Habitable Zones. The Problem: Find Earth 2.0 •  Requirements: need liquid H2O for 4.5 Gyr. •  Implica0ons: limited range of surface temperature and pressure The Habitable Zone in the Solar System LocaFon depends on assumpFons about atmospheric composiFon and albedo •  Inner edge: •  0.84 – 0.95 AU •  Outer edge: •  1.37 – 1.67 AU ____________ 0.9 – 1.5 AU KasFng, J.F., Whitmire, D.P., & Reynolds, R.T. Science, 101, 108 (1993) The ConFnuously Habitable Zone The ConFnuously Habitable Zone The faint young Sun problem: •  Stars evolve -­‐ stars brighten with Fme •  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 Fme? •  Sun is in hydrostaFc balance –  Pressure is proporFonal to nT –  n is density of par0cles –  Pressure depends on total mass •  Nuclear fusion reduces the number of parFcles in the core –  4 protons + 4 electrons -­‐> 1 He nucleus + 2 electrons –  As n decreases, T increases –  ReacFon rates increase, and Luminosity goes up The Faint Young Sun Problem The ConFnuously Habitable Zone The faint young Sun problem: •  Stars evolve -­‐ stars brighten with Fme •  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 ConFnuously Habitable Zone The ConFnuously Habitable Zone •  Details depend on assumed planetary atmosphere, and its evoluFon •  Inner edge at 0.9 x 0.7¼ = 0.8 AU •  Width esFmated to be 0.2 – 0.7 AU •  Earth exits CHZ by 7 Gyr Conclusions •  Earth is in the Habitable Zone •  Earth is in the ConFnuously 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 Life0me •  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 •  ImplicaFons 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 radiaFon –  More mutaFons è faster evoluFon 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 magneFc fields –  Stellar wind stripping of atmosphere •  Slow evolu0on: –  Enhanced ionizing flux for long periods •  Enhanced radiaFon/mutagenic effects –  Enhanced stellar winds for long periods •  Efficient atmospheric stripping Planets of M stars •  Habitable? Maybe •  Earth-­‐like? No Consequences for Drake’s EquaFon •  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 Fme to evolve The GalacFc 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 Fme •  Metal abundance decreases with galacto-­‐
centric radius •  Danger decreases with galacto-­‐centric radius Earth is in the right place at the right 0me!