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2003 UB313: The 10th Planet?
Extra-Solar or Exoplanets
• Planets around stars other than the Sun
• Difficult to observe
• Hundreds discovered (> 2000 so far)
Circa 2013, Courtesy Kepler: Billions!!
• Jupiter to Earth sizes
• Surprise: Some stars with Jupiter mass
planets within 1 AU (revised theory of
planet formation ?)
• Goldilocks Zone: Habitable Distance out to
where water is liquid
Extra-solar planets around distant stars
First image of an exoplanet
Transiting Hot Saturn with a large
dense core
Astronomers have found a weird new creature: a planet
whose core holds 65 Earths' worth of heavy rocky stuff.
Saturn-mass planet transiting
across its star
Discovering Planets with Life
Sunlight is reflected off the
Earth, hits the Moon and
bounces back to Earth
Earthshine is seen in the
faint glow our world gives to
dark areas of the Moon
The light carries information
about Earth's atmosphere
and surface properties
Scientists see details in the
light that betray different
gases, even vegetation
The knowledge can be
applied to the search for
distant worlds
Detect the presence of atoms/molecules
that make up vegetation or life
7.5 Earth-mass planet
Found: 3 'Neptunes' and a 'hot Jupiter'
600 LY’s from the Earth, the planetary system is in the habitable zone with
conditions enabling liquid water to exist
HD 188753 b - A planet in a triple
star system
5.9 Earth mass planet in Gliese
876 system
What are exoplanets?
How do we find them?
What can our detection methods tell us
about them?
How do planets form?
What have we found?
Exoplanet Detection Methods
Radial Velocity (RV)
Observe Doppler shift as star “wobbles” over
planetary orbit
Direct Imaging
Observe light dip as planet moves in front of a
Exactly what it says on the can
Use gravitational lensing to find planets
Observe light dip as planet moves in front
of the star
Find orbital period, radius of the planet
compared to the star, stellar density
Preferentially finds large, short-period
Most prolific detection method today
(Kepler mission)
Transit Video
Transit light curve
Radial Velocity
Uses the Doppler Effect
Observe shift in spectral lines as planet
tugs star to and fro
Preferentially finds massive, short-period
Find mass of planet compared to the star,
orbital period directly
RV video
RV curve
Direct Imaging
Just take a picture of the planet
VERY hard to do; planets are faint and
stars are bright
To do from the ground, need large
telescopes and adaptive optics
Preferentially find large, young planets far
from their star
Very few planets detected this way
HR 8799
Use gravitational lensing to find planets
Star with a planet passes in front of a
more distant star
Lensing star magnifies the light of the
background star; planet can affect this
Preferentially find massive planets in
orbits similar to the Earth's against dense
stellar background (galactic bulge)
Done at OSU!
Microlensing Video
Microlensing Diagram
Planet Detection Demographics
Planet Demographics Figure
Red = detected by transits
Blue = RVs/Doppler shifts
Green = microlensing
Tan = direct imaging
History of Exoplanets
RV method the first to be used
Followed by transits and microlensing
Direct Imaging only possible recently
First Exoplanet Discovered!
51 Pegasi b, using RV method
0.5 Jupiter-mass planet on ~4 day orbit
Enormous surprise; discovering scientists
proposed it wasn't a planet at all!
Opened the floodgates
First Transiting Exoplanet
HD 209458 b
Also hot Jupiter; 0.7 Jupiter masses, 3.5
day orbit
Discovered “right on time”
First planet discovered solely by transits in
Kepler Mission
Designed to stare at one patch of sky for
years on end
Looking for transiting planets
Capable of extremely high photometric
First Earth-sized planets discovered!
Thousands of planet candidates detected
Kepler Field
All Kepler Planets Discovered
So Far
Aside: Planetary Formation
Two dominant models: core accretion and
gravitational instability
Core accretion
Little rocks collide and stick together to form
bigger rocks
Eventually gravity holds big rocks together
and attracts them to each other
Build up each planet bit by bit
Gravitational Instability
Have very massive, cold disk which becomes
So what have we found?
Planets are everywhere!
Other solar systems are NOT like our
Hot Jupiters (how did those get there?)
Hot Neptunes
Many more questions...
Hot Jupiters
Gas giant planets very close to their
parent star
Problem because of planetary formation
Think that giants need to form beyond “snow
Migration? How?
Three ideas: planet-planet interactions, disks,
binary star interactions
Rare: Only about 1% of Sun-like stars
Hot Jupiter Weather
Earth-sized Planets
Very common, even around metal-poor
and less massive stars
Exist in wide range of environments
17% of stars have Earth-sized planet inside
the orbit of Mercury
~20% of stars have Earth-sized planets within
the habitable zone
Where Do We Go From Here?
Planetary atmospheres
Better planetary formation models
Protoplanetary disk evolution