Where we stand N = N* fs fp nh fl fi fc L/T Today we revisit Habitable Zones This affects: • fs • nh Habitable Zones. II. The Con6nuous and Galac6c Habitable Zones The Habitable Zone in the Solar System Loca6on depends on assump6ons about atmospheric composi6on and albedo • Inner edge: • 0.84 – 0.95 AU • Outer edge: • 1.37 – 1.67 AU ____________ 0.9 – 1.5 AU Kas6ng, J.F., Whitmire, D.P., & Reynolds, R.T. Science, 101, 108 (1993) The Con6nuously Habitable Zone The Con6nuously Habitable Zone The faint young Sun problem: • Stars evolve -­‐ stars brighten with 6me • 4.5 Gya, the Sun was 70% of its current luminosity • In 5 Gyr, the Sun will brighten by a factor of 2 Why do stars brighten with 6me? • Sun is in hydrosta6c balance – Pressure is propor6onal to nT – n is density of par&cles – Pressure depends on total mass • Nuclear fusion reduces the number of par6cles in the core – 4 protons + 4 electrons -­‐> 1 He nucleus + 2 electrons – As n decreases, T increases – Reac6on rates increase, and Luminosity goes up The Faint Young Sun Problem The Con6nuously Habitable Zone The faint young Sun problem: • Stars evolve -­‐ stars brighten with 6me • 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 Con6nuously Habitable Zone The Con6nuously Habitable Zone • Details depend on assumed planetary atmosphere, and its evolu6on • Inner edge at 0.9 x 0.7¼ = 0.8 AU • Width es6mated to be 0.2 – 0.7 AU • Earth exits CHZ by 7 Gyr Conclusions • Earth is in the Habitable Zone • Earth is in the Con6nuously 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 (T increases as L¼) 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 • Implica6ons for probability of planets in HZ Width of the Habitable Zone Other Stars and the CHZ • Higher mass stars – Evolve faster than the Sun – If a planet must be in the CHZ for 4.6 Gyr, m<1.1 m¤ – More massive stars have more UV radia6on – More muta6ons è faster evolu6on 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 è atmospheric dynamics – Thick atmosphere è uniform T (like Venus) – Atmospheric collapse? • Tidal locking è Loss of magne6c fields – Stellar wind stripping of atmosphere • Slow evolu&on: – Enhanced ionizing flux for long periods • Enhanced radia6on/mutagenic effects – Enhanced stellar winds for long periods • Efficient atmospheric stripping Planets of M stars • Habitable? Maybe • Earth-­‐like? No Consequences for Drake’s Equa6on • fs: M dwarfs (75% of all stars), may be unsuitable • nH: may be larger for more massive stars The CHZ is important if complex life takes 6me to evolve The Galac6c Habitable Zone Reference: Lineweaver, C.H., Fenner, Y. & Gibson, B.K Science, 303, 59 (2004) I: Metals Metals II: Danger The Habitable Zone The Habitable Zone for Complex Life Conclusions • Metal abundance increases with 6me • 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 Equa6on Let’s add another term fGHZ: • About 10% of stars are solar metalicity of greater • About 2% of stars are far enough out to “safe” fGHZ ~ 0.002 N = N* fs fGHZ fp nh fl fi fc L/T