where%we%stand%habitable%zone(s)%% in%the%solar%system%...
TRANSCRIPT
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Where we stand Today we revisit Habitable Zones
– Standard (Teq) – Modified (greenhouse; albedo) – Extended (tidal heating; subsurface abodes)
This affects: • fs • nh
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Where we stand N = N* fs fp nh fl fREfi fc L/T
This affects: • fs • nh
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The Con/nuous and Galac/c Habitable Zones
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Habitable Zone(s) in the Solar System
Loca/on depends on assump/ons about atmospheric composi/on and albedo • Inner edge: 0.84 – 0.95 au
• Outer edge: 1.37 – 1.67 au ____________ 0.9 – 1.5 au Kas/ng, J.F., Whitmire, D.P., & Reynolds, R.T. Science, 101, 108 (1993)
Plus the icy moons
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The Con/nuously Habitable Zone
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The Con/nuously Habitable Zone
The faint young Sun problem: • Stars evolve -‐ stars brighten with /me • 4.5 Gya, the Sun was 70% of its current luminosity
• In 5 Gyr, the Sun will brighten by a factor of 2
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The Faint Young Sun Problem
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The Con/nuously Habitable Zone
The faint young Sun problem: • Stars evolve -‐ stars brighten with /me • 4.5 Gya, the Sun was 70% of its current luminosity
• In 5 Gyr, the Sun will brighten by a factor of 2
• T = (( [1-‐a] L)/ (σπd2))¼ Temperature increases as L¼
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The Con/nuously Habitable Zone
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The Con/nuously Habitable Zone
• Details depend on assumed planetary atmosphere, and its evolu/on
• Inner edge at 0.9 x 0.7¼ = 0.8 au • Width es/mated to be 0.2 – 0.7 au • Earth exits CHZ by 7 Gyr
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Conclusions • Earth is in the Habitable Zone • Earth is in the Con/nuously Habitable Zone
• Venus is too close to the Sun – suffered a runaway greenhouse
• Mars could be in the habitable zone – (but it lost its atmosphere)
Earth is just right – for now!
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Other Stars
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Habitable Zones of Other Stars
Stellar Luminosity • On main sequence, Luminosity ~ M3 • On lower main sequence, L ~ M4.5 • T = (( [1-‐a] L)/ (σπd2))¼ Stellar Life:me • τ ~ M/L
– τ ~ M-‐2 (upper MS); – τ ~ M-‐3.5 (lower MS)
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Other Stars • All stars have habitable zones • Width ~ √(L)
– More massive stars have wider HZs – Less massive stars have narrower HZs
• Implica/ons for probability of planets in HZ
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Width of the Habitable Zone
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Other Stars and the CHZ
• Higher mass stars – Evolve faster than the Sun – For a planet to be in the CHZ for 4.6 Gyr, m*
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Tidal Locking
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Planets of M stars
• Tidal locking affects atmospheric dynamics – Thick atmosphere è uniform T (like Venus) – Atmospheric collapse?
• Tidal locking è Loss of magne/c fields – Stellar wind stripping of atmosphere
• Slow stellar evolu:on: – Enhanced ionizing flux for long periods
• Enhanced radia/on/mutagenic effects – Enhanced stellar winds for long periods
• Efficient atmospheric stripping
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Planets of M stars
• In the Habitable Zone? Yes • Habitable? Maybe • Earth-‐like? No
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Consequences for Drake’s Equa/on
• fs: M dwarfs (75% of all stars), may be unsuitable
• nH: may be larger for more massive stars – But are in CHZ for less /me
Time in the CHZ is important if complex life takes /me to evolve
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The Galac/c Habitable Zone
Reference: Lineweaver, C.H., Fenner, Y. & Gibson, B.K Science, 303, 59 (2004)
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Shape of the Galaxy
Near-‐IR composite: COBE/DIRBE 1.25, 2.2 3.5 µm
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Shape of the Galaxy
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You are here
• About 28,000 light years from the Center of the Galaxy.
• Our orbital velocity is about 220 km/s. • The Galactic Year is about 220 million
years long. • The Sun is about 21 galactic years old.
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You are here
Monty Python: Galaxy Song
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Mass of the Galaxy
The mass of the Galaxy is 2 x 1044 g, or 1011 solar masses. If the typical star is 1/4 solar masses, there are 4 x 1011 stars in the Galaxy
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The Center of the Galaxy
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The central object Sgr A*
• Orbits è Mass ~ 2.5 x 106 M¤ • Orbits è radius < 1 au • Density > 0.4 g/cm3 • Unseen at any wavelength
A black hole
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Constituents of the Galaxy ~4 x 1011 stars (90% of the visible mass) • Disk population (population I)
– Younger stars – Higher metallicity – Orbits in plane of Galaxy
• Spheroidal population (population II) – Older stars – Lower metallicity – Randomly-directed orbits – Globular Clusters
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5 Million Years of Stellar Mo/ons
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The Neighborhood
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My God, it’s full of stars…
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Galac/c Considera/ons
• The range of metallicity – Are metal-‐poor environments conducive to rocky planets?
• Proximity to supernovae and ionizing radia/on – Cosmic effects can affect life
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I: Metals
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Metals
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II: Danger
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The Habitable Zone
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The Habitable Zone for Complex Life
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Considera/ons • Metal abundance increases with /me • Metal abundance decreases with galacto-‐centric radius
• Danger decreases with galacto-‐centric radius
Earth is in the right place at the right :me!
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Consequences for Drake’s Equa/on
Let’s add another term fGHZ: • About 10% of stars are solar metalicity or greater • About 2% of stars are far enough out to be “safe”
fGHZ ~ 0.002 N = N* fs fGHZ fp nh fl fJ f fEu fm fi fc L/T
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Consequences for Drake’s Equa/on
Let’s add another term fGHZ: • About 10% of stars are solar metalicity or greater • About 2% of stars are far enough out to be “safe”
fGHZ ~ 0.002 N = N* fs fGHZ fp nh fl fJ f fEu fm fi fc L/T N = 32,000