It’s good to have a gentle giant on your side (Image: NASA/JPL-Caltech/SSI)

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.

Booted out

Away from such simulations, the circularity of every planet’s orbit does fluctuate over time. If the orbit is already highly elongated, such fluctuations would allow a planet to escape the sun’s gravity. A 20-degree tilt of Saturn’s orbit could eventually boot Mars out, while Earth would require a 30-degree tilt.

Pilat-Lohinger’s methods are sound and her conclusions well supported, says Rory Barnes at the University of Washington in Seattle. But he notes that the implications for life in the universe are unclear. For one thing, we know the orbital inclination of only two planets outside the solar system: both orbit the star Upsilon Andromedae, with orbits inclined by 30 degrees to the star’s equator.

What the elongation of an orbit means for life is uncertain, too. “At some point, the eccentricity of a planet impacts its potential to support life, but it’s hard to say where that boundary is,” says Barnes. A planet with an orbit shuttling it between Earth’s distance from the sun and that of Mercury would be quite different from the Earth, he says, “but I don’t think it would prevent life from originating”.

Journal reference: International Journal of Astrobiology, DOI: 10.1017/S1473550414000469