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Extrasolar Planets
What’s an Extrasolar Planet?
•An extrasolar planet is a planet that goes around another star
•A star is something that is hot enough to fuse hydrogen
•More on this later
•A brown dwarf is too small to be a star, and too large to be a
planet
•A planet is something that is large enough, but not too large
•Lower limit – irrelevant, we can’t see them that small
•Upper limit – about 13 times Jupiter’s mass
Extrasolar Planets Found = 861, Systems Found = 677
Detection Methods
Methods for finding them
•Direct Imaging
•Gravitational Microlensing
•Astrometry
•Pulsar Timing Dynamical
•Other Timing
Methods
•Radial Velocity
•Transit Method
Direct Imaging
Disadvantages
•Almost impossible to see planet next
to bright star
•Must be done from space
Advantages
•Can estimate size of planet
•Can study spectrum of planet
Score = 32
Gravitational Microlensing
The Technique
•Einstein – Light is bent by gravity
•Any object passing in front of a distant star can cause
it to look brighter
Foreground Star
Distant Star
Planet
•By measuring brightening carefully, can tell if there
is just a star or star plus planet
Gravitational Microlensing
Disadvantages
•Requires lucky alignment
•Tells little about the planet
•Cannot be repeated
Advantages
•Can monitor many stars cheaply
•Sensitive to small masses
Score = 18
Dynamical methods
•Astrometry
•Pulsar Timing
•Other Timing
•Radial Velocity
How they work
•Planet orbits star
•Star also goes in a small circle
•Easier to detect motion of star than of planet
Astrometry
How it works
Star moves left and right as viewed by us
Advantages
•Can see planets
with large orbits
Disadvantages
•Insensitive
•Massive planets
•Large Orbits
•Patience
Score = 1
Pulsar Timing
How it works
•Pulsar sends out very regular signal
•Pulsar is slightly nearer/farther from us
•Pulses come slightly late/early
Advantages
Disadvantages
•Very sensitive
•Works only on Pulsars (weird stars)
•Cheap
•See low mass planets
Score = 5
Other Timing
How it works
•There are other very regular signals from stars
•Orbiting eclipsing binary stars (9)
•Extreme horizontal branch stars (1)
•Signals come slightly late/early
Advantages
•Can see planets
with large orbits
Disadvantages
•Works only in very special
circumstances (rare)
Score = 10
Radial Velocity / Doppler Method
How it works
•Star moves towards and away from us
•Alternating red-shift / blue shift
Advantages
•Currently most
sensitive method
Disadvantages
•Massive planets
•Small Fast Orbits
•No observation of planet
Score = 501
Transit Method
How it works
•Planet blocks light from star, making it dim
•Star blocks light from planet
Disadvantages
•Requires Luck
•False Positives
(requires confirmation)
Advantages
•Works well from space
•Measure spectrum of planet
•Measure atmosphere of planet
Score = 294
Spacecraft for the Transit Method
COROT
•In operation since 2007
•Should see planets ~ Earth size
•17 discovered so far
Kepler
•In operation
since 2009
•Should see
planets < Earth
size
•15 so far
•1000+
candidates
found, more
coming
Latest Candidates from Kepler 01/07/2013
Earth-sized planets from Kepler
What Have We Learned?
•Many large planets – Jupiter size and greater
•Planets as small as Earth and smaller
•Observational bias favors finding large planets
•Often find planets very close to star
•Observational bias
•Gas giants can live very near their stars
•Orbits often highly eccentric
Lessons from Extrasolar Planets?
•We probably need to rethink our understanding of how
stellar systems form
•Planetary migration is often an important factor?
•Giant planets form far away
•Later move inwards
•Eccentric and even retrograde planets are not rare
•Planets are not uncommon
•Certainly > 10% of nearby sun-like stars
•Stay tuned