"(I dont see how you could ever come up with such a %- i was just reading that post about the odds on God's existence; while comical, i dont see how anyone could possibly ever put a line on such a question)"
There are 68 requirements for the galaxy to support life on Earth. While the probabilities for each of them are not that impressive, the chance that they all happen together is astronomical. Borel's Law of Chance states that anything less than 1:10^50 is a statistical impossibility. The probability of all 68 variables happening at once has been computed to be 1:10^99.
The following is a list of each variable and the computed probability. For a more in-depth explanation (complete with references) look
here.
Uniqueness of the Galaxy-Sun-Earth-Moon System for Life Support
galaxy size (p = 0.1)
if too large: infusion of gas and stars would disturb sun's orbit and ignite deadly galactic eruptions
if too small: infusion of gas would be insufficient to sustain star formation long enough for life to form
galaxy type (p = 0.1)
if too elliptical: star formation would cease before sufficient heavy elements formed for life chemistry
if too irregular: radiation exposure would be too severe (at times) and life-essential heavy elements would not form
galaxy location (p = 0.1)
if too close to dense galaxy cluster: galaxy would be gravitationally unstable, hence unsuitable for life
if too close to large galaxy(ies): same result
supernovae eruptions (p = 0.01)
if too close: radiation would exterminate life
if too far: too little "ash" would be available for rocky planets to form
if too infrequent: same result
if too frequent: radiation would exterminate life
if too soon: too little "ash" would be available for rocky planets to form
if too late: radiation would exterminate life
white dwarf binaries (p = 0.01)
if too few: insufficient fluorine would exist for life chemistry
if too many: orbits of life-supportable planets would be disrupted; life would be exterminated
if too soon: insufficient fluorine would exist for life chemistry
if too late: fluorine would arrive too late for life chemistry
proximity of solar nebula to a supernova eruption
if farther: insufficient heavy elements would be attracted for life chemistry
if closer: nebula would be blown apart
timing of solar nebula formation relative to supernova eruption
if earlier: nebula would be blown apart
if later: nebula would not attract enough heavy elements for life chemistry
parent star distance from center of galaxy (p = 0.2)
if greater: insufficient heavy elements would be available for rocky planet formation
if lesser: radiation would be too intense for life; stellar density would disturb planetary orbits, making life impossible
parent star distance from closest spiral arm (p = 0.1)
if too small: radiation from other stars would be too intense and the stellar density would disturb orbits of life-supportable planets
if too great: quantity of heavy elements would be
insufficient for formation of life-supportable planets
z-axis range of star's orbit (p = 0.1)
if too wide: exposure to harmful radiation from galactic core would be too great
number of stars in the planetary system (p = 0.2)
if more than one: tidal interactions would make the orbits of life-supportable planets too unstable for life
if fewer than one: no heat source would be available for life chemistry
parent star birth date (p = 0.2)
if more recent: star burning would still be unstable; stellar system would contain too many heavy elements for life chemistry
if less recent: stellar system would contain insufficient heavy elements for life chemistry
parent star age (p = 0.4)
if older: star's luminosity would be too erratic for life support
if younger: same result
parent star mass (p = 0.001)
if greater: star's luminosity would be too erratic and star would burn up too quickly to support life
if lesser: life support zone would be too narrow; rotation period of life-supportable planet would be too long; UV radiation would be insufficient for photosynthesis
parent star metallicity (p = 0.05)
if too little: insufficient heavy elements for life chemistry would exist
if too great: radioactivity would be too intense for life; heavy element concentrations would be poisonous to life
parent star color (p = 0.4)
if redder: photosynthetic response would be insufficient to sustain life
if bluer: same result
H3+ production (p = 0.1)
if too little: simple molecules essential to planet formation and life chemistry would never form
if too great: planets would form at the wrong time and place for life
parent star luminosity (p = 0.0001)
if increases too soon: runaway green house effect would develop
if increases too late: runaway glaciation would develop
surface gravity (governs escape velocity) (12) (p = 0.001)
if stronger: planet's atmosphere would retain too much ammonia and methane for life
if weaker: planet's atmosphere would lose too much water for life
distance from parent star (p = 0.001)
if greater: planet would be too cool for a stable water cycle
if lesser: planet would be too warm for a stable water cycle
inclination of orbit (p = 0.5)
if too great: temperature range on the planet's surface would be too extreme for life
orbital eccentricity (p = 0.3)
if too great: seasonal temperature range would be too extreme for life
axial tilt (p = 0.3)
if greater: surface temperature differences would be too great to sustain diverse life-forms
if lesser: same result
rate of change of axial tilt (p = 0.01)
if greater: climatic and temperature changes would be too extreme for life
rotation period (p = 0.1)
if longer: diurnal temperature differences would be too great for life
if shorter: atmospheric wind velocities would be too great for life
rate of change in rotation period (p = 0.05)
if more rapid: change in day-to-night temperature variation would be too extreme for sustained life
if less rapid: change in day-to-night temperature variation would be too slow for the development of advanced life
planet's age (p = 0.1)
if too young: planet would rotate too rapidly for life
if too old: planet would rotate too slowly for life
magnetic field (p = 0.01)
if stronger: electromagnetic storms would be too severe
if weaker: planetary surface and ozone layer would be inadequately protected from hard solar and stellar radiation
thickness of crust (p = 0.01)
if greater: crust would rob atmosphere of oxygen needed for life
if lesser: volcanic and tectonic activity would be destructive to life
albedo (ratio of reflected light to total amount falling on surface) (p = 0.1)
if greater: runaway glaciation would develop
if less: runaway greenhouse effect would develop
asteroid and comet collision rates (9) (p = 0.1)
if greater: ecosystem balances would be destroyed
if less: crust would contain too little of certain life-essential elements
mass of body colliding with primordial earth (0 = 0.002)
if greater: Earth's orbit and form would be too greatly disturbed for life
if lesser: Earth's atmosphere would be too thick for life; moon would be too small to fulfill its life-sustaining role
timing of above collision (p = 0.05)
if earlier: Earth's atmosphere would be too thick for life; moon would be too small to fulfill its life-sustaining role
if later: Earth's atmosphere would be too thin for life; sun would be too luminous for subsequent life
oxygen to nitrogen ratio in atmosphere (p = 0.1)
if greater: advanced life functions would proceed too rapidly
if lesser: advanced life functions would proceed too slowly
carbon dioxide level in atmosphere (p = 0.01)
if greater: runaway greenhouse effect would develop
if less: plants would be unable to maintain efficient photosynthesis
water vapor quantity in atmosphere (p = 0.01)
if greater: runaway greenhouse effect would develop
if less: rainfall would be too meager for advanced land life
atmospheric electric discharge rate (p = 0.1)
if greater: fires would be too frequent and widespread for life
if less: too little nitrogen would be fixed in the atmosphere
ozone quantity in atmosphere (p = 0.01)
if greater: surface temperatures would be too low for life; insufficient UV radiation for life
if less: surface temperatures would be too high for life; UV radiation would be too intense for life
oxygen quantity in atmosphere (p = 0.01)
if greater: plants and hydrocarbons would burn up too easily, destabilizing Earth's ecosystem
if less: advanced animals would have too little to breathe
seismic activity (p = 0.1)
if greater: life would be destroyed; ecosystem would be damaged
if less: nutrients on ocean floors from river runoff would not be recycled to continents through tectonics; not enough carbon dioxide would be released from carbonate buildup
volcanic activity
if lower: insufficient amounts of carbon dioxide and water vapor would be returned to the atmosphere; soil mineralization would be insufficient for life advanced life support
if higher: advanced life would be destroyed; ecosystem would be damaged
rate of decline in tectonic activity (p = 0.1)
if slower: crust conditions would be too unstable for advanced life
if faster: crust nutrients would be inadequate for sustained land life
rate of decline in volcanic activity (p = 0.1)
if slower: crust and surface conditions would be unsuitable for sustained land life
if faster: crust and surface nutrients would be inadequate for sustained land life
oceans-to-continents ratio (p = 0.2)
if greater: diversity and complexity of life-forms would be limited
if smaller: same result
rate of change in oceans-to-continents ratio (p = 0.1)
if smaller: land area would be insufficient for advanced life
if greater: change would be too radical for advanced life to survive
distribution of continents (p = 0.3)
if too much in the Southern Hemisphere: sea-salt aerosols would be insufficient to stabilize surface temperature and water cycle; increased seasonal differences would limit the available habitats for advanced land life
frequency and extent of ice ages (p = 0.1)
if lesser: Earth's surface would lack fertile valleys essential for advanced life; mineral concentrations would be insufficient for advanced life.
if greater: Earth would experience runaway freezing
soil mineralization (p = 0.1)
if nutrient poorer: diversity and complexity of lifeforms would be limited
if nutrient richer: same result
gravitational interaction with a moon (p = 0.1)
if greater: tidal effects on the oceans, atmosphere, and rotational period would be too severe for life
if lesser: orbital obliquity changes would cause climatic instabilities; movement of nutrients and life from the oceans to the continents and vice versa would be insufficient for life; magnetic field would be too weak to protect life from dangerous radiation
Jupiter distance (p = 0.1)
if greater: Jupiter would be unable to protect Earth from frequent asteroid and comet collisions
if lesser: Jupiter’s gravity would destabilize Earth's orbit
Jupiter mass (p = 0.1)
if greater: Jupiter’s gravity would destabilize Earth's orbit 9
if lesser: Jupiter would be unable to protect Earth from asteroid and comet collisions
drift in (major) planet distances (p = 0.1)
if greater: Earth's orbit would be destabilized
if less: asteroid and comet collisions would be too frequent for life
major planet orbital eccentricities (p = 0.05)
if greater: Earth's orbit would be pulled out of life support zone
major planet orbital instabilities (p = 0.1)
if greater: Earth's orbit would be pulled out of life support zone
atmospheric pressure (p = 0.1)
if smaller: liquid water would evaporate too easily and condense too infrequently to support life
if greater: inadequate liquid water evaporation to support life; insufficient sunlight would reach Earth's surface; insufficient UV radiation would reach Earth's surface
atmospheric transparency (p = 0.01)
if greater: too broad a range of solar radiation wavelengths would reach Earth's surface for life support
if lesser: too narrow a range of solar radiation wavelengths would reach Earth's surface for life support
chlorine quantity in atmosphere (p = 0.1)
if greater: erosion rate and river, lake, and soil acidity would be too high for most life forms; metabolic rates would be too high for most life forms
if lesser: erosion rate and river, lake, and soil acidity would be too low for most life forms; metabolic rates would be too low for most life forms
iron quantity in oceans and soils (p = 0.1)
if greater: iron poisoning would destroy advanced life
if lesser: food to support advanced life would be insufficient
if very small: no life would be possible
tropospheric ozone quantity (p = 0.01)
if greater: advanced animals would experience respiratory failure; crop yields would be inadequate for advanced life; ozone-sensitive species would be unable to survive
if smaller: biochemical smog would hinder or destroy most life
stratospheric ozone quantity (p = 0.01)
if greater: not enough LTV radiation would reach Earth's surface to produce food and life-essential vitamins
if lesser: too much LTV radiation would reach Earth's surface, causing skin cancers and reducing plant growth
mesospheric ozone quantity (p = 0.01)
if greater: circulation and chemistry of mesospheric gases would disturb relative abundance of life-essential gases in lower atmosphere
if lesser: same result
frequency and extent of forest and grass fires (p = 0.01)
if greater: advanced life would be impossible
if lesser: accumulation of growth inhibitors, combined with insufficient nitrification, would make soil unsuitable for food production
quantity of soil sulfur (p = 0.1)
if greater: plants would be destroyed by sulfur toxins, soil acidity, and disturbance of the nitrogen cycle
if lesser: plants would die from protein deficiency
biomass to comet-infall ratio (p = 0.01)
if greater: greenhouse gases would decline, triggering runaway freezing
if lesser: greenhouse gases would accumulate, triggering runaway greenhouse effect
quantity of sulfur in planet's core (p = 0.1)
if greater: solid inner core would never form, disrupting magnetic field
if smaller: solid inner core formation would begin too soon, causing it to grow too rapidly and extensively, disrupting magnetic field
quantity of sea-salt aerosols (p = 0.1)
if greater: too much and too rapid cloud formation over the oceans would disrupt the climate and atmospheric temperature balances
if smaller: insufficient cloud formation; hence, inadequate water cycle; disrupts atmospheric temperature balances and hence the climate
dependency factors (estimate 100,000,000,000)
longevity requirements (estimate .00001)
Total Probability = 1:10^99