Lecture 22: Habitable Zones Around Stars

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Lecture 22 Habitable Zones Around Stars
Meteo 466
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Liquid Water is Essential for Life(as we know it)

• Clever biochemists have suggested that
  non-carbon-based, non-water-dependent life could
  possibly exist
• Nonetheless, the best place to begin the search
  for life is on planets like the Earth
• This means that we should look within the
  conventional habitable zone around nearby stars

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Definitions(from Michael Hart, Icarus, 1978)

• Habitable zone (HZ) -- the region around a star
  in which an Earth-like planet could maintain
  liquid water on its surface at some instant in
  time
• Continuously habitable zone (CHZ) -- the region
  in which a planet could remain habitable for some
  specified period of time (e.g., 4.6 billion
  years)

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Michael Harts calculations(Icarus, 1978, 1979)

• 4.6-Gyr CHZ around our own Sun is quite narrow
• 0.95 AU Runaway greenhouse
• 1.01 AU Runaway glaciation
• CHZs around other spectral types are even
  narrower
• Corollary Earth may be the only habitable planet
  in our galaxy

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Finding the boundaries of the habitable zone

• Inner edge determined by loss of water via
  runaway or moist greenhouse effect
• Venus is a case in point

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J. F. Kasting, Icarus (1988)
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Implications Inner edge of the HZ is at S/So ?
1.1, corresponding to a semi-major axis of 0.95
AU - This could be overly pessimistic if (as
seems likely) clouds provide negative climate
feedback
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Finding the boundaries of the habitable zone

• Inner edge determined by loss of water via
  runaway or moist greenhouse effect
• Outer edge depends on how large a planets
  greenhouse effect might be
• Conservative approach Assume that CO2 and H2O
  are the only important greenhouse gases
• Less conservative approach Consider CH4 as well

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The Carbonate-Silicate Cycle
(metamorphism)

• This cycle regulates Earths atmospheric CO2
  level
• over long time scales and has acted as a
  planetary
• thermostat during much of Earths history
• It also ensures that the liquid water habitable
  zone
• around the Sun and other stars is fairly wide

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• However, CO2 itself begins to condense as one
  moves farther away from the Sun ? outer edge is
  at 1.6 to 2.0 AU

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Mars Flux calculations at Ts 273 K

• Minimum solar flux to support
• 273 K on Mars is S/S0 0.86
• The solar flux at Mars orbit is
• 0.43 times that at Earth
• ? Outer edge of the HZ is at
• S/S0 0.86?0.43 0.37
• The solar flux varies as 1/r2,
• so the HZ outer edge is at
• r (1/0.37)0.5 ? 1.6 AU
• Warming by CO2 clouds might
• extend this to 2.0 AU

SEFF FIR/FS
J.F. Kasting, Icarus (1991)
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Boundaries of the 4.6-b.y. CHZ

• CHZ boundaries are somewhat different because the
  planet must remain habitable for (in this case)
  4.6 b.y.
• Inner edge remains at 0.95 AU because the Sun is
  as bright now as it ever has been
• Outer edge moves in because the Sun was 30 less
  bright early in its history
• Assume that the HZ outer edge is at 1.8 AU
• CHZ outer edge is at 1.8?(0.7)0.5 ? 1.5 AU
• Is this broad, or is this narrow?

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Titius-Bode Law

• Planets are geometrically spaced for reasons of
  orbital
• stability (although this statement applies more
  directly to
• giant planets than to terrestrial ones)
• If other planetary systems have similar spacing,
  then the
• number of planets within the habitable zone may
  be large

Ref. J. K. Beatty et al. (1999), Ch. 2.
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We can also calculate HZs and CHZs for other
types of stars
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Hertzsprung-Russell (HR) Diagram
See also The Earth System, p. 191
http//observe.arc.nasa.gov/nasa/space/stellardeat
h/stellardeath_1ai.html
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ZAMS habitable zones

• Gold strip indicates the habitable zone
• ZAMS means zero age main sequence

Kasting et al., Icarus (1993)
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Planets around early-type stars

• Planets orbiting stars earlier than about F0 have
  at least two problems
• Short main sequence lifetime (2 b.y. for an F0
  star)
• High levels of UV radiation (about 20 times that
  of the Earth)

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Earth-like planets around F, G, and K stars

• The Earth is assumed to be at a distance
  equivalent to 1 AU in the
• extrasolar planet system. First, scale the
  orbital radius by ?L, then
• move the planet inward or outward until its
  calculated surface
• temperature is 288 K.

Segura et al., Astrobiology (2003)
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Earth-like planets around F, G, and K stars
Temperature
Ozone number density

• The planet around the F star develops a super
  ozone layer because of
• the abundance of short-wavelength UV radiation
  (? lt 200 nm) that can
• dissociate O2 ? UV is not a problem once O2
  levels become high

Segura et al., Astrobiology (2003)
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Possible problems for planets around M stars

• Tidal locking
• But this can probably be overcome by atmospheric
  and oceanic heat transport
• Lack of magnetic field ? atmospheres may be
  removed by flares and sputtering (H. Lammer et
  al., Astrobiol., 2007)
• Initial deficiency in volatiles due to smaller
  orbits, hotter accretion environment, more
  energetic collisions (J. Lissauer, Ap. J., 2007)

GRIN Great Images in NASA http//www.fanboy.com/s
cience/
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The galactic habitable zone

• There may also be a preferred time and location
  within the galaxy for habitable planets to exist
• Stars that are too close to the center of the
  galaxy are subject to frequent near-collisions
  and more supernovae and gamma ray bursts
• Stars that are too far out in the galaxy (or that
  evolved too early in its history) may be too
  metal-poor
• Fortunately, though, the GHZ is probably large
  compared to the local solar neighborhood

Ref. Lineweaver et al., Science (2004)
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Conclusions

• Habitable zones around F, G, and early K stars
  are relatively wide
• Stars earlier than about F0 are bad candidates
  for harboring habitable planets, primarily
  because of their short main sequence lifetimes
• M stars have good CHZs but their planets are
  beset with many other problems (tidal locking,
  atmospheric loss, lack of initial volatiles?)