The frantic search for extraterrestrial life
The odds are stacked against it!
16 October 2018; updated 18 October 2018
I am constantly amused by the ongoing vigorous efforts by many scientists to find some sort of life at places other than the earth. Daily in the media, there are conjectures about microbes in deep lakes on the Saturnian moons Enceladus or Titan. Or maybe the source of organic type compounds on Mars. And on and on.
Let’s approach this from two angles: how does life originate? And how does life survive, advance and propagate?
If life starts from some sort of single cell, how could that cell form, and how could it beget life? Even the simplest cell is an immensely complex factory and object. It is suggested that a cell, over the course of extremely long timeframes, could just ‘come together’ from apparently inorganic chemicals, to form the original building blocks of life in some sort of primordial soup, via chemical evolution (aka abiogenesis).
A cell consists of a number of vital parts. It needs a membrane to contain the internal parts and to gather nutrients and expel waste products. It needs genetic information which controls the operations of the cell, and this is stored on the DNA molecule. It needs transport proteins to move and control operations, and enzymes that are the ‘tools of life’. These are at the very least, crucial elements. Each protein comprises hundreds or thousands of building blocks arranged in a precise sequence, controlled by the information on the DNA, which must be decoded. But DNA has the instructions for its own decoding machines, still a huge problem for the origin of first life!
And, if they did, what jump-starts this process which we call life? Life means that all these mechanisms start working, producing energy, metabolizing, growing, taking in nutrients, expelling waste products the very first time it appeared. The accidental production of a complex cell would defy any reasonable mathematical odds. Consider two cells, identical in structure and composition, one alive and one not so. What was lost to cause the cell to die? Why can’t it jump start again? It is because of the arrangement of information in the cell. Information comes from a greater source of information. It does not happen by chance. For a thorough demolition of chemical evolution, see Origin of life: An explanation of what is needed for abiogenesis (or biopoiesis).
But let us be generous and assume that somehow a live cell exists somewhere in the universe, what conditions are necessary to allow it to survive, to propagate, and to expand its functions—that is, to evolve to higher life forms? We can examine the one example we know about for sure, and that is spacecraft earth. We know a great amount about the earth, and know many features, conditions, and elemental substances available to allow life.
In searching for life elsewhere, it is necessary to find all the enabling conditions and raw materials first even before jumping to the conclusion that just because some organic molecules are found, the life must be there.
So when searching for life the seekers hypothesize the existence of simple cell organisms, but many resources are dedicated to looking for higher forms (intelligent) of life. But what conditions and substances must be there for life to exist? When looking at the earth we do know there are essential minimal requirements. And not just a few of these conditions, but all of them must be there.
Life, but not life as we know it?
Former NASA/JPL specialist, David Coppedge, and I spend a good part of a chapter of our book Spacecraft Earth – A Guide for Passengers, examining the earth and identifying necessary features.2 Life needs to be carbon-based to have all the different organic molecules and compounds available for life forms. We know of no other element that has all the features and versatility that carbon has. Some have suggested a silicon-based ecology, but although there are some silicon compounds that duplicate the carbon ones, the selection is very limited—(see below). Carbon has literally millions of compounds available.
However, many of them are fragile, susceptible to damage from heat, cold, energetic particles, ultraviolet light, chemical attacks, etc. This is particularly true of organic compounds in life forms, particularly DNA (although this has been found in dinosaur bones!). The survival of life forms centers on protections against damaging forces. The other main consideration is the availability of beneficial compounds and chemicals to allow and promote growth—and the absence of other compounds that would destructively react with the beneficial ones, including other beneficial ones!
A number of carbon atoms are needed to form proteins, amino acids, esters, alcohols, enzymes, fats, carbohydrates, and so on.
A suitable planet needs the following:
A ‘habitable zone’ is the orbital radius around a star where liquid water—and presumably life—could exist. As we shall see, there’s a lot more required for life than just being ‘in the zone’. Earth’s distance from the sun—ranging from 147.1 million to 152.1 million km (average about 149.6 million km)—keeps it always within the habitable zone. That zone is pretty narrow. Venus is well outside the inner edge and Mars is outside the outer edge. If the Earth’s average distance from the sun were 5% percent greater, temperatures would drop such that most of the Earth’s water would freeze in a ‘runaway ice age’. If the Earth were just 5 percent closer to the sun, on the other hand, the polar caps would melt, more water would evaporate, and a ‘runaway greenhouse effect’ would ensue, turning Earth into an inhospitable hothouse.
But that’s just one of the numbers in the ‘cosmic lottery’ that our Spacecraft Earth got right. More information about habitable zones has added further requirements. From the literature of astrobiology, we can identify ten or more other zones required for habitability, in addition to circumstellar distance:
- Galactic Habitable Zone: the solar system must be localized in a narrow band within the galaxy. Our sun is at an ideal distance from the galactic centre, called the co-rotation radius, where a star’s orbital speed matches that of the spiral arms. In other places, the sun would cross the arms too often and be exposed to supernovae.3
- Continuously Habitable Zone: the habitable zone must not vary significantly.
- Temporal Habitable Zone: the habitable zone must last long enough for life to persist.
- Chemical and Thermodynamic Habitable Zone: the planet’s chemistry and heat transfer mechanisms must permit liquid water to persist.
- Ultraviolet Habitable Zone: the planet must filter out ionizing radiation from its star.
- Tidal Habitable Zone: the star must not tidally “lock” its habitable planet to force one hemisphere to always face the star (this rules out red dwarfs, which means most stars).
- Obliquity Habitable Zone: the star must not “erase” its habitable planet’s tilt through tidal forces. (While not eliminating the possibility of life, a planet without a tilt would have no seasons, drastically reducing its habitable surface area.)
- Eccentricity Habitable Zone: the planet must have a nearly circular orbit so that it stays in the proper place in the zone.
- Stellar Chemistry Habitable Zone: the star must have the right chemical composition to remain quiet and well-behaved. A G2 main-sequence star like our sun is ideal.
- Stellar Wind Habitable Zone: the star must not be given to extreme ‘space weather’ that might strip off a habitable planet’s atmosphere.
- Inhabited Zone: recently, two astrobiologists suggested that to be habitable, a planet needs inhabitants! “…there is a growing amount of evidence supporting the idea that our Planet will not be the same if we remove every single form of life from its surface,” a news report said.
In thinking over all the factors, Patrick Young, a planetary scientist at the University of Arizona, said:
“Habitability is very difficult to quantify because it depends on a huge number of variables, some of which we have yet to identify.”4
It’s likely, therefore, that this is only a partial list. Spacecraft Earth scores an “A” on them all.
A special sun and solar system
First of all, to be in the habitable zone, the planet must generally be orbiting a main sequence G-type star (our sun is G2V). Only about 1% of all stars fall into this class, and many are not as quiet as our sun. The associated star, furthermore, must be stable in its energy output. Any significant temporal variation of the heat output would cause catastrophic swings in the temperature of the planet that would destroy life. Our sun is actually a remarkably stable star.
The orbit in which a candidate planet travels must be nearly circular in the habitable zone, again to reduce variations in temperature. Life depends on what might be called ‘reasonable’ chemical reactions, and these are very temperature-dependent. The temperature at which most life-related reactions occur is above the freezing point of water and well below the boiling point, so the location in the habitable zone must be such as to result in moderate temperatures.
The rate of rotation is important to have a reasonable time of night and day. A rotation rate of one hour would be too short for life cycles to get rest and activity. A rotation rate of once a month would be too long for night-to-day changes. A rotation rate of 24 hours seems just about right – how about that?
The mass of the planet needs to be in the proper range to produce gravity large enough to keep an atmosphere, but not too much where the atmosphere would have crushing pressures.
One important thing about Spacecraft Earth is its moon. It’s not only beautiful; it may be required for life to flourish. Approximately one quarter of a million miles away, with an orbital period of approximately 28 days (a ‘moon-th’ or month), our moon has just the properties to benefit life in several ways. As moons go, ours is extraordinarily large in relation to its planet’s diameter. This is another unique feature of Spacecraft Earth that is quite remarkable. Our moon is an enormous object with significant gravity, roughly one-sixth of that of the Earth, as you may recall from watching videos of Apollo astronauts cheerfully leaping high from the surface. No other planet in the solar system has a moon so large relative to its size (Pluto has a large moon, Charon, but Pluto is no longer considered a planet but dwarf planet 134340 Pluto). Mercury and Venus have no moon; Mars has two tiny satellites. The gas giants have some moons larger than ours, but they are small relative to the planets themselves.
The moon stabilizes the Earth’s axis, preventing wild swings in obliquity that would wreak havoc with the seasons. Another very noticeable benefit of having a large moon is creation of the tides. The moon is much smaller than the sun, but because it is much closer, its gravity has a more pronounced effect, causing both land and sea to rise and fall in a daily rhythm. Because rock is more resistant to tidal pull than water, the oceans are affected the most. As the earth rotates, and the moon revolves around it, gravity sets up enormous ocean currents we only partially notice at high tide and low tide. These currents are amplified when the moon and sun are in line with the Earth. Because the bodies are constantly moving, there is a time delay in their effects on the Earth, so that high tides are some hours after the alignment. A variety of marine organisms, from fish to turtles to worms, regulate their spawning very precisely by the lunar tides.
No tides. No air to breathe!
Oxygenated by the tides, the ocean waters are then transported and mixed at various depths around the globe by strong currents, where numerous creatures await the bounty. Fish can extract this oxygen by passing seawater through their gills, similar to the way land animals extract oxygen with their lungs. The moon therefore plays a very significant role in the vitality of the marine ecosystem. Ocean life, without direct access to the oxygen in the atmosphere, is able to obtain vital elements for extracting energy from food—thanks to the tidal engine provided by our moon.
It would be a very stale world without a moon like ours. Most likely, the biosphere would be severely impoverished. The opposite extreme—too large a moon, or too close—would be devastating as well, with tides that might run clear across continents, causing massive erosion making much of the earth uninhabitable, and driving extreme weather that could destroy life. Here we see another ‘Goldilocks’ factor that came together just right at Spacecraft Earth. But there’s more: some things about the moon seem particularly beneficial for humans.
Water must not just be abundant, but available. It cannot be all buried under the crust, for example. The water must be pure enough to allow life; only then can it provide a source of food. The water must be of a temperature favorable to the existence and growth of living cells. The ratio of water to land mass is also important to providing a habitat for complex life.
Along with water, an atmosphere is necessary. Our atmosphere gives us oxygen for utilizing the fuels in our cells. The ratio of gases in the atmosphere is also crucial. An atmosphere of pure oxygen would be too rich for cells, making them self-destruct, and would also lead to catastrophic wildfires on the land as well. The mixture of roughly 20% reactive oxygen and 80% neutral nitrogen seems ideal for life as we know it. The atmosphere also protects us from harmful radiation from the sun. The sun gives off damaging ultraviolet light, most of which is filtered out by the upper atmosphere. Our atmosphere shields us from the electrons and protons in the solar wind which could be destructive to cells. The composition and ratios of elements are critical if we are to have atmospheres conducive to life on other worlds. As you can see, there is a stringent set of criteria that must accompany any exoplanet under consideration for life.
Another key feature that is needed to protect the surface of a planet from destructive effects of a star is a magnetic field. Our magnetic field deflects the streams of solar electrons and protons, and thus they do not hit the Earth. It’s clear that any planet must have a magnetic field to support complex life, and, moreover, a field that is not too strong and not too weak. If too strong, it might disrupt the planet’s crust. If too weak, it would not deflect charged particles.
Only the best chemicals will do
There must be an abundant source of carboniferous matter on the surface of the planet to provide organic chemicals, the basis of living cells. We mentioned silicon as a possible alternative to carbon, but the vast number of useful compounds that we know can be made from carbon has not anywhere been duplicated by silicon. Some have been made in the laboratory, but they are not found in nature. That’s evident from the fact that silicon is abundant on the Earth (start with sand, SiO2), but silicon-based organic-like molecules have not formed here. Silicon, however, does perform an important function for Spacecraft Earth as a structural element in many crustal minerals. Silicate rocks make up some 90% of Earth’s crust, providing a firm continental foundation for plants and animals. For more, see Silicon based bugs: Scientists discover the first silicon-based life forms … in their imagination!
As we consider the many important features that support complex life on Earth, such as the presence near the surface of trace elements used by cells, these pile on the requirements for any exoplanet to be seriously considered a candidate for life. Let’s recap just a few of the features that are absolutely essential for even simple life (ignoring the question of how life started) even if all the physical requirements are met, and calculate the probability that all these characteristics could be found at a remote planet.
What are the odds?
To put some numbers, say there are 100 billion galaxies, and each has 100 billion stars, that is 1022 stars. Let’s say that one in 10,000 is a G2 main-sequence star like our sun. That amounts to 1018 stars. Let’s say that one in 10,000 of these stars has a planet in the habitable zone; that now gives us 1014 candidate planets. Let’s further grant a generous 5% chance that any of the required features would ‘happen’ to be present in any one planet (I think even a 1% chance would be high). Everything has to be present simultaneously for there to be any chance of complex life existing.
- Located within the galaxy habitable zone 5%
- A stable star with constant energy output 5%
- A planet formed within the habitable zone around the star 5%
- A planet in a stable orbit maintaining a steady distance from the star 5%
- A rotation speed of about 24 hours 5%
- A planet with a suitable atmosphere: oxygen rich, depth, circulation 5%
- A planet with the appropriate mass 5%
- A planet with abundant water 5%
- A reasonable ratio of water to land mass 5%
- A magnetic field within the proper strength range 5%
- A moon of the proper size, distance, and orbit around the planet 5%
- A readily available source of carbon compounds 5%
- Trace elements of the right type and quantity 5%
One could go on and on, adding more factors, but these are a few of the essential features to consider.
So let’s multiply that out: 0.05 × 0.05 × 0.05 × 0.05 × 0.05 × 0.05 × 0.05 × 0.05 × 0.05 × 0.05 × 0.05 × 0.05 × 0.05 = 0.0513 ≈ 10-17. This probability times 1014 candidate planets leaves 10-3 planets, far less than one!
Beyond that, if a habitable planet did exist somewhere, could we expect undirected evolution to once again bring about anything on the level of the beauty and complexity of life we find here on Spacecraft Earth? I maintain that it could not have happened once by accidental means here, much less a second time elsewhere.
I know a bit about planets. I worked in JPL/NASA from 1955 to 1960 in the field of space exploration. There are a lot of wishful thinkers out there who are pouring a lot of money into a search for life against all odds. They refuse to think this through because an underlying philosophy that time and chance (evolution) could produce it all. But this is nothing new. A wise man once said, “There is nothing new under the sun” (Ecclesiastes 1:9). Since the beginning of time, there has been and will always be people who are willingly ignorant of the evidence of a Creator all around us (Romans 1:20).
References and notes
- Sarfati, J., Evolution’s Achilles’ Heels, CMI 2014, chapter 3. Return to text.
- Richter, H. and Coppedge, D., Spacecraft Earth, a Guide for Passengers, CBP, 2017. Return to text.
- Chown, M., What a star! New Scientist 162(2192):17, 1999. Return to text.
- Young, P., cited in: ASU, Stellar makeup impacts planet habitability, https://asunow.asu.edu, 6 September 2012. Return to text.