Download Planets Beyond the Solar System

Planets Beyond the Solar System:
Is our Solar System Unusual or Normal?
Eric Jensen
Swarthmore College
Alumni 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 present
1995 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
Sun’s diameter
Figure from Planet Quest, © 1997 by Ken Croswell
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:
The amount of light blocked by the planet is
2
Planet's area πRPlanet
=
2
Star's area
πRStar
€
We know Rstar, so measuring the dimming
gives us Rplanet!
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
Model map of planet surface brightness
Knutson et al.,
Nature, May
2007
What have we learned so far?
Not all stars are created equal
Abundance of ‘heavy’ elements matters
<3% of stars observed
have detected planets
Sun’s
abundance
25% of stars
observed
have
detected
planets
Log fraction of elements > helium, relative to Sun
The more ‘heavy’ elements the
parent star has, the more likely
it is to host a planet
Plot courtesy of Greg Laughlin
Low-mass planets are more common
Planet mass (Jupiter masses)
Planets found so far are
Jupiter-like, not rocky
Planets
Stars
Saturn
Bakos et al. 2007
Jupiter
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.
Convection on Sun’s surface
• Motion of Sun due to Earth is 0.1 meter/second.
Limitations of Doppler shift
Radial 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 2007
Figure: Greg Laughlin
Rotation plus active regions can mimic transits
The future
The era of Earth-like planet detection?
• 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?)
Are we typical?
Are we alone?
• 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.
The universe is infinitely wide.
Its vastness hold innumerable atoms,
Beyond all count, beyond all possibility of number
Flying along their everlasting ways.
So it must be unthinkable that
Our 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 rest
Alone, 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
2:1
5:3
3:2
4:3
50
Orbital Inclination [deg]
40
30
20
10
N < 1 Opposition
N ! 1 Opposition
0
30
Image from Dave Jewitt
35
40
Semi-Major Axis [AU]
45
50