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Lecture 34
ExoPlanets
Astronomy 1143 – Spring 2014
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Key Ideas:Many planets around other stars foundTwo methods:• Transits of host star• Gravitational microlensing
Finding Earth-mass planets is tough!• All methods more sensitive to bigger planets
Not all solar systems look like ours• Hot Jupiters – massive planets near star• Planets on highly elliptical orbits
More familiar situations• Multiplanet systems• Rocky Planets in habitable zones (not yet around Sun-like star)
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Are we alone?
The question of the existence of other planets beyond the Solar System is anold one in Astronomy.
• Are there solar systems around other stars?• Are such solar systems like ours or
different?• Are any of the planets like the Earth?• Has life arisen on other planets?• Has intelligent life arisen on other planets?
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Scientific Questions
Finding other solar systems test our ideas of how solar systems form
•Do all have terrestrial and Jovian planets?
•What are the periods and distances of the planets?
•Does the kind of solar system depend on the type of star?
•What about systems with multiple stars?
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What is a planet?
Star: massive enough to ignite 1H fusion in its center. Mass > 0.08 Msun
Brown Dwarf: no 1H fusion, but is massive enough to ignite 2H fusion. Mass between 0.05 Msun and 0.08 Msun
Planet: No nuclear fusion Orbits star Shape determined by gravity Dominates its local gravity
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Searches for ExoPlanets
There are two basic search strategies:
Light Detection:• Images of planets orbiting other stars.• See planets transit their parent star, causing a
characteristic drop in brightness.
Gravitational Detection:• Orbital motions of the star because of the
planet's gravity.• Gravitational microlensing by the planet.
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Direct Imaging: Not this easy!
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ProblemsStar is very bright (at all wavelengths!!!), planet is very dim•A million to 10 billion times brighter
Planet is very close to star on the sky because of distance
Planet usually lost in the glare/diffraction spikes/Airy disk of star
Not a planet
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Direct Imaging
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Planetary Transits
Planet's orbital plane along the line of sight:• The planet periodically crosses (transits) the face
of its parent star.• Star dims slightly (fraction of a %) during transit.• Biased towards close-in Jupiter-sized planets.
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Earth in Transit
We are on a distant planet, looking back at the Earth-Sun system. If the Earth transits the star from our perspective, what fraction of the surface area of the Sun will it block?
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Earth in Transit
0.000084 of the Sun will be blocked. Therefore the Sun will appear 0.000084 less bright when the Earth transits it.This is a very small change, and difficult to detect.
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CoRoT and Kepler Missions
Measuring changes in the brightness of stars is much easier above the Earth’s atmosphere
•No blurring/extinction from atmosphere
•Can always be looking (no bright blue sky)
In recent years two spacecraft took repeated observations of nearby stars, looking to detect Earth-sized planets in transit
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Transiting Planet HD 209458b• Mass of ~0.7 Jupiters• ~0.045 AU from its star• Orbital Period of ~3.5 days
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Smaller Planets, Harder to Find!
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Some Planets Discovered by Kepler
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Gravitational MicrolensingTwo stars line up:
Light from the background star is amplified by the gravity of the foreground"lensing" star.
• Brief brightening of thebackground star asthey pass
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Microlensing by Planets
If there is a planet around the lensing star, it will amplify the light as well.
22 planets have been found this way by the MicroFUN team led by OSU!
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Jupiter-mass planetdiscovered in 2005by the MicroFUNCollaboration
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Detected ExoplanetsAs of 17 April 2014, --1782 planets had been confirmed• Planetary Transits• Radial velocity variations• Microlensing• Direct Imaging
Transits are the most effective method• 1132 planets so far• Kepler has many more candidates• Other upcoming missions
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We can’t “see” all planets
Planetary transits• Larger radius = better• Closer in =better• Edge-on orientation = necessary
Gravitational microlensing• More massive = better• More distant = better• Alignment = necessary
Bigger (in mass or radius) planets are always easier to find, regardless of technique. Most techniques favor detection of close-in planets as well
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Exoplanets compared to Solar System
Exoplanets as of 1 year ago
S. Seager, Science
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Exoplanet radius & mass
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Exoplanet radius & mass
Earth Mass
Earth Radius
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Exoplanet Semi-major Axis & Mass
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Exoplanet Semi-major Axis & Mass
Earth’s Semi-major Axis
Earth Mass
Lots of massive planets
Not very many distant planets
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Picture of Our Solar System
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Planets to Scale
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Upsilon Andromedae – Hot Jupiters
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Upsilon Andromedae
Comparison of our Solar System and Upsilon Andromeda
o Extremely close inner planet
o More elliptical orbits
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HD 80606b – Very EllipticalPlanet mass = 4 Jupiter masses
Originally discovered by Doppler wobble
e=0.95
periastron=0.03 AU
apastron=0.88 AU
When star eclipsed it, we could learn about the light from the planet
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Every 111 days, things get interesting
When the planet is at periastron
• T spikes by 682 K in six hours
• Goes from 980oF to 2240oF
• Then drops by same amount
Creates weather on planet
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Habitable Zone
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Planetary Properties
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Kepler 37 – small planets!
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Kepler 186f
Planet about the size of the Earth
Likely to be rocky, but don’t know its mass yet
Orbital period – 130 days
In a system with 4 inner planets
Host star is ½ the size and mass of the Sun• Habitable zone much closer than 1 AU• Receives about 1/3 as much energy as Earth
Very exciting to discover a rocky planet in the habitable zone around a star
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Kepler 186f
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Kepler 186
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Comparison with 55Cnc
5 Planets detected so far1. ~ mass of Uranus with a period=3 Earth days
2. ~ mass of Jupiter with a period=14.7 Earth days
3. ~ mass of Saturn with a period=44 days
4. ~ ½ mass of Saturn with a period=260 days
The first four planets are closer to the star than 1 AU
5. ~ 4 times the mass of Jupiter with a period=14 years, at a distance of 5 AU
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OGLE-2006-109 System
Our Solar System
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The Present & Future
Continuing search for other systems
Find systems more like our own
How common are planetary systems?
Future Goals:
Find Earth-sized planets.
Find Earth-sized planets in orbits where liquid water is possible
Search for Life Markers like O2 & O3 in their atmospheres