have some random variability due to, e.g., sunspots.

• An Earth-mass planet dims a Sun-like star by about 0.005%.

• Only about 1 Earth-like planet in 100 would be oriented so that an eclipse would be visible to us.

Limitations of photometry:small planets are hard to see!

Data: first planet detected by the COROT mission, May 2007Figure: Greg Laughlin

Rotation plus active regions can mimic transits

The future

• Photometric (transit) detection: NASA’s Kepler mission (launch November 2008)

• Astrometric detection? NASA’s Space Interferometer Mission

• Direct imaging? NASA’s Terrestrial Planet Finder (launch 2014?)

The era of Earth-like planet detection?

Are we typical?

• We don’t know yet! But we’ll find out soon...• Current techniques are biased toward close-in,

giant planets• Detecting planets like our giant planets takes a

long time (Jupiter: 12 years; Saturn: 29 years)• Detecting Earth-like planets is hard! But

Kepler mission has a chance in next five years.

Are we alone?

The universe is infinitely wide.Its vastness hold innumerable atoms,Beyond all count, beyond all possibility of numberFlying along their everlasting ways.So it must be unthinkable thatOur sky and our round world are precious and unique…Out beyond our world there are, elsewhere,Other assemblages of matter making other worlds.Ours is not the only one in air’s embrace…You’ll never find one single thing,Completely different from all the restAlone, apart, unique,Sole product, single specimen of its kind…There are other worlds, more than one race of people,And many kinds of animals. Lucretius, 70 BCE

What about Pluto?

Image from http://threadless.com/

The newest dwarf planet:Eris, larger than Pluto

Image from Mike Brown

Pluto shares many properties with other Kuiper Belt Objects

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30 35 40 45 50

N < 1 Opposition

N ! 1 Opposition

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Semi-Major Axis [AU]

3:2

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Image from Dave Jewitt