Planets Elsewhere? Protoplanetary Disks and universality suggest many stars have planets First...
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Transcript of Planets Elsewhere? Protoplanetary Disks and universality suggest many stars have planets First...
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Planets Elsewhere?• Protoplanetary Disks and universality suggest
many stars have planets• First discovery in 1988. Now 853 around 672
stars• Finding planets is tough: dim, small, near bright
star. 32 planets in 28 systems detected by imaging
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Transiting Planets• If planet eclipses star can observe light curve• Shape of curve helps find size, mass, even
properties of atmosphere of planet• 290 planets in 235 systems detected via transit• Kepler has 2321 candidate planets in 1290
systems
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Other Methods• Gravitational lensing of starlight by planet. 16
planets in 15 systems• Transit Timing Variation uses discrepancies in
transit times of eclipsing planet to predict others in same system
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What Have We Found?• 1-40% of (Sunlike) stars
have planets. Planets are ubiquitous!
• Our methods are most sensitive to hot Jupiters so these are mostly what we find
• Migration is common as are strongly interacting orbits
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What Are They Like?• Taking selection bias
into account, super Earths outnumber Jupiters
• Some SuperJupiters• Kepler-16b orbits two
stars
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The Sun Shines – but How?• Sun is big and hot so luminous
• How does it stay hot?• Chemical (rearrange electrons -
electromagnetic) burning produces per atom, or per kg.
• Need to burn so run out in
• Kelvin-Helmholtz (gravitational) energy would last
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Nuclear Physics• Why don’t nuclei break
up under electric repulsion?
• A strong attractive force binds nucleons
• Short-range since atoms do not collapse
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Nuclear Energy• Rearranging nucleons
recover nuclear energy• In large nuclei distant
nucleons barely attract• Breaking up – fission – or
emission recover electromagnetic energy
• Heats planets powers reactors
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Fusion?• In small nuclei, less
attractive interactions• Liberate nuclear energy by
fusion to Helium• Problem: Hydrogen is all
protons• Strong interactions cannot
change a proton to a neutron
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Weak Interactions• Something can do this!• And the inverse• A free neutron decays in
15min • Weak nuclear force
mediates this decay
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Some Questions and Answers• Can a force change one
particle into another?
• Is a neutron just a tiny Hydrogen atom?
• What is ?• Are there any rules?
• Conservation Laws– Mass-Energy– Momentum– Angular Momentum– Electric Charge– Electron Number
• Weak interaction: rare
Yes
No
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Particle PhysicsParticle Q Ne
1 0
0 0
-1 1
0 1
-1 0
0 0
1 -1
0 -1
0 0
• Antiparticle: same mass opposite charges
• Neutrinos almost massless, weakly interacting
• Discovered as missing energy in decay
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What it Takes• To initiate fusion, protons must overcome
electric repulsion• One proton must inverse decay before
highly unstable breaks up• Requires temperatures of - only in core • Inefficient because weak process required
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How Do We Know?• Theory (Eddington,
Bethe 1932) first• Davis, Bahcall (1968):
Detect the• Pro: Penetrate Sun• Con: Penetrate detector• Flux at Earth:
• Put a tank with of Chlorine in Homestake Gold Mine
• Requires high-energy produced in other processes
• Expect one atom per six days
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Where Are the Neutrinos?• Flux Found is less than
predictions• Is Solar Model wrong?• Is detector model
wrong?• Decided in 2001 by
SNO: particle physics
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More Particles, More ChargesParticle Q Ne Nμ Nτ Mass
1 0 0 0 935
0 0 0 0 938
-1 1 0 0 0.511
0 1 0 0 ?
-1 0 1 0 106
0 0 1 0 ?
-1 0 0 1 1777
0 0 0 1 ?