Facing Reality About Life on Other Planets is a nifty series of articles from Dr Henry Richter, basically outlining the impossibility (no, not “high improbability”, that’s just a cop-out) of randomly occurring life in this universe.
I’m giving an excerpt from every article below the title & link.
Let me start with the first: location within the parent galaxy. We ride in the Milky Way galaxy, about half way between the center and the edge. That is a safe place to be, as the center has all sorts of things going on such as black holes and supernovae. The center is a place with a high radiation environment which would be destructive to life, and even to basic chemistry processes. On the other hand, being close to or at the edge of the galaxy would be in a region of low density of materials, probably not enough to collect into a planetary body. The half-way position results in a reasonable sized habitable zone around stars.
Our galaxy has spiral arms, and we don’t want to be located in one of those. We are between the Sagittarius and Perseus arms of the galaxy. These are regions of star building activity, where we do not want to be. We are in a nice safe gap between the arms.
The star needs to be within a fairly narrow range of: size, heat output, stability, and spectral composition. Stars are classified into seven main types. In order of decreasing temperature (and luminosity): O, B, A, F, G, K, and M. The temperatures are:
- O – over 25,000 K
- B – 11,000- 25,000 K
- A – 7,500-11,000 K
- F – 6,500 -7,500 K
- G – 5,000-6,000K
- K – 3,500-5000 K
- M – under 3,400 K
So you see there is really a wide variety of star types, most of which are unsuitable to heat a planet to a livable temperature. Habitable zones are just not possible with most. Our G2 yellow dwarf is really the only possibility. And, how many of these are there? It is estimated that in the billion galaxies with a billion stars each, that there are about 1022 stars in the known universe. That’s 1 followed by 22 zeros. That is more stars in the universe than there are grains of sand on earth – deserts, shorelines, and elsewhere. Estimates are that only one in 10,000 is a G2 yellow dwarf, but that is still a lot of stars that can host planets on which life conditions are possible. Present estimates are that there are maybe 512 G2 stars within 100 light years of us, and these are the ones most easily examined for planets. But more about that in a later writing.
It has been known for some time that the magnetic field affected cosmic rays impinging the earth. That is why cosmic ray measurements have been made at a wide range of longitudes. Not much was known about the solar wind until earth satellites were developed and measurements outside the earth’s atmosphere became possible. Measurements by rocket-borne detectors were made outside the atmosphere, but it was mainly measurements in the polar regions, where the magnetic field was lowest, that many measurements of the solar wind were made.
So, among the criteria of conditions crucial to life on an exoplanet, the existence of a suitable magnetic field is high on the list. The strength of the field is important: strong enough to produce a magnetic bubble of the right size, but not too strong to affect surface processes.
Earth has a satellite moon, about a quarter of a million miles away. It is about one-fourth the size of the earth, and in a nearly circular orbit around the earth. Each orbit takes just under thirty days – a month (from “moon-th”). This moon does a number of things for us, some of which are important to the support of higher forms of life.
Moving in even closer we come to the earth’s atmosphere. The atmosphere is very thin and is held in by gravity. The amount of atmosphere results in a gas pressure that is low enough to not crush delicate life and chemical structures. Some planets have an atmosphere that is destructively dense with correspondingly high pressures. Venus, for example, has a dense atmosphere of carbon dioxide with such pressure that it crushes spacecraft that have landed on the planet. That particular notorious ‘global warming’ gas also creates a greenhouse effect, making the atmosphere unsurvivably hot. Earth’s atmospheric pressure is high enough to supply oxygen to tissues in a quantity sufficient to facilitate metabolism.
Again, we see other factors that are just right, such as the composition of the earth’s atmosphere. Three main components comprise our atmosphere: 21% oxygen, 78% nitrogen and 1% carbon dioxide. The oxygen is necessary for metabolism. Too much oxygen would overwhelm the metabolic chemical reactions; i.e., they would in essence burn up. Too little oxygen would not support metabolic reactions. The nitrogen, although somewhat inert, provides enough substance to the atmosphere to maintain its thickness.
Water is essential to life – and not just liquid water, but with the proper purity, neutral acidity, and chemical content. Water is an essential ingredient in living cells, both plant and animal. And there must be ample water. In consideration of a human colony on the moon or mars, possible sources of water have been found, mainly small deposits of water ice, but these are not sufficient for the development of life forms, at best—just the sustenance of existing life.
Rocks and Minerals
In addition to water, dry land is required for an ecosystem. True, there are plants and animals that thrive in a watery environment, but that makes for a limited set of forms and creatures. The movie Waterworld (1995) portrayed humans trying to exist in a planet completely covered by water. They had to develop a pseudo-land like environment to operate. It was, of course, quite weird and awkward. To have higher forms of life like humans, some fraction of the planet needs to be solid land—and not just solid land, but a rocky surface. This is another necessary criterion for life.* I do not know if anyone is able to compute the optimum ratio of sea surface to land surface on a planet to best support higher forms of life, but a good quantity of both types of surfaces are undoubtedly necessary.
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.