A Rare Earth
A Rare Earth?
In my last 'Esoterica' article, The Probability of God, I discussed ways to answer the age-old question 'Does God Exist?' and came to the conclusion that if we accept an idea of 'Guided Evolution' then yes, a lot of blocks do fall into place.
Since writing that article, I have been watching a very interesting television series by Dr Iain Stewart, entitled 'Earth: the Power of the Planet'. In the final episode, Dr Stewart, a geo-scientist, discusses just how unusual the Earth really is, and what an extraordinary number of lucky breaks and coincidences it has taken for our home planet to be able to nurture and sustain complex life. As the program progressed, I started to hear a louder and louder 'quacking' (q.v. The Probability of God). Are these lucky breaks and coincidences really such a coincidence after all, or could the idea of a 'Guided Evolution' be again coming to the fore? Let's see what Dr Stewart, and my own research, have to say...
The Earth is certainly unique in the solar system, and possibly in the Universe as well - the so-called Rare Earth Hypothesis. Although the Universe contains many billions of stars, and no doubt many billions of accompanying planets as well, the likelihood of any of these duplicating the Earth's diversity of complex life-forms is smaller than you think. Why is this? Surely given a suitable environment with water and sunlight, life will evolve anywhere eventually? Apparently this is not the case, and were it not for a series of lucky breaks and coincidences, the Earth would contain at best a light covering of bacterial 'soup' rather than the complex and beautiful life that is here today.
So what are the extraordinary sequence of events that got us to where we are today, and why are they so extraordinary? Lets start right at the beginning, shortly after the Earth was formed. It was a much smaller planet than it is today. Is that a problem? Yes it is, because if a planet is too small, it won't have sufficient gravity. Without sufficient gravity, our atmosphere will 'leak away' into space, leaving behind a dead and desolate planet with no chance of supporting life, as has already happened on Mars. Water will either freeze, boil away, or decompose under the action of UV radiation. There won't be enough air to breathe. Shortly after the Earth was formed, however, our sister planet 'Theia' collided with the Earth. Evidence for this comes from rocks collected during the Apollo Moon landings. The impact did two things: it brought Earth to a critical mass needed to support an atmosphere, and it formed the Moon, our nearest neighbour, that plays a crucial part in Earth's story - more about that later.
What's special about the atmosphere? As well as providing air to breathe, and weather to keep the planet cycling through its natural seasons, the atmosphere has another vital task: it protects the Earth from collisions by meteors that would otherwise smash into the Earth's surface. It does this by providing resistance to an object as it hurtles towards us. This resistance causes the object to heat up and in most cases, to burn up rather than smash into the surface of our planet - you can watch this happening with the shooting stars we all love to see at night. Objects that don't burn up completely are severely reduced in size, lessening their ultimate impact (in both senses of the word!). (They also become 'meteorites' - def. a meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and impact with the ground without being destroyed.)
Are collisions by meteorites a problem? Yes, very much so, because the effect on us of even a small object when combined with the speed it is travelling at can be catastrophic. The Barringer Meteorite Crater, in the northern Arizona desert of the United States, is a gigantic hole in the middle of an arid desert. It is nearly a mile wide, and 570 feet deep. A rim of smashed and jumbled boulders, some of them the size of small houses, rises 150 feet above the level of the surrounding plain. Yet this crater was caused by a meteorite just 50 metres in diameter! The local environment may have taken over 100 years to recover. A larger crater, or rather the remains of a crater now worn away by natural erosion, can be seen at Chicxulub in the Yucatan, Mexico. This crater is over 180 kilometres (110 miles) in diameter, making the feature one of the largest confirmed impact structures in the world, yet the asteroid or comet whose impact formed it was only 15km in diameter. The effect of this collision on the climate was so great that it wiped out the dinosaurs! There are plenty of other comets and meteors of that size travelling about in space.
Luckily, we have been spared from many meteorite impacts on Earth. A look at the Earth Impact Database shows the current total number of confirmed impact structures as just 174. That's less than we might statistically expect to see. The atmosphere is partly responsible for this, but the other major reason is the presence of the Jupiter. This neighbour-planet is enormous: some 300 times the size of Earth. With it comes a huge gravitational field that is responsible for attracting many of the dangerous flying objects in our vicinity. We have actually seen this in action when in 1994, astronomers witnessed the collision of Comet Shoemaker-Levy 9 with Jupiter. A similar collision on Earth would be devastating to life. Unfortunately, the surface of Jupiter is continually covered by clouds, so we can't count the craters that may have accumulated, but mathematical modelling confirms Jupiter's role as Earth's protector.
The Moon, formed by the collision with Earth by our once sister-planet 'Theia', is also vital to life on Earth. Without it, we wouldn't be here - its as simple as that. A lot of planets have moons, but they are generally tiny in comparison with our Moon. Ours is unusually large, and not only that, its quite close to Earth, so its gravity is constantly influencing us, pulling the Earth's oceans towards it, creating tides which act as stabilisers to the climate and hence to life on Earth. This mechanism has been working for 4.5 billion years, since oceans first formed on the Earth, and is vital in the slow time period that complex life takes to evolve. If the Moon wasn't here, our climate would swing wildly, baking hot one day and freezing cold the next. Evolving life could not survive such unpredictability.
Another important factor in the possibility of complex life is Earth's position in relation to the Sun. There is just a narrow band of space where the Earth must reside. Any closer to the sun would be too hot: the water would all evaporate, and we'd all be burnt alive. Any further away and the Earth would be in eternal winter. Our Solar System has to be in the right part of the Universe too. Much of the known universe, including large parts of our own galaxy, cannot support complex life for a number of reasons. For one thing, as the distance from the galactic centre increases, the metal content of stars declines, and metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets. Conversely, if you are too close to the black hole at the galactic centre then the X-ray and gamma ray radiation increases, becoming too dangerous to support complex life. So not only is the Earth in the right position in relation to the Sun, but our Solar System is in the right bit of the Universe. Those coincidences are starting to stack up, aren't they!
We are lucky enough to orbit around the right kind of Sun, too. For life to evolve in all its complexities, it is important that our Sun burns at a steady and constant rate, with just the right amount of energy landing on Earth. Too much ultraviolet radiation would kill everything but a few underground microbes. Any changes in the Sun's output must occur gradually, too. Other suns are bigger than ours, but burn up too quickly. Ours is the right size to burn slowly, allowing complex life to slowly evolve.
Oh, and lets not forget Plate Tectonics. Put simply, this is all about how the surface of our planet moves about on its molten core of liquid rock due to the convection currents caused by the heat of radioactive decay occurring deep in the Earth's interior. Scientists argue that in order for a rocky planet to support complex lifeforms, its crust must experience plate tectonics. Why is that? Firstly, such movements enable the magnetosphere - a protective magnetic 'blanket' that shields us from the deadly effects of the magnetic Solar Wind. You can actually see this in action when the 'Aurora Borealis' or 'Northern Lights' appear. Secondly, it allows dry land to be created by pushing areas of crust above the surface of the oceans, yet also moves it around to stop stagnation in the world's oceans, and create biodiversity. Thirdly, it regulates the temperature of the atmosphere by adding to and then removing carbon-dioxide, a greenhouse-gas that is necessary in the right quantities, yet deadly if there's too much of it.
To recap, lets list some of the extraordinary events that have been needed to get us to where we are today:
1/ Collision of 'Thea' to increase the Earth's mass and gravity.
2/ Formation of the Moon - correct size and distance from the Earth.
3/ Size and position of Jupiter, acts as Earth's protector.
4/ Earth's position in relation to the Sun - just the right amount of heat and light.
5/ The right type of Sun, not too big, not too small, and the right amount of stable energy output.
6/ Plate Tectonics, that create a shielding magnetic field and an atmosphere with the right amount of CO2.
Are all these *really* just a series of lucky coincidences, or does it point to something else? Just how many coincidences are needed before you come to the conclusion that something else is involved? Do you start to hear quacking yet?
For those of you interested in taking this discussion to a deeper level, there is a WikiPedia page all about the Rare Earth Hypothesis. Also worth reading is a WikiPedia page that discusses the crucial Giant Impact Hypothesis. Both of these are useful for a greater understanding of just how rare our Earth might be, but neither will say, or even consider, whether this long line of coincidences vital to our existence is just that: a long line of coincidences, or whether, as I believe, it is just too unlikely for all this to have happened by itself, and makes me suspect that a Supreme being of type and form that I am unable to explain or even visualise, has set the processes in motion and now steps back to see what we will make of it.
As a footnote, I would like to say that I find it strange, though sadly predictable, that there is such animosity between the Creationists and the Evolutionists. Perhaps its time for both sides to call a halt and wonder instead whether a Middle Way, such as the idea of a Guided or Theistic Evolution is not possible. What is certain is that neither side can truthfully rule out the possibility that it might be true, any more than I can rule out the possibility that it is not. However, I *do* hear quacking...
Tim Makins
January 2008
You can find some further information on Dr Iain Stewart and his television series Earth: the Power of the Planet at the following sites:
University of Plymouth - Faculty of Science
BBC Science and Nature
Dr Iain Stewart's book
