Where we stand Today we revisit Habitable Zones – Standard (Teq) – Modified (greenhouse; albedo) – Extended (tidal heating; subsurface abodes) This affects: • fs • nh Where we stand N = N* fs fp nh fl fREfi fc L/T This affects: • fs • nh The Con/nuous and Galac/c Habitable Zones Habitable Zone(s) in the Solar System Locaon 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 The Con/nuously Habitable Zone 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 The Faint Young Sun Problem 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 The Con/nuously Habitable Zone 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 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! Other Stars 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) Other Stars • All stars have habitable zones • Width ~ √(L) – More massive stars have wider HZs – Less massive stars have narrower HZs • Implicaons for probability of planets in HZ Width of the Habitable Zone 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*<1.1 m¤ – More massive stars have more UV radiaon – More mutaons è faster evolu/on of life? • Lower mass stars – Evolve more slowly than the Sun – HZ is closer to star – Tidal locking for m*<0.5 m¤ Tidal Locking 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 evoluon: – Enhanced ionizing flux for long periods • Enhanced radiaon/mutagenic effects – Enhanced stellar winds for long periods • Efficient atmospheric stripping Planets of M stars • In the Habitable Zone? Yes • Habitable? Maybe • Earth-like? No Consequences for Drake’s Equaon • 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 The Galac/c Habitable Zone Reference: Lineweaver, C.H., Fenner, Y. & Gibson, B.K Science, 303, 59 (2004) Shape of the Galaxy Near-IR composite: COBE/DIRBE 1.25, 2.2 3.5 µm Shape of the Galaxy 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. You are here Monty Python: Galaxy Song 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 The Center of the Galaxy The central object Sgr A* 6 • Orbits è Mass ~ 2.5 x 10 M¤ • Orbits è radius < 1 au • Density > 0.4 g/cm3 • Unseen at any wavelength A black hole 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 5 Million Years of Stellar Mo/ons The Neighborhood My God, it’s full of stars… Galac/c Consideraons • The range of metallicity – Are metal-poor environments conducive to rocky planets? • Proximity to supernovae and ionizing radiaon – Cosmic effects can affect life I: Metals Metals II: Danger The Habitable Zone The Habitable Zone for Complex Life Consideraons • 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! Consequences for Drake’s Equaon 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 Consequences for Drake’s Equaon 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 .