Rare Earth Factors
From: Rare Earth: Why Complex Life is Uncommon in the Universe,
Peter D. Wood & Donald Brownlee, 2000
Astrobiologists, Wood & Brownlee postulate that the following 18 factors
prohibit or hinder bacteria from evolving into complex life, but first, the
Timeline of Evolution:
4.5 Billion Years Ago: Earth formed
4 BYA: "Life" (bacteria/archaea)
1 BYA: Multicelluar algae and seaweed
i.e., "complex life" took 3.5 billion years!
600 Million YA: Sponges/Worms
475 MYA Plants/Fungi
450-365 MYA Non-flying insects, arthropods and fish
300 MYA: Reptiles on land
250 MYA: Mass extinction wipes out 95% of animal species
220 MYA: First mammals
200 MYA: Birds
65 MYA: 50% of animal species die from meteor cooling planet
1800 Thousand YA: Homo erectus
130 TYA: Neanderthals (Homo Neanderthenis)
100 TYA: Homo sapiens
"intelligent life"
30 TYA: Man migrates to North America
27 TYA: Neanderthals extinct
4 TYA: Humans record history
Rare Earth Factors:
Why Complex Life is Uncommon in the Universe
Right distance from star
Habitat for complex life.
Liquid water near surface.
Far enough to avoid tidal lock.

Right planetary mass
Retain atmosphere and ocean.
Enough heat for plate tectonics.
Solid/molten core.

Plate tectonics
CO2-silicate thermostat.
Build up land mass.
Enjance biotic diversity.
Enable magnetic field.

The right tilt
Seasons not too severe.

Atmospheric properties
Maintenance of adequate temprature,
composition and pressure for plants
and animals.

Right kind of galaxy
Enough heavy elements.
Not small, elliptical, or irregular.

Right mass of star (sun)
Long enough lifetime.
Not too much ultraviolet.

Jupiter-like neighbor
Clear out comets and asteroids.
Not too close, not too far.

Not too much.
Not too little.
Giant impacts
Few giant impacts.
No global sterilizing impacts after an
initial period.

Biological evolution
Successful evolutionary pathway to
complex plants and animals.

Right position in galaxy
Not in center, edge or halo.

Stable planetary orbits
Giant planets do not create chaos.

A Mars
Small neighbor as possible life source
to seed earth-like planet, if needed.

Large Moon
Right distance.
Stabilizes tilt.

Right amount of carbon
Enough for life.
Not enough for Runaway greenhouse.

Evolution of oxygen
Invention of photosynthesis. Not too
much or too little. Evolves at the right

Wild Cards
Snowball Earth. Cambrian explosion.
Inertial interchange event.
Another proof: Aliens don't exist
By Dennis Overbye, New York Times, 11/11/03
Suppose, for example, that the first alien spaceship that sets out for another star takes a
million years to arrive there and become established. Then the new colony and the original
civilization each send a spaceship on a similar voyage, and so forth, so that a wave of new
colonies is formed, doubling the total number every million years.

After 10 million years, there will be 1023 alien settlements, plus the original. After 20 million
years, there will be a million. After 40 million years, if they keep it up, there would be a trillion
-- more than there are stars in the galaxy.

By now, after 10 billion years, if there were more than one spacefaring civilization in the
galaxy, they would be tripping over one another or one another's artifacts [and raidowaves].
But in fact there seems to be nothing.
The Radiation Factor
New York Times, 12/09/03
Alien beings from outer space would not only have to contend with traveling many light years
from home but with radiation: Americans receive an annual average dose of 200 millirems of
radiation. A chest x-ray is 8 to 50 mrems. A person in Kerala, India (where there's lots of sun)
receives 1300 mrems/year. A person on the space station for 84days receives 17,800 mrems
but a one year trip to and from Mars would entail a fatal 80,000 mrems.
Design and the Anthropic Principle: The unlikely-hood of intelligent life elsewhere.
Rare Earth Hypothesis: This article is based on the Rare Earth book, cited above.
It is estimated that Earth-like planets in the Milky Way started forming 9 Gya BP, and that
their median age is 6.4 ± 0.7 Gya. Moreover, 75% of stars in the galactic habitable zone are
older than the Sun. This makes the existence of potential planets with evolved intelligent life
more likely than not to be older than that of the Earth (4.54 Gya). This creates an
observational dilemma since interstellar travel even of the "slow" kind—that nearly within the
reach of present Earth technology—if had arisen elsewhere could in theory take only from 5
million to 50 million years to colonize the galaxy. This leads to a conundrum first posed in
1950 by the physicist Enrico Fermi in his Fermi's paradox: "Why are no aliens or their
artifacts physically here?"