18 - Habitable Zones around Stars

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18 - Habitable Zones around Stars
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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!

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1970s Michael Hart re-posed Fermis 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!
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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
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Planet radiates (roughly)
In EQUILIBRIUM
More Simply
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For our own solar system Planet d(AU) a
Predicted T Observed T Mercury 0.39 0.056 440 1
00-620 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 - ?????
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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 ?
lt 1.
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If a 0.39 and ? 0.5 (add some greenhouse)
If a 0.39 (Earths reflectivity) but ? 1 (no
greenhouse)
If a 0 and ? 1 (blackbody planets)
The HZ (ecoshell) depends on the properties of
the planet As stars L changes and planets
atmosphere evolves, the HZ MOVES!! - Related to
Faint Sun Problem - how was life on Earth
possible when Lsun was 25 less???
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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 (2FeOO ?
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 lt 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.
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• MAIN RESULTS
• His "best" initial gas composition was 84 H2O,
  14CO2, 1CH4, 0.2N2
• 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.
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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)
lt 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!!
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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 gt1.20 Red Giant Too
Soon 1.20 F7 1.616 1.668 0.054 1.15 F8 1.420 1.4
81 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
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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.
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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?"
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Some shortcomings of Hart models addressed by
James Kasting others
Newer models are somewhat more optimistic
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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?