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18 - Habitable Zones around Stars

18 - Habitable Zones around Stars. Why is the Earth Habitable?. Solid surface - useful for concentrating chemicals & reactions Not located in a “bad” neighborhood - low supernova rate, no nearby gamma ray bursters or “death rays”

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18 - Habitable Zones around Stars

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  1. 18 - Habitable Zones around Stars

  2. Why is the Earth Habitable? • Solid surface - useful for concentrating chemicals & reactions • Not located in a “bad” neighborhood - low supernova rate, no nearby gamma ray bursters or “death rays” • Sun had plenty of heavy elements to make terrestrial planets from (not Pop II star) • Relatively low major impact rate - major “killers” only every 100 Myr - planet-sterilizers less frequent • Large Moon stabilizes rotation axis to prevent some huge changes in climate • Temperature, temperature, temperature!

  3. 1970’s Michael Hart re-posed Fermi’s Question: “Where IS Everyone?” Are the conditions for life common or rare? How does one decide the issue? What is necessary for a terrestrial planet to have suitable conditions for the emergence of life as we understand it? Hart decided to model why it is that the Earth is currently capable of sustaining life!

  4. Habitable Zones around Stars Criterion: existence of liquid water Question: at what distances from a star will water be liquid (within a reasonable value of atmos. pressure) Depends on the Equilibrium Temperature T where the energy absorbed by the planet equals the energy it loses Energy absorbed by planet: Which can also be re-written as:

  5. Planet radiates (roughly): In EQUILIBRIUM: More Simply:

  6. For our own solar system: Planet d(AU) a Predicted T Observed T Mercury 0.39 0.056 440 100-620 Venus 0.72 0.76 230 750 (and very uniform!) Earth 1.00 0.39 250 180-330 (290 avg.) Mars 1.52 0.16 220 130-290(Sub-Solar Equatorial) Jupiter 5.2 0.51 104 160 (cloud tops) Saturn 9.5 0.61 81 90 (cloud tops) The freezing point of water is 273 K, and the boiling point is 373 K, under 1 Atm pressure. Venus is currently too hot for liquid water. Mars is too cold. The Earth is "just right". We can do the same calculation for other types of stars as well. Question: Why are most of the planets hotter than this? J & S - internal heat source - emit more than they absorb! V & E - ?????

  7. At what distance will water freeze & boil? Set L* = Lsun and calculate d for T = 273 K (water freezes) and T = 373 K (water boils, std atm pressure). With a greenhouse effect, need an additional term -   = 1 means no greenhouse effect. Otherwise  < 1.

  8. If a = 0 and  = 1 (blackbody planets) If a = 0.39 (Earth’s reflectivity) but  = 1 (no greenhouse) If a = 0.39 and  = 0.5 (add some greenhouse) The HZ (“ecoshell”) depends on the properties of the planet As star’s L changes and planet’s atmosphere evolves, the HZ MOVES!! - Related to “Faint Sun Problem” - how was life on Earth possible when Lsun was 25% less???

  9. CONTINUOUSLY HABITABLE ZONES First calculated for Earth-Sun by Hart (1978 Icarus, 33, 23-39) Included the following processes: Rate of outgassing of volatiles (H, C, N, O) from the interior Condensation of H2O vapor into oceans Solution of atmospheric gases into oceans Photodissociation of H2O in the upper atmosphere Escape of H from the uppermost atmosphere (exosphere) Chemical reactions in atmospheric gases Presence of life and variations in biomass Photosynthesis and burial of organic sediments Urey reaction (CaSiO3 + CO2  CaCO3 + SiO2) Oxidation of surface minerals (2FeO+O  Fe2O3) Variations in the luminosity of the Sun Variations in the albedo (reflectivity) of the Earth Greenhouse effect The criteria he assumed for life to arise were: Liquid water with T < 42 C for 0.8 Byr Concurrent presence of C and N in atmosphere and oceans Absence of free O in atmosphere His starting conditions: No atmosphere Albedo (reflectivity) = 0.15 (rock) Start 4.5 by ago His process: Using time steps of 2.5 Myr, vary the composition of juvenile volatiles until the best fit to present conditions is reached.

  10. MAIN RESULTS: • His "best" initial gas composition was 84% H2O, 14%CO2, 1%CH4, 0.2%N2 • Most of the H2O vapor condensed promptly into oceans • Early atmosphere was dominated by CO2 • CO2 was later removed by the Urey Reaction • O released by photolysis of H2O vapor and later by photosynthesis. This O destroys the CH4 (the O being consumed in the process, of course). By 2 by ago, most of the CH4 was gone, leaving N2 as the dominant gas. Since then, there has been a slow buildup of O2. By 420 Myr ago, enough O2 and O3 had built up to provide protection from solar UV, making life on land tolerable.

  11. OTHER IMPORTANT RESULTS AND LIMITS In these models, once CH4 was gone and the luminosity of the Sun reached its current value, if T(surface) < 278K, Runaway Glaciation occurs, and in none of the simulations is it ever reversed. This occurs 2 Byr ago if the Earth were located 1.01 AU from the Sun, a mere 1% further away! If the earth were at 0.95 AU from the Sun, a Runaway Greenhouse Effect occurs 4 by ago, and in none of the simulations is it ever reversed! These results, which include runaway effects, provide only a very narrow (0.06 AU) CHZ for the Earth. CHZ IS VERY NARROW!!

  12. What are the CHZs like for other stars? - Hart (1979 Icarus, 37, 351-357) Thickness goes to ZERO for masses less than 0.8 solar masses, and for masses greater than 1.2 solar masses. Stellar Mass SpT Rin Rout Thickness >1.20 Red Giant Too Soon 1.20 F7 1.616 1.668 0.054 1.15 F8 1.420 1.481 0.061 1.10 F9 1.240 1.310 0.069 1.05 G0 1.086 1.150 0.064 1.00 G2 0.958 1.004 0.046 0.95 G5 0.837 0.867 0.030 0.90 G8 0.728 0.743 0.015 0.85 K0 0.628 0.629 0.001 0.835 K1 0.598 0.598 0.000

  13. SKIP THIS SLIDE UNLESS YOU REALLY MUST KNOW MORE DETAILS! d  Rinner Planet always too hot for oceans to condense d  Rinner Oceans exist in early stages. Buildup in atmospheric gases and increase in stellar luminosity lead to a Runaway Greenhouse Effect after 1by. It is L*(t  1 Byr) that determines Rinner. d  Router Runaway Glaciation occurs as soon as most of the CH4 (etc.) is gone - usually occurring at t = 2.5 Byr. d  Router Runaway Glaciation does not occur until after 3.5by. It is L*(t  3.5 Byr) that sets the value of Router.

  14. OVERALL PICTURE The evolution of other terrestrial planets will be similar to that of the Earth if inside the CHZ CHZs are widest around G0 main sequence stars, and shrink to zero at F7 at the hot end, and K1 at the cool end. In all cases d  0.1 AU, suggesting that the average planetary system only had a ~1% chance for an Earth-like planet in the CHZ. Whew! Finally! "It appears therefore, that there are probably fewer planets in our galaxy suitable for evolution of advanced life than had been previously thought." M. Hart (1979). Did I say "Finally?"

  15. Some shortcomings of Hart models addressed by James Kasting & others: Newer models are somewhat more “optimistic”

  16. CONCLUSIONS Based on these models, the likelihood that a planetary system similar to our own has a terrestrial planet with conditions suitable for life is 1-10% or so. BUT WHAT ARE OTHER PLANETARY SYSTEMS REALLY LIKE?

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