The (Un)Stable Planets
If orbital chaos exists, its effects can't be seen over short time frames. But astronomers are gathering other clues about the instability of planetary motion. In February 2012, the European Space Agency's Venus Express spacecraft peered through the dense Venusian clouds expecting to see certain surface features that should have been there, based on data from Magellan taken 16 years earlier. Instead, those features were displaced by 12 miles (20 kilometers), suggesting that the planet's rotation is slowing down. Astronomers point to the planet's high atmospheric pressure and strong winds, which create friction on the surface, as a possible cause. If the data is right, one day on Venus may now be almost 250 Earth-days long [source: Atkinson].
Computer Code and Chaos
To answer that question, you would need to account for the movements of all the planets, as well as all of the forces being exerted as that movement occurs. Then you'd need to let the solar system run, like a clock, so that the planets cycled through hundreds of thousands of orbits. As this occurred, you would need to track key data about each planet. One of the most important pieces of data to collect would be orbital eccentricity -- the measure of how far a planet deviates from a perfectly circular shape -- because eccentricity determines whether two planets occupy the same airspace and run the risk of having a close encounter.
Think you would be able to run such a simulation in your head or with a desktop model of the solar system? Probably not. A supercomputer can though, which is why Laskar and Gastineau selected the JADE supercomputer to do their heavy lifting. Their inputs consisted of 2,501 orbital scenarios, where each one altered Mercury's orbit by just a few millimeters [source: Laskar and Gastineau]. They chose Mercury because, as the runt of the solar system, it's the biggest pushover and because its orbit synchronizes with Jupiter's to create changes that ripple across the entire solar system.
For each hypothetical scenario, they tracked the motion of all planets for more than 5 billion years (the estimated life span of the sun), letting the computer make all of the complex calculations. Even with the high-powered CPU in the JADE unit, each solution required four months of computing to generate results.
Luckily for life on Earth, the solar system remains stable in 99 percent of the French pair's scenarios -- no planets get set on collision courses or get ejected from their orbits [source: Laskar and Gastineau]. But in 1 percent of them, where the orbital chaos has the greatest cumulative effect, Mercury's orbit becomes eccentric enough to cause catastrophic changes in the solar system. Some of those catastrophes only involve Mercury, which could either crash into the sun or get dislodged from its orbit and flung out into space. But other, more troubling scenarios play out with Earth colliding into either Mars or Venus. A collision with Venus would occur through five steps, all of which illustrate the cumulative effects of orbital chaos [source: Laskar and Gastineau]:
- First, interaction between Jupiter and Mercury in about 3.137 billion years causes the eccentricity of the latter planet to increase. This transfers noncircular angular momentum from the outer planets to the inner planets.
- This transfer destabilizes the inner planets, increasing the eccentricities of Earth, Venus and Mars.
- Earth has a near miss with Mars, which disturbs the eccentricity of Mars even more.
- Subsequent resonances, or synchronized, reinforcing interactions, between the inner planets decrease the eccentricity of Mercury and increase the eccentricities of Venus and Earth even more.
- Venus and Earth have several near misses until, at 3.352891 billion years, the two planets collide in an epic explosion that would destroy both worlds.