Yet Newton himself had doubts about that. He appreciated that although the Sun dominates its gravitational environment, the planets exert small but persistent gravitational forces on one another. He suspected that these interactions might sooner or later upset the solar system’s clockwork predictability, but he was unable to calculate their effects. “To define these motions by exact laws admitting of easy calculation exceeds, if I am not mistaken, the force of any human mind,” he wrote.
He was right. It took the power of modern computation to reveal that all planetary systems, even those as seemingly serene as the Sun’s, are infected by potential chaos. Computer simulations indicate, for instance, that Jupiter’s gravity has repeatedly altered the polar axis of Mars and may one day pull Mercury into an orbit so elliptical that it might collide with Venus or Earth. (Even a near miss between Mercury and Earth would generate enough tidal friction to transform both planets into balls of lava.) Troubled by his intimations of chaos, Newton wondered aloud whether God might have had to intervene from time to time to keep the solar system running so smoothly. Today it might be said that only an act of God could save strict determinism.
Bidding good night to planet-bearing stars that increasingly looked like ports of call, I closed up the observatory and paused to scratch a few numbers on a scrap of paper. NASA estimates that the Milky Way galaxy contains at least 100 billion planets, not counting the lonely free-floaters. If the “instrumental and technical improvements” I read about back in 1959 eventually attain such a state of excellence that astronomers are finding new planets every minute, day and night, they would be at it for 100,000 years before they’d mapped half the planets in our galaxy. And ours is one among more than 100 billion galaxies.
In short, we stand at the onset of a great age of adventure—and always shall, so long as we keep doing science.