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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 Where we stand

N = N* fs fp nh fl fREfi fc L/T

This affects:

• fs

• nh The Connuous and Galacc Habitable Zones Habitable Zone(s) in the Locaon depends on assumpons about atmospheric composion and albedo • Inner edge: 0.84 – 0.95 au

• Outer edge: 1.37 – 1.67 au ______0.9 – 1.5 au

Kasng, J.F., Whitmire, D.P., & Reynolds, R.T. Science, 101, 108 (1993)

Plus the icy moons The Connuously Habitable Zone The Connuously Habitable Zone

The faint young problem: • evolve - stars brighten with me • 4.5 Gya, the Sun was 70% of its current • In 5 Gyr, the Sun will brighten by a factor of 2 The Faint Young Sun Problem The Connuously 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 Connuously Habitable Zone The Connuously Habitable Zone

• Details depend on assumed planetary atmosphere, and its evoluon • Inner edge at 0.9 x 0.7¼ = 0.8 au • Width esmated to be 0.2 – 0.7 au • exits CHZ by 7 Gyr

Conclusions • Earth is in the Habitable Zone • Earth is in the Connuously Habitable Zone

• 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 , Luminosity ~ M3 • On lower main sequence, L ~ M4.5 • T = (( [1-a] L)/ (σπd2))¼

Stellar Lifeme • 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 in HZ

Width of the Habitable Zone Other Stars and the CHZ

• Higher mass stars – Evolve faster than the Sun

– For a to be in the CHZ for 4.6 Gyr, m*<1.1 m¤ – More massive stars have more UV radiaon – More mutaons è faster evoluon of life? • Lower mass stars – Evolve more slowly than the Sun – HZ is closer to

– 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 magnec fields – 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 Galacc Habitable Zone

Reference: Lineweaver, C.H., Fenner, Y. & Gibson, B.K Science, 303, 59 (2004) Shape of the

Near-IR composite: COBE/DIRBE 1.25, 2.2 3.5 µm Shape of the Galaxy You are here

• About 28,000 light from the Center of the Galaxy. • Our orbital velocity is about 220 km/s. • The Galactic 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 • 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 – Orbits in plane of Galaxy

• Spheroidal population (population II) – Older stars – Lower metallicity – Randomly-directed orbits – Globular Clusters 5 Million Years of Stellar Moons The Neighborhood My God, it’s full of stars… Galacc 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 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 fƒ 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 fƒ fEu fm fi fc L/T N = 32,000