Facing Reality About Life on Other Planets, 6: Chemistry and Probability
by Dr Henry Richter
My past few articles on “Facing Reality About Life on Other Planets” have dealt with the necessary conditions for the existence of life on a planet—any planet. With the almost fanatical drive of the scientific community to find and prove the existence of life elsewhere in the universe, it is important to ponder the requirements for habitability. We’ve looked at the location in a galaxy, the location in a stellar system, the type of star required, and the physical characteristics of an exoplanet such as mass, rotation rate, atmosphere, magnetic field, water, a partial rocky surface and so on. This final article will consider the composition of planet: what materials and chemical building blocks must be present to sustain life.
We talked about life being based on carbon chemistry. A readily accessible and usable supply of carbon must be available in the planet’s chemical composition. Carbon itself, with its four bonds, is a remarkable element having a flexible bonding capability, giving it hundreds of thousands of possible molecular structures. The only other close element is silicon which can form a few somewhat complex molecules, but only a few compounds—nowhere near as many as carbon. Star Trek fantasies aside, astrobiologists generally admit that silicon-based life is just not possible. Carbon is known to be the basis of the acids, bases, enzymes, proteins, alcohols, esters, ethers, amino acids, and much more. A large number and variety of these are involved in building and maintaining living cells. So a ready supply of carbon is necessary. Where can it come from?
The earth has usable carbon available in the atmosphere in the form of carbon dioxide gas, in carbonate rocks, as carbon dioxide dissolved in the oceans and lakes, and secondarily in plant tissues and juices (converted from atmospheric carbon dioxide). Both plants and animals are involved in the earth’s carbon cycle. Plants convert carbon dioxide gas into a wide variety of carbon compounds. These plants are consumed by animals, used as fuel, and the metabolic product being carbon dioxide. It’s a remarkable process!
Many other elements are required for life in addition to carbon. These probably can’t be ranked in order of importance since all are essential. The next that comes to mind is calcium. This is used in bones and structural frameworks, and is an important signaling molecule. Calcium is also important in eggshells and seashells. Nitrogen is another essential element. Nitrogen compounds are needed for plant growth, and are in proteins. Phosphorus plays an important role in cell structure, cell activity, and the genetic code molecules, DNA and RNA. A variety of other elements, even some rare earths, are involved in life’s structures and processes. All you have to do is look at the label of a bottle of mineral supplements in a health food store to find selenium, magnesium, copper, and several others. We even see non-minerals such as iodine and bromine. Could we exist if any of these were absent from the earth, or weren’t readily accessible? I don’t know, but it raises the question whether complex life could exist elsewhere if any of these—or some combination of these elements—were missing. That brings us to the consideration of probability: what is the chance that all the required factors would exist simultaneously on an alien world?
Probability: Running the Numbers
Let’s look at the big picture now – the really big picture: the universe. It is estimated that there 100 billion galaxies (1011), each with 100 billion stars. That results in 1022 stars. Say that only one in 10,000 is a dwarf main sequence G2 star which, as we saw, is the most stable star for a habitable zone. That leaves 1018 possible host stars. That’s a quintillion—still a lot of stars! Let’s say that only one of 10,000 of these stars has a planet in the habitable zone; that now gives us 1014 candidate planets (a hundred trillion). Let’s further grant a generous 10% chance that any of the required features would “happen” to be present in any one planet (I think a 1% chance would even be high). All of these features have to be present simultaneously for there to be any chance of complex life existing. The factors below are listed in the documentary The Privileged Planet, mentioned earlier.
- Located within the galaxy habitable zone 10%
- A stable star with constant energy output 10%
- A planet formed within the habitable zone around the star 10%
- A planet in a stable orbit maintaining a steady distance from the star 10%
- Protected by gas giant planets in the solar system 10%
- A rotation speed of about 24 hours 10%
- A planet with a suitable atmosphere: oxygen-rich, depth, circulation 10%
- A planet with the appropriate mass 10%
- A planet with abundant water 10%
- A reasonable ratio of water to land mass 10%
- A crust capable of plate tectonics 10%
- A magnetic field within the proper strength range 10%
- A moon of the proper size, distance, and orbit around the planet 10%
- A readily available source of abundant carbon compounds 10%
- Trace elements of the right type and quantity 10%
One could go on and on, adding more factors, but these are a few of the most essential features to consider. So let’s multiply that out: 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 × 0.1 = 10-15. This probability times 1014 candidate planets leaves 10-1 planets, less than one! If I had used a 1% probability instead of 10% (more reasonable), that would have reduced the overall probability to 10-30, yielding 10-16 habitable planets out of the hundred trillion candidate planets. This implies that even one habitable planet in the whole universe has less than one quadrillionth a chance of being found! With a probability this small, changing the order of magnitude of our estimates for the number of stars is not going to make much difference.
Beyond just the requirements for habitability, could we expect undirected evolution to bring about a second form of complex life anything like the beauty and complexity of life we find here on Spacecraft Earth? Could life even start by chance, before evolution’s natural selection comes into play? I maintain that it could not have happened once by accidental means here, much less than a second time elsewhere!
So, to wrap up, the outlook is bad for avid hunters of populated planets. There aren’t likely to be any other habitable planets in the universe. The only reasonable conclusion, given the evidence we have considered, is that our earth was specially and wonderfully made to be inhabited.
Dr Henry Richter, a contributing science writer to Creation-Evolution Headlines, was a key player at NASA/JPL in the early days of the American space program. With a PhD in Chemistry, Physics and Electrical Engineering from Caltech), Dr Richter brings a perspective about science with the wisdom of years of personal involvement. His book America’s Leap Into Space: My Time at JPL and the First Explorer Satellites (2015), chronicles the beginnings of the space program based on his own records and careful research into rare NASA documents, providing unequaled glimpses into events and personnel in the early days of rocketry that only an insider can give. His next book, Spacecraft Earth: A Guide for Passengers, is due out later in 2017. For more about Dr Richter, see his Author Profile.