Our Solar System's Surprisingly Unstable Planets

It isn't every day that you read an astronomy paper that talks about the "grisly" details of changes in planets' orbits, triggering a tongue-in-cheek warning: "suffice it to say here that Earth does not fare well in the resulting interplanetary melee." But a pair of papers in today's issue of Nature are not your everyday offerings.

An embarrassing little secret of astronomy (dating to the days of Newton) is that the mathematical equations that describe the orbits of the planets have—how to put this?—a lot of wiggle room. That is, the gravitational pull on a planet of everything but the sun is incalculable (literally; see the three-body problem. Combine that with chaos theory, which shows that a slight perturbation in an object's motion can explode into wild careering, and you have the makings of at least one such "grisly scenario," as astronomer Gregory Laughlin of the University of California, Santa Cruz, puts it. For instance, Mercury's orbital eccentricity (eccentricity is a measure of how stretched and noncircular an orbit is) may "increase to the point at which it intersects the orbit of Venus, setting the stage for catastrophe," notes Laughlin.

Such possibilities emerge from an exhaustive analysis of future planetary orbits. Astronomers Jacques Laskar and Mickael Gastineau of the Paris Observatory improved on the standard statistical simulations by, among other things, incorporating general relativity, which tweaks planetary motions just enough to make it interesting. The constant, and constantly changing, pull of one planet on another degrades their regular, predictable orbital motion, so that like an unbalanced car tire "that tears itself off the axle," explains Laughlin, "planets might fling each other out into space or into their parent star, or collide with each other."

Considering what a planetary squirt it is, Mercury "poses the greatest risk to the present order," notes Laughlin. Of the 2,501 scenarios covering the next 5 billion years that the Paris astronomers examined, 25 led to a disruption of Mercury's orbit sufficient to make it collide with Venus or the sun. One scenario led to a collision of Mercury with Earth. In what Laughlin calls a "small but disturbing subset of possible future trajectories, Mercury becomes trapped in a 'secular resonance' with Jupiter, a state of affairs in which the elliptical figure of Mercury's orbit rotates in synchrony with Jupiter's orbital precession."

If this happens, one scenario finds, Mars and Earth come within 500 miles of one another in 3.3 billion years, putting little green men practically within hailing distance and proving "disastrous for life on the Earth," write the Paris astronomers. (What that life will be, 3 billion years on, is anyone's guess.) In five cases, Mars is ejected from the solar system. (Given how upset people got when Pluto was demoted from planethood, I can't wait to see how they'll react to Mars's exile.) In 196 scenarios, two celestial bodies collide within a mere 100 million years: sun-Mercury, 33 times; sun-Mars, 48 times; Mercury-Venus, 43 times; Mercury-Earth, once; Mercury-Mars, once; Venus-Earth, 18 times; Venus-Mars, 23 times; Earth-Mars, 29 times (that's 48 in which one of the colliders is Earth). And in one particular "grisly" scenario, Mercury's orbit becomes so destabilized 3.34 billion years from now that it leads to "a wholesale exchange of angular momentum between the inner and outer Solar System," with the above-mentioned interplanetary melee in which Earth may collide with Mercury, Mars or Venus.

The chances of cosmic billiards are fairly low, though. The simulations show that the orbits of the inner planets (Mercury, Venus, Earth and Mars) have about a 99 percent chance of staying on their current, orderly paths for another 5 billion years, which is when the sun evolves into a red giant and swallows the inner solar system, incinerating the planets and thereby rendering orbital calculations somewhat beside the point. But the chances of orbits changing with less-than-catastrophic results are greater, notes Laughlin: "the planetary orbits will indeed become chaotic," with "the time required for chaos to significantly degrade the predictability of a system [on] the order of 5 million years." For a 4.5 billion-year-old solar system, that's practically tomorrow. So while the odds of any one of these violent scenarios coming to pass are about 1 percent, notes Laughlin, that small but nonzero probability is enough to bring astronomers "a vicarious thrill of danger." Whatever turns you on.