Is Earth home to "Rare Intelligent Life"?

N* x fs x fp x ne x fi x fc x fl = N

N* x fp x fpm x ne x ng x fi x fc x fl x fm x fj x fme = N

Some Technical Reasons (why complex life might be rare)

Note: many of the following points were derived from the book "Rare Earth: Why Complex Life is Uncommon in the Universe" written by Peter D. Ward and Donald Brownlee (published by Copernicus in 2000). Be sure to pick up a copy of the book for a more complete list. I was surprised at how many scientists are listed as advisors for various chapters. This book is much more than 2 guys working alone in a vacuum.

This is only a partial list

Planet Orbital Position

1. The planet must be at the right distance away from its sun so that water remains liquid. Specifically, in the temperature range of: 0C to 45C (32F-113F) which is also known as the zone of habitability. At least on our planet, the mitochondria found in mammals do not properly function above 45C. While many forms of life can live above this temperature, it's not the kind of life that could develop technology.

2. Like Earth, the planet must be initially formed near the colder edge of the zone of habitability. This is necessary because all suns get brighter as they age which means warmer temperatures for the planet in question. Not much good would come from a world where life would evolve for 3-4 billion years only to be cooked by a warming sun. Note: without biology generated green house gases, the Earth's natural temperature is below freezing.

3. The planet must be in a relatively circular orbit around the sun. While it is true that orbits are really elliptical, an ellipse of high eccentricity would produce thermal distributions too great to support the evolution of life.

1. The planet must rotate. A planet that orbits a sun in the zone of habitability but does not spin will be too hot on one side and too cold on the other. See: "Sun Size" Item #3 for more information
2. A large moon is required to keep the spin of the planet stable with respect to the sun. Without a similar celestial companion, the planet would wobble rather than spin which would produce thermal distributions too great to support the evolution of life. Note: our moon is 1/4 the size of earth and we acquired it by a very rare accident. We are the only planet in our solar system with such a large moon/planet mass ratio.

1. The planet must be large enough to keep its atmosphere from escaping into space
2. The planet must be the right size (not too large or small) to allow plate tectonics so that minerals (including metal bearing ores) are close to the surface. A society without these raw materials will not be able to build radio telescopes.
3. The planet must be small enough so that mountains can grow and creatures can walk upright. On worlds without mountains, the planet might be totally covered with water. Although the evolution of higher forms of life in an aquatic only environment is probable, metallurgy is only possible where fire can exist. An aquatic dolphin society can not build a rocket to protect itself from the next "asteroid caused" mass extinction.

1. Some solar systems are composed of two or three suns. Planets orbiting such a system would probably have a very strange shape (almost square or even petal shaped with lobes) based upon the complex gravitational forces present.
2. In our solar system large gas giants (like Jupiter) sweep out a zone of protection which, after a time, allows the inner planets to be almost impact free. (it is still believed that large gas giants are responsible for pulling comets from the Ort cloud but this is safer than than rocky asteroids)
3. There have been recent discoveries of large Jupiter sized planets orbiting very close to their respective suns. If the earth were part of such a system we would probably not have the necessary conditions described in items #1 + 2.

1. Size wise, our Sun is the top 10% of those found in the galaxy. A large yellow sun provides a relatively even heat source required for the evolution of life. Our Sun has a G classification and is stable for approximately 10 billion years.
2. A Sun only 50% larger than ours is classified as G2 and is only stable for 2 billion years. Even if a planet was the right distance from this kind of sun, there wouldn't be enough time for evolution to take place.
3. When the sun is smaller than ours, the zone of habitability will be closer. When a planet (or moon) gets too close to its companion, its spin can become synchronized to its orbital period causing the same side to face the companion (like our moon which is locked to Earth now, or the planet Mercury which seems to be locking to the Sun with a 58-day rotation and an 88-day orbit which results in a solar day of  176 Earth days). On these worlds the bright side cooks while the dark side freezes.

1. The sun must not be in a position in the galaxy that is too dense because...
    
   • asteroids will hit the planet too frequently and will exterminate life before it can evolve.
   • the probability of being exposed to a super nova would be too high (life on planets 1 light year away would be exterminated; life on planets 30 light years away would be affected).
   • the probability of being exposed to neutron star or any other large radiation sources would be too high. This excludes the center of the galaxy (as viewed from above) as well as stellar nurseries and globular clusters
      
2. The sun must not be in a position in the galaxy that is too sparse. Otherwise, asteroids will hit the planet too infrequently and initial life (like the dinosaurs) will dominate the surface.
    
3. The previous extermination took place 65 million years ago. About 5 mass exterminations have taken place on Earth at an average interval of 100 million years.
   Note: It may be a requirement for intelligent life to develop space technology in order to protect itself from the next impact. A strictly agrarian society is a world awaiting a death sentence. Despite common belief, we do not yet possess the technology to protect ourselves from this type of extinction.

The Follow-up Book

 In 2003, authors Peter D. Ward and Donald Brownlee released a follow-up book to "Rare Earth" titled "The Life and Death of Planet Earth" (subtitled: "How the New Science of Astrobiology Charts the Ultimate Fate of Our World"). We all know that the Earth will be destroyed in 5-7 billion years when our Sun becomes a red giant but these authors give convincing arguments why all animal and plant life may be extinct 500 million years from now while the oceans will be gone in just a billion.

Teaser: Carbon dioxide levels are already 20 times lower then it was during the time of the dinosaurs. This is causing the slow death of deciduous and conifer trees which are being replaced by grasses which don't require as much of the gas. Meanwhile, our sun is now 30% hotter than it was when Earth was young and heating will continue. Since almost all the carbon dioxide is gone, what green house gases can we remove to counter the increase in solar output? 

This book was a very good read but be sure to check out the Millennial Project as soon as you're done because Earth is just a nest and we should start preparations to leave it ASAP. The Moon and Mars will buy us some time until technological improvements will make star travel practical.