Lecture 33: The Habitability of Worlds
Astronomy 101/103
Terry Herter, Cornell University
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Lecture
Topics
  • The question of life
  • The balance of powers
    • What controls the Earth's temperature?
  • Ecospheres and Habitable Zones
  • The Greenhouse Effect
  • Influence of the stellar spectrum
  • The lifetimes of stars
  • Searching for other worlds

Life
Elsewhere?
  • How can we find out about:
    • Other planetary systems
    • Planets like earth
    • Other intelligent species
  • If other civilizations exist can we:
    • communicate with them?
    • travel there?

What kind?
  • We are relegated to discussing life as we know it.
  • That is, we can only consider physical conditions somewhat similar to earth.
  • There could be life on Jupiter and we would never know it!

What kind
of Star?
  • For the existence of life as we know it, must there be a star like our sun?
  • Issues
    • Temperature of the planet
    • Spectrum of the radiation from the star
    • Lifetime of the star

Temperature
  • Temperature is the main quantity which can influence the feasibility of life.
    • Too hot: water is not liquid
    • Too cold: water is frozen
      (chemical reactions slow)
  • Temperature is governed by:
    • distance from the planet to the star
    • thickness of the planet's atmosphere


What Keeps
the Earth
Warm?
  • The power received from the Sun is balanced by emission by the Earth.

 

Power
Received
  • The power reaching the earth from the sun is:
  • This is the inverse square law.

Power
Radiated
Away
  • The power radiated by the Earth is:
  • This is the Stefan-Boltzmann law!

A Balance
of "Powers"
  • Setting these two equal gives:

  • Te ~ 300 K, but if we quadrupled the Earth's distance from the Sun we would have T = 150 K (-120 C).
  • The oceans would be frozen!

Changing
the Sun
  • The star Vega is 3 times as massive as the Sun and 58 times more luminous.

  • If we place a planet 1 AU from Vega, then
  • No oceans, no people! Too hot!!!

Moving
Further
Away
  • We get back to the temperature of the earth if it is moved 8 times further away:

Habitable Zones
and
Ecospheres
  • The habitable zone is the range of distances around a star in which "comfortable" temperatures are possible.
  • It is also called the Ecosphere.
  • Let us choose this range to be where water is liquid.
    • Inside this zone, water boils.
    • Outside this zone, water freezes.

The Ecosphere
  • The distance of the habitable zone from the star will vary depending on the type of star.
  • More luminous stars => the habitable zone is further away than that of the Sun.
  • Less luminous stars => the habitable zone is closer than that of the Sun.

Sample
Ecospheres

 

Ecospheres
for Stars


Ecospheres
(cont'd)
  • This criterion is not strict.
  • Venus at 0.72 AU is within the Sun's ecosphere!
    • But its surface temperature is 745 K!
    • A runaway greenhouse effect.
  • The Earth is helped by a mild greenhouse effect!

The Greenhouse
Effect

  • Photons (blue) from the sun penetrate the glass.
  • Infrared photons (red) are trapped inside by the glass.
  • So the greenhouse heats up.

Greenhouse
Effect
on the Earth


  • Photons from the sun reach the surface of the earth.
  • Carbon dioxide prevents infrared photons from radiating energy to space.
  • The Earth can't cool.
  • Too much CO2 will cause the earth to get too hot.

Temperature is
Not Everything
  • More luminous stars have a larger habitable zone; however they emit a lot of UV radiation!
  • The peak of the spectrum is given by Wien's Law

Comparing
Some Stars

Star
Type
T (K)
Peak Wavelength (microns)
Sun
G2
5,800
0.5
Vega
A0
10,000
0.29
Barnard's
M5
2,800
1.0
  • Vega would give you a bad sunburn!
  • Barnard's star could not support photosynthesis.

 

The Spectrum
of the
Star
  • If the star is too hot it will emit lots of UV photons (high energy!)
    • Life will be damaged
  • If the star is too cool, it will emit mostly infrared photons (low energy!)
    • Star cannot support chemical reactions essential for life
    • Photosynthesis not possible

The Lifetime
of Stars

  • Stars must live long enough for life to evolve.


    Star
    Mass (Msun)
    Lifetime (109 years)
    A0
    3.5
    0.44
    F0
    1.7
    3.0
    G0
    1.1
    8.0
    K0
    0.8
    17.0




Lifetimes
  • The is evidence that (simple) life existed on Earth 3.5 billion years ago!
  • O, B, and A type stars do not survive long enough for life to evolve.
  • Only F, G, K and M stars survive long enough.

Where to
Look for Life
  • To find life as we know it:
    • Look around F, G and K stars.
  • They have relatively long lives, emit photons with right (suitable for life) energy, and have moderately large ecospheres.

The Solar
System from
Afar
  • Alpha Cen, a nearby star ( 1.3 pc = 4.3 lyr )
    • G2 V star, mv = 0.0
  • How would the solar system appear if it were there?

    Planet
    Separation from alpha Cen
    mv
    Factor fainter than alpha Cen
    Earth
    0.76 "
    24
    4 x 109
    Jupiter
    3.9 "
    22
    6 x 108

Finding other
Planets?

Four Possible Methods

  1. Direct observation
    • Reflected light from star
    • Intrinsic infrared radiation
  2. Search for brightness variations
  3. Measure wiggles on the sky
  4. Doppler spectroscopy

Analogy to
Binary
Stars
  • These techniques are similar to those used to detect binary stars.


    Search Technique
    Binary Type
    Direct observation
    Visual
    Brightness variations
    Eclipsing
    Wiggle on sky
    Visual
    Doppler shifts
    Spectroscopic

Patience...
  • If other planetary systems are like our solar system then we must observe the planet long enough to see orbital motion.
  • Less massive stars will have longer periods.
    Planet
    Period (years)
    Jupiter
    11.9
    Saturn
    29.5
    Uranus
    84.0


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