Either Luck or Design

Yesterday's post discussed the very narrow range of values to which the masses of subatomic particles must be restricted in order for a universe that can give rise to living things to exist.

Today's post addresses just a few of the amazing facts about the earth and our solar system that have to be almost precisely what they are in order for life to exist on earth.

The improbability of a planet having so many of these properties is so high that some scientists have speculated that life, at least complex life, might exist nowhere else in the universe no matter how many other planets are out there. This is the thesis of such books as Rare Earth by Ward and Brownlee and Privileged Planet by Gonzalez and Richards.

Here are just a few of the parameters a planet must satisfy in order to be able of giving rise to, and sustaining, life:

1. A life-bearing planet has to located in a region of the galaxy not too close to the center nor too far from the center, and where there aren't too many other objects that could collide with the planet.This is called the galactic habitable zone.
2. A life-bearing planet has to orbit a central star of just the right age, size, and energy output and at just the right distance so that the planet is not too hot or too cold (the circumstellar habitable zone).
3. A life-bearing planet has to be the right size, with the right period of rotation so that nights don't get too cold nor days get too hot.
4. A life-bearing planet has to have a large moon to stabilize its wobble on its axis and protect it from meteoric bombardment, it has to have a magnetosphere to protect it from cosmic radiation and it has to have plate tectonics to recycle minerals.
5. A life-bearing planet must have a lithosphere, atmosphere, and oceans of a particular size and chemical composition.

There are many more such conditions that a planet must meet to be life-bearing, but here's one that's particularly interesting because we might never have suspected it. New Scientist reported on some research that shows that, as strange as it may seem at first, the orbit of Saturn has to be almost exactly as it is for life to exist on earth:

Earth's comfortable temperatures may be thanks to Saturn's good behaviour. If the ringed giant's orbit had been slightly different, Earth's orbit could have been wildly elongated, like that of a long-period comet.

Our solar system is a tidy sort of place: planetary orbits here tend to be circular and lie in the same plane, unlike the highly eccentric orbits of many exoplanets. Elke Pilat-Lohinger of the University of Vienna, Austria, was interested in the idea that the combined influence of Jupiter and Saturn – the solar system's heavyweights – could have shaped other planets' orbits. She used computer models to study how changing the orbits of these two giant planets might affect the Earth.

Earth's orbit is so nearly circular that its distance from the sun only varies between 147 and 152 million kilometres, or around 2 per cent about the average. Moving Saturn's orbit just 10 percent closer in would disrupt that by creating a resonance – essentially a periodic tug – that would stretch out the Earth's orbit by tens of millions of kilometres. That would result in the Earth spending part of each year outside the habitable zone, the ring around the sun where temperatures are just right for water [to exist in a liquid state].

Tilting Saturn's orbit would also stretch out Earth's orbit. According to a simple model that did not include other inner planets, the greater the tilt, the more the elongation increased. Adding Venus and Mars to the model stabilised the orbits of all three planets, but the elongation nonetheless rose as Saturn's orbit got more tilted. Pilat-Lohinger says a 20-degree tilt would bring the innermost part of Earth's orbit closer to the sun than Venus.

In other words, our solar system is like a delicately balanced ecosystem, all the parts of which seem to be important in making earth the sort of place where life can arise and be sustained. The odds of such a system existing elsewhere in the universe would seem to be very small.

It might be mentioned in passing that it's not just Saturn's orbit that makes life possible on earth. Scientists have shown that massive outer planets like Saturn, Jupiter, Uranus, and Neptune act as gravitational vacuum sweepers sucking up a lot of debris that would otherwise invade the inner reaches of the solar system and threaten earth with constant collisions.

It really is astonishing how many factors must all be just right for life to exist on this one little planet. We're either unimaginably lucky or we're the product of intelligent engineering.