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The Galactic Habitable Zone. Guillermo Gonzalez Iowa State University. Fermilab. Acknowledgements: Don Brownlee Peter Ward. August 21, 2002. Starting Assumptions. Chemical life requiring carbon and liquid water,
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The Galactic Habitable Zone Guillermo Gonzalez Iowa State University Fermilab Acknowledgements: Don Brownlee Peter Ward August 21, 2002
Starting Assumptions • Chemical life requiring carbon and liquid water, • The surface of a terrestrial planet in the CHZ is the favored habitat for complex life, • Our present understanding of stellar nucleosynthesis and Galactic chemical evolution are generally correct (though incomplete in details).
Relevant Factors Two broad categories: • Requirements for a habitable planet* • Survival of existing complex life *See Gonzalez, Brownlee, and Ward (2001, Icarus, 152, 185).
Terrestrial Requirements What does it take to build an Earth-like planet? The Earth is composed mostly of iron and oxygen, followed by silicon and magnesium. The absolute and relative abundances of these elements are not uniform in the Milky Way Galaxy in time or in space. Therefore, the distribution of Earth-like planets in the Galaxy will not be uniform in place and time.
Galactic Metallicity Trends The metallicity of the interstellar medium has been increasing since the Galaxy first formed. We can determine it from observations of nearby Sun-like stars. Mean trend is: [Fe/H] = -0.035 t dex, where t is age in Gyrs. The metallicity of the local interstellar medium is solar. The scatter in [Fe/H] among stars forming near Sun is about ± 0.08 dex. Also expect “cosmic noise” in formation of planets.
Galactic Metallicity Trends The Galaxy’s disk has a radial metallicity gradient. It is seen with several tracers: B stars, cepheids, Sun-like stars, open clusters, planetary nebulae, and H II regions. Average value is -0.07 dex/kpc. Other spiral galaxies like the Milky Way are observed to have similar metallicity gradients.
Evolution of planet mass based on interstellar metallicity increase with time at the Sun’s location.
Evolution of planet mass based on interstellar metallicity increase with distance at the present time.
Other Compositional Factors • Radioactive isotopes for terrestrial planets, • Giant planet metallicity dependence (mass, migration, eccentricity), • Mg+Si/Fe ratio for terrestrial planets, • Comets The functional metallicity dependencies of terrestrial planet, giant planet, and comet formation are probably distinct.
Radioactive Isotopes To power plate tectonics and a magnetic field on an Earth-like planet for several billion years, a sufficient concentration of radioactive elements are required. The most important ones are: 40K, 232Th, 235U, 238U.
Evolution of heat production in Earth-mass planet 4.5 Gyr after its formation versus formation time.
Causes of Metallicity Difference What are the possible causes of the high metallicities in the stars with giant planets? • The high metallicities are primordial and make it more likely that planets will form, • The high metallicities are primordial and make it more likely that planets will undergo large migration (and therefore be easier to detect), • The high metallicities are due to accretion of planetary material (“self-enrichment”).
Possible Consequences • If giant planets in ~1-10 year eccentric orbits are more likely to form from metal-rich clouds, then high metallicity disfavors the formation of habitable Earth-like planets, • If low metallicity results in low mass giant planets, they may not provide sufficient protection from comets.
Survival of Complex Life • Transient Radiation Events (supernovae, gamma ray bursts, Galactic nucleus outbursts), • Comet showers resulting from perturbation of Oort cloud comets by nearby stars, giant molecular clouds, and Galactic tide.
Evolution of SN rate with time. Timmes et al. (1995)
Radial Variation of SNe. Data from Case and Bhattachrya (1998)
Stars are more densely packed towards the center of the Galaxy, resulting in more frequent perturbations of the Oort cloud.
Dynamical Issues • Corotation radius -- the Sun is near it; the Sun also has a relatively “cold” orbit. • Orbital diffusion -- stellar orbits become “hotter” with time, making it more likely that they will cross spiral arms. Mid-plane crossing speed may also be an issue. • Spiral arms -- contain most of the star formation in the disk.
Vallée (2002) Sun is located midway between two major spiral arms.
Summary of GHZ • Compositional requirements for habitable planetary system formation. • Galactic-scale threats to complex life. • Both limit the places and times in the Galaxy where habitable planets can exist. • The GHZ has fuzzy boundaries, due to cosmic scatter of metallicity at given time and location in Galaxy and cosmic scatter of terrestrial planet mass at a given metallicity. Take home message: Galactic environment is relevant to habitability.
