news global warming: northern hemisphere warms faster, affecting rainfall patterns. hungry
TRANSCRIPT
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News• Global Warming: Northern hemisphere warms faster,
affecting rainfall patterns. http://www.sciencedaily.com/releases/2013/04/130402162559.htm
• Hungry black hole wakes up for a planet-sized
snack
• Supernova Remnant 1987A Reveals Its Secrets
• New Insights on How Spiral Galaxies Get Their Arms
• Monkeys Show Why Middle Managers Get Stressed
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AST101 Lecture 18
Extra Solar Planets
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Finding Planets1. Direct Imaging
HR 8799
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Direct Searches
Direct searches are difficult because stars are so bright.
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How Bright are Planets?
Planets shine by reflected light.
The amount reflected is the amount received (the solar constant)
- Times the area of the planet
- Times the albedo (reflected), or - Times (1-albedo) (emitted)
Lp = L*/4πd2 a πRp2 ~ L* (Rp/d)2
For the Earth, (Rp/d)2 ~5 x 108
For Jupiter, (Rp/d)2 ~108
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How Bright are Planets?
You gain by going to long wavelengths,where the Sun is relatively faint, and the planet is relatively bright.
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How Far are Planets from Stars?
By parallax, 1 AU = 1“ at 1 pc•1 pc (parsec) = 3.26 light years•1“ (arcsec) = 1/3600 degree
As seen from α Centauri (4.3 LY):•Earth is 0.75 arcsec from Sol•Jupiter is 4 arcsec from Sol
Can we see this? Yes, but it takes special techniques, and is not easy.
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HR 8799(A5V)
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Finding Planets
2. Transits
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Finding Planets
2. Transits
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Transits
Transits requires an edge-on orbit.•Jupiter blocks 2% of the Sun's light•the Earth blocks about 0.01%.
Artist’s ConceptionVenus, 8 June 2004
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Finding Planets
3. Astrometric Wobble
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Finding Planets
4. Doppler Wobble
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OrbitsPlanets do not orbit the Sun - they both orbit the center of mass.
The radius of the orbit is inversely proportional to the massThe radius of the Sun’s orbit with respect to the Earth is 1/300,000 AU, or 500 km
R1 M1 = R2 M2; a = R1 + R2
This is Newton’s law of equal and opposite reactions.
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Orbital Velocity V = 2πr/P• r is the radius of the orbit• P is the orbital period• V is the orbital velocity
How fast does the star “wobble”?
Kepler’s 3rd law: P2 = a3 a ~ rp (M* >> Mp)r* = mp/m* rp (center of mass)
V* = 2π mp/m* /(rp)1/2
V⊕ = 2 cm/s; VJ = 3 m/s
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51 Pegasi b
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Doppler Wobble: Gliese 876
The three planets of Gl 876: masses = 2.5 MJ, 0.8 MJ, and 7.5 M⊕
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Gliese 876
M4V star3 planets, including the least massive known (0.75 M⊕)
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Extrasolar Planetary Systems
55 Cancri (G5V): 5 planets•1 MU 0.4 AU•1 MJ 0.15 AU•1 Ms 0.25 AU•0.5 MJ 0.8 AU•4 MJ 5 AU
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Biases and Limits
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Extrasolar Planets
Planets are preferentially found around metal-rich stars
- mostly younger than the Sun.
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Habitable Zones
Refer back to our discussion of the Greenhouse Effect.
Tp ~ (L*/D2)0.25
The habitable zone is the region where the temperatureis between 0 and 100 C (273 and 373 K), and water canbe liquid.
Tp depends on both the solar luminosity L* and thedistance D.D does not change, but L* does, as the star evolves.
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Faint Young Sun Problem
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