Just Right

Scientists have cataloged dozens of parameters that have to be just right for life to appear 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.

A few such parameters are:

...a galactic habitable zone, a central star and planetary system having the requisite character, the circumstellar habitable zone, a right sized terrestrial planet, the advantage of a gas giant guardian and large satellite, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts, and whatever led to the still mysterious Cambrian explosion of animal phyla. The emergence of intelligent life may have required yet other rare events.

New Scientist has a report on some research that shows yet another cosmic coincidence that allows life to exist on earth. It turns out 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 right for liquid water.

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.