The true nature of our ambiguous ally

Nearby Jupiter exerts a powerful influence over life on Earth, and I obviously don’t mean that in the sense of astrology. The planet’s gravity dominates the orbital dynamics of other objects in the Solar System. But is that for better or worse as far as we’re concerned?
The prevailing theory is that Jupiter shields Earth from comet and asteroid impacts; either its gravity slingshots them away from the inner Solar System or the planet’s bulk takes the hit itself. In this way, Jupiter is like a cosmic goalkeeper, protecting us from massive impacts that might otherwise have hampered the development of life on Earth, or perhaps prevented it altogether. But is the situation really that simple? If Jupiter’s gravity can swing the path of some incoming objects away from the inner Solar System, couldn’t it just as easily cause others to cross our planet’s orbit?

Jonti Horner and Barrie Jones have spent the past few years trying to tease apart all the different possible influences of Jupiter. In this paper they summarise the results of a series of dynamic simulations of the Solar System.

There are actually three distinct populations of objects that pose an impact risk to Earth. Long-period comets, which have orbits of thousands or even millions of years, can be nudged out of the Oort Cloud and onto a Sun-plunging trajectory by passing stars. Short-period comets, on the other hand, originate from the region around Jupiter and return to the inner Solar System every five or six years. But the majority of the impact threat to our world is from near-Earth asteroids. Horner and Jones studied the effect that varying the mass and orbit of Jupiter would have on the orbital dynamics of each of these classes of objects to see how the rate of life-threatening impacts on Earth would change. The answer is clear-cut for long-period comets: Jupiter offers a significant level of shielding. But
these represent only a small proportion of the impact hazard. For short-period comets and asteroids the situation seems to be much more complicated. If Jupiter’s mass is slimmed to that of Saturn, the Earth impact rate jumps up, but if the simulated mass is cranked right down or Jupiter is plucked out of the Solar System altogether, impacts actually fall dramatically.

So, what’s the overall effect? Is Jupiter more of a friend or foe to life on Earth; is it actually an ally or more of an enemy? Horner and Jones found that although the situation is very complex, on balance giant planets like Jupiter probably represent an increased hazard to life on habitable planets, a finding that will be important for assessing the likelihood of life on Earth-like planets in other solar systems.


This article first appeared in the November 2011 issue of Sky at Night Magazine.

 

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