Check out the scientific information and conclusions about whether the earth really is finely-tuned for life, or if there are likely to be numerous hospitable planets in the universe, many of them home to life.
Earth
There are many aspects relating the earth, our solar system, our sun and our galaxy, which appear to make the earth "fine-tuned" for life as we know it:
- Our solar system is in the right position in the best type of galaxy (spiral). Elliptical galaxies and the centre of spiral galaxies are too unstable for the long process of evolution of life. Elliptical galaxies and the edge of spiral galaxies have far less of the heavier elements which are necessary to form planets like earth, and for life to form.
- Our sun, a yellow dwarf, is just the right size and type, and very stable. The more numerous red dwarfs are much less likely to allow life on any orbiting planets, and larger stars are too short-lived for earthlike planets and life to evolve.
- The structure of our solar system is optimal for life. The earth is within the "circumstellar habitable zone" that allows liquid water to form on the earth's surface - 5% closer or further away from the sun and we wouldn't have surface water, nor the optimum amount of carbon and oxygen. The earth's orbit is nearly circular, which provides a stable temperature. The larger outer planets' orbits are also nearly circular, which reduces the gravitational pull on earth and the potential to destabilise earth's orbit. The larger outer planets act as a comet and asteroid shield protecting earth from fatal collisions.
- The moon is unusually large, which creates larger tides necessary for oceanic processes, but not too large.
- Finally, many features of the earth are serendipitous, including plate tectonics, which are seen by some scientists to being crucial for a whole range of important earth processes, the earth's albedo which ensures that just the right amount of sunlight is reflected, and the 'just right' mass of the earth to retain our beneficial atmosphere and retain heat stored in the earth's core.
There is no doubt that the earth is in the right place and has the right characteristics to allow life - after all, we wouldn't be here if it wasn't! However there is contention about whether this is surprising - after all there are zillions of stars (perhaps 1022) and probably billions of planets (we don't really know how many), so it is likely that life has evolved wherever it was able to. So the question is, how many planets are there that meet the requirements of life?
The probabilities
Unfortunately, no-one can agree on the answer. The following are some of the estimates:
- The Drake Equation attempts to estimate the likely number of civilisations in our galaxy, and (according to Wikipedia) gives answers in the range of 2-10. This suggests that, if our galaxy is representative of the approximately 1011 others, life is not so rare in the universe and the earth is not particularly fine-tuned. However the Drake equation does not consider many of the factors discussed above and considered by other approaches, and the extrapolation to other galaxies takes no account of the apparent greater suitability of our galaxy.
- Many others take a similar line - e.g. Carl Sagan, and those in the SETI (Search for Extra Terrestrial Intelligence) movement. Some estimate that as many as 1020 planets in the known universe may be habitable.
- On the other hand, Ward and Brownlee, investigating the same questions, developed the Rare Earth Equation, and came to the conclusion that the earth is indeed a rare place and that life is "exceedingly rare in our galaxy and in the universe".
- Others have followed their lead. Barrow & Tipler concluded that we are the only life in our galaxy and possibly in the universe. Ross has listed more than 150 factors supporting a rare earth conclusion, and others have used his information to make probability estimates leading to the conclusion that life on earth may be unique.
So, is the earth rare?
It seems that we are unable to draw a clear conclusion. The information is not available to make very accurate estimates, so people tend to get the answers they would like. Theists tend to say the earth is uniquely fine-tuned for life, and therefore points to a creator. Non-theists tend to argue that life is likely to be found all over the universe, evolving naturally in the many locations where conditions are suitable.
Perhaps the most we can do is to take most notice of those that do not appear to have allowed their philosophical views to affect their scientific judgment. In this case, perhaps Ward and Brownlee may be the most credible, and we may, perhaps, slightly lean towards the rare earth hypothesis. But it would be unwise to be too dogmatic. Perhaps better data will be available in the future.