Habitability: Making a
habitable planet
26 January 2016
Introduction
 Define Habitability
 The Habitable Zone
 Environment of early Earth
Defining Habitability
Defining Habitability
 What do we mean when we say
habitable?
 Earth-like animal life: specific requirements
 Microbial life - broader set of conditions
Defining Habitability
Defining Habitability
 What do we mean when we say
habitable?
 Earth-like animal life: specific requirements
(oxygen, water, dry land, temperature
range)
 Microbial life - broader set of conditions
(more extreme conditions ok)
Defining Habitability
Common basic requirements
for life
 Water
 Stable climate
Defining Habitability
What stabilizes the climate?
 Size - long-term heat source
 Stellar evolution - incoming solar energy
 Impact rate - could result in climate
change
 Presence of large, natural satellite prevents large swings in obliquity
 Oceans - regulate global temperatures
The Habitable Zone
Habitable Zones
 Why is Earth the only (as far as we
know) habitable planet in our solar
system?
 2 main properties:
 Abundant liquid water
 Environmental conditions that maintain
liquid water
The Habitable Zone
Liquid Water is the Key!
 Required temperature: 273-373 K
 Use this as simple requirement for
identifying possibly habitable planets
 Where do planets in this temperature
range orbit?
The Habitable Zone
Liquid Water
 Where do planets in
this temperature
range orbit?
 Called the Habitable
Zone
The Habitable Zone
How does star type affect HZ?
 Different sized
stars have
different
luminosities
 T goes as L1/4
 Brighter stars
have HZs
farther out
The Habitable Zone
How does star type affect HZ?
 Main sequence stars
have different
luminosities
throughout their
lifetimes
 Continuously
Habitable Zone:
maintains conditions
suitable for life
throughout the
lifetime of star
The Habitable Zone
Is it that simple?
 Albedo, a
 Atmosphere
greenhouse –
chemical absorbers
act like a warm
blanket
Moves HZ
inwards
Moves HZ
outwards
The Habitable Zone
Role of the Carbon Cycle
 Kasting proposed
the Carbon Dioxide
Thermostat
 Extends to HZ for
Earth-like planets
 Keeps off
temperature
extremes
 Carbon sources:
 Volcanic
outgassing
 Decarbonation
 Organic carbon
 Carbon sinks:
 Calcium
carbonate
formation
 Photosynthesis
How the CO2 cycle buffers the planet’s temperature
The Habitable Zone
Continuously Habitable Zone
 Inner edge: 0.95 AU
 Outer edge: 1.15 AU
 Were other
planets habitable
in the past?
 Will other planets
be habitable in
the future?
The Habitable Zone
Mars:
Once Habitable?
Still Habitable?
 Early Mars
 Evidence of large
amounts of flowing liquid
water
 Warmer temperatures:
 Heat from interior would
have been higher
 Warm climate from
greenhouse gases or
CO2 clouds
 Current Mars
 Gullies may be due
to underground
water
 Carbon cycle not as
active as on Earth
The Habitable Zone
Characteristics that make a
habitable planet
•Size of planet
•Internal heat comes
from
•Accretional heat
•Differentiation
•Radiogenic decay
•Allows for plate
tectonics
•Mars cooled
quickly, so no
plate tectonics at
present
•Other Heat sources to
sustain liquid water
•Geothermal
•Iceland
•Tidal
•Europa
The Habitable Zone
Characteristics that make a
habitable system
 Star Type: stable
luminous stars
necessary
 Sufficiently long
lifetime for life to
evolve
 Large enough so
planets don’t tidally
lock
•Star system
•Single star: allows for
stable orbit
•Binary system:
•Fewer stable orbits
exist
•HZ calculated on
individual basis
The Habitable Zone
Characteristics that make a
habitable neighborhood
 Galactic Habitable Zone
 Area of high metallicity (elements w/ Z>2)
 Outer region of galaxy
 Lower stellar density
 Lower radiation levels
Discussion Topics:
• How common are habitable zones?
• What does this say about the
likelihood of life?
• What stars are the best to search?
Early Earth
Environment of early
Earth
• Evidence of a habitable planet 3.8
Ga
– Geological evidence near Isua,
Greenland
– Limestone and sandstone
– We can infer presence of liquid water
– Earth must have had temperatures
similar to today’s
Early Earth
Liquid water 3.8 Ga?
• Faint young Sun
– Sun was 25-30% less luminous
– Simple energy balance shows Earth’s
surface temperature would have been below
273 K
• Other heat sources
– Geological activity
• More internal heat from radioactive decay and
primordial heat
• Plate tectonics release CO2 - greenhouse traps
heat
Early Earth
Snowball Earth
• Global glaciations brought on by disruptions in
the carbon cycle
– Up to 4 occurred between 750 Ma and 580 Ma ago
– Geological record shows layered deposits in tropics
attributable to glacial erosion
• CO2 sinks would cease, but sources would
continue. 350 times current CO2 levels would
accumulate to create a severe greenhouse,
causing the ice to melt w/in a few hundred
years.
• All eukaryotes today are from the survivors of
snowball earth
Summary
 Habitable Zone: area around a star
where liquid water can exist on the
surface
 Continuously Habitable Zone: the region
where water would exist over the entire
star lifetime