The Galactic Habitable ZoneThe Galactic Habitable Zone

Guillermo GonzalezGuillermo GonzalezIowa State UniversityIowa State University

FermilabFermilab

August 21, 2002August 21, 2002

Acknowledgements:Acknowledgements:

Don BrownleeDon Brownlee

Peter WardPeter Ward

Starting AssumptionsStarting Assumptions

• Chemical life requiring carbon and liquid Chemical life requiring carbon and liquid water,water,

• The surface of a terrestrial planet in the The surface of a terrestrial planet in the CHZ is the favored habitat for complex life,CHZ is the favored habitat for complex life,

• Our present understanding of stellar Our present understanding of stellar nucleosynthesis and Galactic chemical nucleosynthesis and Galactic chemical evolution are generally correct (though evolution are generally correct (though incomplete in details).incomplete in details).

Relevant FactorsRelevant Factors

• Requirements for a habitable planet*Requirements for a habitable planet*• Survival of existing complex lifeSurvival of existing complex life

*See Gonzalez, Brownlee, and Ward (2001, Icarus, 152, 185).

Two broad categories:Two broad categories:

Terrestrial RequirementsTerrestrial Requirements

What does it take to build an Earth-like planet?What does it take to build an Earth-like planet?

The Earth is composed mostly of iron and The Earth is composed mostly of iron and oxygen, followed by silicon and magnesium. oxygen, followed by silicon and magnesium. The absolute and relative abundances of these The absolute and relative abundances of these elements are not uniform in the Milky Way elements are not uniform in the Milky Way Galaxy in time or in space. Therefore, the Galaxy in time or in space. Therefore, the distribution of Earth-like planets in the distribution of Earth-like planets in the Galaxy will not be uniform in place and time.Galaxy will not be uniform in place and time.

Galactic Metallicity TrendsGalactic Metallicity TrendsThe metallicity of the interstellar medium has The metallicity of the interstellar medium has been increasing since the Galaxy first formed. been increasing since the Galaxy first formed. We can determine it from observations of We can determine it from observations of nearby Sun-like stars. Mean trend is:nearby Sun-like stars. Mean trend is:

[Fe/H] = -0.035 [Fe/H] = -0.035 dex, dex,

where where is age in Gyrs. The metallicity of is age in Gyrs. The metallicity of the local interstellar medium is solar.the local interstellar medium is solar.The scatter in [Fe/H] among stars forming The scatter in [Fe/H] among stars forming near Sun is about ± 0.08 dex. Also expect near Sun is about ± 0.08 dex. Also expect “cosmic noise” in formation of planets.“cosmic noise” in formation of planets.

Galactic Metallicity TrendsGalactic Metallicity Trends

The Galaxy’s disk has a radial metallicity The Galaxy’s disk has a radial metallicity gradient. It is seen with several tracers: B gradient. It is seen with several tracers: B stars, cepheids, Sun-like stars, open clusters, stars, cepheids, Sun-like stars, open clusters, planetary nebulae, and H II regions. Average planetary nebulae, and H II regions. Average value is -0.07 dex/kpc.value is -0.07 dex/kpc.

Other spiral galaxies like the Milky Way are Other spiral galaxies like the Milky Way are observed to have similar metallicity observed to have similar metallicity gradients.gradients.

Evolution of planet mass based on interstellar Evolution of planet mass based on interstellar metallicity increase with time at the Sun’s location.metallicity increase with time at the Sun’s location.

Evolution of planet mass based on interstellar Evolution of planet mass based on interstellar metallicity increase with distance at the present time.metallicity increase with distance at the present time.

Other Compositional FactorsOther Compositional Factors

• Radioactive isotopes for terrestrial planets,Radioactive isotopes for terrestrial planets,

• Giant planet metallicity dependence (mass, Giant planet metallicity dependence (mass, migration, eccentricity),migration, eccentricity),

• Mg+Si/Fe ratio for terrestrial planets,Mg+Si/Fe ratio for terrestrial planets,

• CometsComets

The functional metallicity dependencies of The functional metallicity dependencies of terrestrial planet, giant planet, and comet terrestrial planet, giant planet, and comet formation are probably distinct.formation are probably distinct.

Radioactive IsotopesRadioactive Isotopes

To power plate tectonics and a magnetic To power plate tectonics and a magnetic field on an Earth-like planet for several field on an Earth-like planet for several billion years, a sufficient concentration of billion years, a sufficient concentration of radioactive elements are required. The most radioactive elements are required. The most important ones are:important ones are:

4040K, K, 232232Th, Th, 235235U, U, 238238U.U.

Evolution of heat production in Earth-mass planet Evolution of heat production in Earth-mass planet 4.5 Gyr after its formation versus formation time.4.5 Gyr after its formation versus formation time.

What about Giant Planets?What about Giant Planets?

Exo-planets Metallicity DistributionExo-planets Metallicity Distribution

Normalized Metallicity DistributionNormalized Metallicity Distribution

Causes of Metallicity DifferenceCauses of Metallicity Difference

What are the possible causes of the high What are the possible causes of the high metallicities in the stars with giant planets?metallicities in the stars with giant planets?

• The high metallicities are primordial and make it The high metallicities are primordial and make it more likely that planets will form,more likely that planets will form,

• The high metallicities are primordial and make it The high metallicities are primordial and make it more likely that planets will undergo large more likely that planets will undergo large migration (and therefore be easier to detect),migration (and therefore be easier to detect),

• The high metallicities are due to accretion of The high metallicities are due to accretion of planetary material (“self-enrichment”).planetary material (“self-enrichment”).

Possible ConsequencesPossible Consequences

• If giant planets in ~1-10 year eccentric If giant planets in ~1-10 year eccentric orbits are more likely to form from metal-orbits are more likely to form from metal-rich clouds, then high metallicity disfavors rich clouds, then high metallicity disfavors the formation of habitable Earth-like the formation of habitable Earth-like planets,planets,

• If low metallicity results in low mass giant If low metallicity results in low mass giant planets, they may not provide sufficient planets, they may not provide sufficient protection from comets.protection from comets.

Survival of Complex LifeSurvival of Complex Life

• Transient Radiation Events (supernovae, Transient Radiation Events (supernovae, gamma ray bursts, Galactic nucleus gamma ray bursts, Galactic nucleus outbursts),outbursts),

• Comet showers resulting from perturbation Comet showers resulting from perturbation of Oort cloud comets by nearby stars, giant of Oort cloud comets by nearby stars, giant molecular clouds, and Galactic tide.molecular clouds, and Galactic tide.

Evolution of SN rate with time.Evolution of SN rate with time.

Timmes et al. (1995)

Data from Case and Bhattachrya (1998)

Radial Variation of SNe.Radial Variation of SNe.

Stars are more densely packed towards the center of the Galaxy, resulting in more frequent perturbations of the Oort cloud.

Dynamical IssuesDynamical Issues

• Corotation radius -- the Sun is near it; the Corotation radius -- the Sun is near it; the Sun also has a relatively “cold” orbit.Sun also has a relatively “cold” orbit.

• Orbital diffusion -- stellar orbits become Orbital diffusion -- stellar orbits become “hotter” with time, making it more likely “hotter” with time, making it more likely that they will cross spiral arms. Mid-plane that they will cross spiral arms. Mid-plane crossing speed may also be an issue.crossing speed may also be an issue.

• Spiral arms -- contain most of the star Spiral arms -- contain most of the star formation in the disk.formation in the disk.

Vallée (2002)

Sun is located midway between two major spiral arms.

Summary of GHZSummary of GHZ• Compositional requirements for habitable Compositional requirements for habitable

planetary system formation.planetary system formation.• Galactic-scale threats to complex life.Galactic-scale threats to complex life.• Both limit the places and times in the Galaxy Both limit the places and times in the Galaxy

where habitable planets can exist.where habitable planets can exist.• The GHZ has fuzzy boundaries, due to The GHZ has fuzzy boundaries, due to

cosmic scatter of metallicity at given time cosmic scatter of metallicity at given time and location in Galaxy and cosmic scatter of and location in Galaxy and cosmic scatter of terrestrial planet mass at a given metallicity.terrestrial planet mass at a given metallicity.

Take home message: Galactic environment is relevant to habitability.Take home message: Galactic environment is relevant to habitability.

The Galactic Habitable Zone -- Summary