The Anthropic Earth

Written by Jeff Yeager

In a previous article , we discussed the anthropic principle as it relates to the cosmos.  This article will detail the fine-tuning required for a life-sustaining planet.

Rare Earth

The first astronomers to discuss the intricacies required for life were Frank Drake, Carl Sagan and Iosef Shklovskii {1}. They attempted to estimate the number of planets in the universe able to support life. In 1966 they had determined that it takes a certain type of star with a planet located at the right distance from the star to provide minimal conditions for life. Working with just these 2 parameters, they estimated that .001% of all stars could have a planet capable of supporting life.

Subsequent evidence has shown they overestimated the range of viable star types and the range of viable planetary distances and they also ignored dozens of other significant parameters. But their estimate of a million+ possible life sites in our galaxy endures today. It is this optimistic estimate of the number of possible life sites that many naturalists cling to when arguing about the odds of a naturalistic origin of life.

In fact, the list has grown from 2 parameters in 1966, to 8 by 1970, to 23 by 1980, to 40 by 1995, to more than 120 today and the list shows no sign of leveling off {2}.

The amount of factual information on this topic is immense, so in the interest of brevity, we will summarize only some of the information here.  Note that all of the probabilities listed are generous to the other side in the debate.

Characteristic	Probability
Galaxy type capable of sustaining life	.1
Location of star in galaxy	.2
Correct number of stars in the system	.2
Correct star birthdate	.2
Correct age of the star	.4
Correct mass of the star	.001
Star luminosity relative to speciation	.0001
star color	.4
supernovae rates and locations	.0001
white dwarf binary types, rates, locations	.01
planetary distance from star	.001
inclination of planetary orbit	.8
planet axis tilt	.3
planet rotation period	.1
planet rate of change in rotation period	.05
planet orbit eccentricity	.3
planet surface gravity	.001
planet tidal force	.1
planet magnetic field	.01
planet albedo	.01
planet density	.1
planet's crust thickness	.01
land/water ratio	.2
rate of change in land/water ratio over time	.1
global distribution of continents	.3
asteroidal and cometary collision rate	.1
rate of change in asteroid and comet collision rate over time	.1
position and mass of Jupiter relative to Earth (protection from collisions)	.01
eccentricity and regularity of Jupiter and Saturn's orbits	.05
atmospheric transparency	.01
atmospheric pressure	.1
atmospheric electric discharge rate	.1
atmospheric temperature gradient	.1
carbon dioxide level in atmosphere	.1
oxygen quantity in atmosphere	.1
ozone quantity and location in atmosphere	.1
water vapor level in atmosphere	.1
oxygen to nitrogen ratio in atmosphere	.1
quantity of greenhouse gases in atmosphere	.01
soil mineralization	.1
seismic activity in the right amount	.1

It is easy to see where this leads.  A calculation of the odds of a life-sustaining planet according to these generous estimates, of just this fraction of the known parameters, is 10^-53.   In fact, the latest calculations taking all of the conditions into account come up with a likelihood of 10^-140{3}.  Add to this, the fact that a natural origin of life requires these life-supporting conditions to remain within their life-friendly limits for extremely long periods of time (for life development, presummably by naturalistic means) and naturalists are left with an insurmountable problem.

It's important to note that the likelihoods of some of these life-enabling properties are known precisely, while others are, admittedly, not known precisely.  This is because the number of known planets from which to calculate probabilities is rather small.  There are currently only 76 known planets (9 of those in our solar system).  Current scientific evidence points to the conclusion that planets are extremely rare.  Scientists estimate that in our Milky Way galaxy, only 2% of stars are metal-rich enough to support planet formation {4}. In addition, it appears that the Milky Way galaxy is extremely friendly to planet formation in comparison to other galaxies in our universe.

The most generous estimate of the number of planets is around 10²⁰.  This leaves the likelihood of a life-sustaining planet anywhere in our universe at roughly 10^-120 which is beyond the realm of possibility, naturalistically speaking.

Theistic Implications

While it can't be denied that the anthropic principle is real, naturalists won't agree that a theistic conclusion is required.  When contemplating the odds against a chance ocurrence of a life-sustaining planet the theistic conclusion is the most rational one. 

From the creation of the world His invisible attributes, that is, His eternal power and divine nature, have been clearly seen, being understood through what He has made. As a result, people are without excuse. (Rom 1:20)

Notes:

1. Iosef S. Shklovskii and Carl Sagan, Intelligent Life in the Universe (San Francisco: Holden-Day, 1966), 343-52. Guillermo Gonzalez, private communication, 1991.
2. Hugh Ross, The Creator and the Cosmos, 3d ed. (Colorado Springs, CO: NavPress, 2001), 195-99.
3. Ross
4. The 2 percent figure was determined from the minimum metal richness observed in stars with planets and the maximum age of stars with planets. Interestingly, Carl Sagan came up with the same figure in 1966 (Shklovskii and Sagan, 344).

© 2008 LifeWay Christian Resources

Note: Unless otherwise noted, all Scripture references are taken from the Holman Christian Standard Bible®.

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