Be careful what you read! Recently a planet was found to be circling Proxima Centauri, a red dwarf star very near to Alpha Centauri, and the closet star to our solar system, about 4+ light years away. In many of the accounts of the discovery the headline read something like this: "Potentially Habitable Planet Found Orbiting Star Closest to Sun." But what they tell you is the planet is "earth-like" because it is terrestrial (a rocky planet). But this planet has other things that make it less like earth and more likely to be uninhabitable. For a planet to be a possible life sustaining place, it does not have to be just like earth but must fit within a matrix of conditions that makes most, if not all, contenders unsuitable. In fact the odds are such that no planet can meet the requirements without outside help.
As mentioned in my last blog, Reasons to Believe has listed at least 154 independent environmental factors that must be fine tuned in order for a planet to be capable of sustaining life. That list was from 2004, they have since expanded it to over 300 tune-able factors. The fine tuning is not just for the planet either. Many of these deal with the stars, the galaxies, galaxy clusters and the Universe as a whole.
Just looking at a few of these conditions that deal with the Heavens - galaxies, stars, and planets - should give you an idea how rare our earth truly is. The probabilities listed come from Reasons to Believe and relate to the probability of the existence of space objects that meet the requirement. Thus a probability of 0.001 means that only 0.1% of all existing stars, for example, would be the right size. You must then multiply the probabilities of all factors times the number of stars, since each factor is independent of the others, to get the number of stars that would possibly meet all of the included conditions.
First a few for the galaxies. For a galaxy to be a suitable location for a solar system and ultimately a planet that can sustain life, here are just a few characteristics it must have.
It must be the right type - If too elliptical or irregular insufficient heavy elements (elements heavier than Hydrogen and Helium) for life chemistry would not be available due to limited star formation. It appears that spiral galaxies (like our own) are the most suitable. (probability of 0.1)
It must be in the right location - Too close to a rich galaxy cluster or a very large galaxy, the host galaxy would be gravitationally disrupted, impacting the formation of a suitable host solar system. If it is too far away from other, smaller galaxies it can pull into itself, there would not be a source for gas and dust to sustain star formation long enough for heavy element formation. (probability of 0.1)
Assuming there are 100 trillion galaxies in the universe, just these three factors reduce the possible life supporting galaxies to 100 billion.
Now lets look at supernovae eruptions within the galaxies. Supernovae are exploding stars that have come to the end of their life and will be seeding the galaxy, the solar system, and specifically the host planet with heavy elements.
If too many supernovae - Too many heavy elements, resulting in planets not able to sustain life plus too many collision events, exterminating any life that may have been able to start. (probability of 0.1)
Also, if supernovae are too soon or too late during the life span of the host planet, the heavy elements may be too few or too many to sustain life. Likewise, if the supernovae are too close or too far from the host solar system. (probability of 0.2)
Orbiting a Nearby Star?
Now if we take the 100 billion galaxies from above, adding these factors reduces the possible life supporting galaxies to 200 million.
Next we will look at the host star. This next group of factors are requirements of the central star, like our sun, that will be supporting the host planet.
The star must be the right age - If too old or too young, the star's luminosity could change rapidly and impact the surface temperature of the host planet so as to prevent life. (probability of 0.4)
The star must be the right size - If too big the star could burn too quickly, changing the luminosity and burn out before life had a chance to form. If too small, the host planet would have to rotate closer to the star and possibly become tidal locked to the star - the planet would rotate like our moon and one side would always face the star - getting too hot for life on one side and too cold on the other. This is the suspected condition of the planet recently discovered around Proxima Centauri. Proxima Centauri is a Red Dwarf and is most likely not suitable as a life supporting star. (probability of 0.001)
The star must be a constant source of light - If the star's luminosity changes too quickly or too much, the planet will not have a consistent environment in which to sustain life. (probability of 0.0025)
The star must be the right color - If too red or too blue in its light output, photosynthesis could not occur. Another reason that Proxima Centauri is most likely not suitable as a life supporting star. As a Red Dwarf, its light is much more in the Infrared end of the visible light spectrum. (probability of 0.4)
Now if we take the 200 million galaxies from above and multiply by the average number of stars in a galaxy (100 billion), we get 20 million trillion possible stars. Using the factors just discussed reduces the number of possible stars to 160 billion. This is just considering so far a total of 10 of the possible 300+ factors.
Finally we will look at some of the factors directly affecting the host planet. This will just include some of those that relate to the planet's relationship to its host star and other planets within its solar system. There are many others related specifically to conditions about the planet itself that will be considered in a later blog.
The planet must have a close to circular orbit around the host star - If the planetary orbit is too elliptical, the seasonal temperature differences would be too extreme, thus prohibiting life. (probability of 0.3)
The planet must have a rotational period (day) that is not too long or too short - If the day is too long, surface temperatures would vary too much between the day and the night, thus limiting the possibilities of life. If the day is too short, atmospheric wind speeds would be too great. (probability of 0.1)
The planet must have a small axial tilt - The earth tilts approximately 24 degrees on its axis. This is one of the factors that creates seasonal weather variations. If the tilt was less or zero, there would be no seasons and daily temperature variations could be too high to sustain life. Days would be warmer and nights would be colder. If the tilt is greater than 24 degrees, then seasonal temperature variations would be greater as well. The tilt of the earth has been relatively stable for millions of years, varying from 22.0 to 24.6 degrees. Excessive tilt could produce weather fluctuations much like we are starting to see now. Could a change in the tilt be the cause of "Climate Change?" (probability of 0.3)
The planet must have a large moon at the proper distance - Back in 1993, French astronomer Jacques Laskar and his team showed that the tilt of the earth’s axis has been stabilized over long periods of time because the earth has a large moon (the moon is about 25% the size of earth). They demonstrated that the earth’s axial tilt varied only between 22.0 and 24.6 degrees over many millions of years because of the moon. Without the moon, the earth’s axial tilt would vary between 0 and 85 degrees. Variations in Earth’s axial tilt of much more than a couple of degrees could generate climate changes (as mentioned above) that would be catastrophic for advanced life. (probability of 0.01)
The planet must have a gas giant planet in its solar system, orbiting farther from its star. This gas giant must be the right size and distance from the host planet - If the gas giant is too big or too close to the host planet, the orbit of the host planet could be unstable due to gravitational effects between the two bodies. If the gas giant is too small or too far from the host planet, the host planet could be subject to many asteroid and comet collisions. We saw an example of this with Comet Shoemaker–Levy 9 between July 1992 and July 1994 when the comet broke apart and ultimately collided with Jupiter. This collision highlighted Jupiter's role in reducing space debris in the inner Solar System. (probability of 0.01)
Now if we take the 160 million stars from above and multiply by the average number of planets suspected to revolve around each star (2.5 on average), we get 400 million possible planets. Using the planetary factors discussed above reduces the number of possible planets to just 360 in the entire universe! This is just considering 17 of the possible 300+ factors. As you can see, it doesn't take too many more conditions to reduce this number to less than one. In fact, when all 300+ independent environmental factors are considered, Reasons to Believe concludes:
There is less than 1 chance in a million trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion (10282) that such a planet would exist anywhere in the universe without invoking divine miracles. These highly fine-tuned features form one of the cornerstones of the evidence for a supernatural, super-intelligent Creator.
Psalm 19:1 (NIV) - The heavens declare the glory of God; the skies proclaim the work of his hands.
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