Habitable Zone
ASTR 1420
Lecture 8
Sections 10.1-10.4
Habitable Zone = Zone of liquid water
Goldilocks’ zone
This porridge is too hot…
This porridge is too cold…
Ahhh, this porridge is just
right!!
Habitable Zone
The range of distances from a star at which a planet could potentially have
surface temperatures right for liquid water!
Important facts
1. range of distances
2. is not a sufficient condition (e.g., Moon is in the H.Z.)
3. HZ is changing over time
? Surface habitability only?
While it is possible for us to find sub-surface life in our solar system, it is
nearly impossible to find subsurface life around other stars. Therefore, in
the search for life beyond solar system, we will only focus on worlds with
possible surface water!
Free floating Earths
Ejected Earth-size planets during planet formation
with a thick atmosphere
can trap (or insulate) heat for billions of years
can have surface water without a sun!
But, we cannot detect these! Likewise, subsurface water-worlds.
So, we will ignore these possibilities from now on.
Venus and Earth
• Venus is only 30% closer to the Sun than Earth, but with surface
temperature of 880°F!
• Strong greenhouse effect due to 200,000 times more CO2 in the
atmosphere
• Why so much CO2 than Earth?
o Lack of CO2 cycle  due to the lacking surface water (ocean)
• Most water on Earth were brought by impacts, and similar impacts must
have happened in Venus
 Then, Venus once had an ocean but lost her water over time?
• Yes! From the fossil record of
ocean from High Deuterium to Hydrogen ratio…
If we “move” Earth to the Venus orbit…
Runaway greenhouse effect?
Not so simple…
• Higher temperature  more evaporation  more clouds  more
reflection of incoming sunlight  balancing effect!?!
So, the true inner boundary of Habitable zone should be b/w Venus and Earth orbits!
Ancient Venus : Lost Tropical Paradise?
• Young Sun was about 30%
fainter than the current one
(young fainter Sun)!
• Young Venus did not have high
concentration of greenhouse
gases from outgassing yet…
 That 30% reduced sunlight +
lesser amount of greenhouse
gases in the past might have
produced ancient oceans on
the Venusian surface.
Changing Habitable Zone!
 Over time, Sun will get
brighter eventually engulfs the
Venus
Surface Habitability Factors : Summary
1.
distance from a central star
o
2.
when we consider the minimum distance, in addition to the solar heating, we
must account other processes (e.g., greenhouse effect) also.
role of planetary size
o
o
o
3.
Mars is a good example…
loss of atmosphere  loss of magnetic field  loss of internal heat due to a
smaller size …
Is there a minimum size? Smaller than Earth, larger than Mars…
role of an atmosphere
o
o
o
o
sources of gases  outgassing of trapped gas from ancient impacts.
possibility of forming rocky planets without gases?
importance of magnetic field  slowly rotating planets..
planets around binary stars or even binary planets?
Habitable Zone around our Sun
• Inner boundary is not so hard to guess (somewhere near to the Venus’
orbit).
• Outer boundary is fuzzy.
o Mars: Had it been in the HZ? Mars might have never had an extended period of
surface water.
Inner Boundary
Detailed calculations predict that if we “put” the Earth at around 0.84 AU, it
will go through a runaway greenhouse effect.
However, a moderate
additional warming
stronger air circulation
taller clouds
break up of H2O by
solar UV radiation
gradual lose of H2O
Moist Greenhouse Effect
even at 0.95AU!!
Outer Boundary
The distance from the Sun where even a strong greenhouse effect could not
warm the planet enough to above the freezing temperature.
• If Mars were larger and with a thicker atmosphere, Mars could have strong
enough greenhouse warming to have surface liquid water.
• Calculations show that the outer boundary is ~1.7AU (Mars is at 1.52AU)
• However, in cold atmosphere, CO2 could condense on snowflakes and fall
onto the ground  reducing greenhouse effect!  reducing temperature
Sun’s present Habitable Zone
• optimistic case : 0.84 to 1.7 AU
• conservative case:
0.95 to 1.4 AU
Different Kinds of Stars
• massive (hotter) stars
• about 30-40 times less
abundant than Sun-like
stars.
• ~1000 times shorter life
• Sun-like stars
• about 10 billion yrs of life
• less massive (cooler) stars
• ~10 times more abundant
and 100 times longer life
times than Sun-like stars
Understanding Stars…
Less Massive Stars are
better…
more abundant
longer-lasting…
Habitable Zone around Different types of Stars
• HZ is closer to lower mass stars
• HZ is further away from more massive stars
Life Cycle of Stars
As stars age,
their sizes and
temperatures
are changing
drastically!
 cause a
changing HZ
with time
Changing habitable zone in our solar system over time
Changing
Habitable Zone
• Continuously habitable
zone : range of distances
that remain always
habitable over time.
Galactic Habitable Zone
• Inner boundary : threats to life (too high radiation such as Super Novae,
gamma-ray bursts, etc.)
• Outer boundary : existence of heavy elements such as Carbon
How long can we survive?
Phase of Giant star: 5+ Gyrs
Habitable zone will move outward over time due to more dumping of energy from
the Sun
building a sunshade?
slowly moving Earth outward
Planetary nebula & white dwarf phase: 6-7 Gyrs
Sun will eventually die and white dwarf will vanish slowly over billions of years
 hopping to other nearby young stars…
Galaxy: 50+ Gyrs
All gas will be used up in our Galaxy and no more new stars will be born
migrate to nearby galaxies
Universe: 100s Gyr
Universe is ever expanding faster … then?
Isaac Asimov’s Last Question:
In summary…
Important Concepts
Important Terms
• Runaway greenhouse effect
• Surface habitability factors
• Habitable zone is changing over
time due to the life-cycle of Stars
• HZs are closer to less massive stars
and further away around massive
stars.
• Best host stars for life = less
massive stars. Why?
• Habitable Zone
o around different stars
o Galactic
• Continuously Habitable Zone
Chapter/sections covered in this lecture : 10.1 – 10.4