The path of true love never runs smooth, it is said. Especially on Valentine's Day. And for young planets, that path turns out to be an inward-moving annulus. A simulation by scientists in France and USA appears to show that Jupiter once strayed to flirt with the inner Solar System, before being "jilted" and sent back to its present-day position. The effect of this was to form the inner planets, according to the theory, which comes up with mass ratios for Earth and Mars similar to that observed today and which, remarkably, also accurately depicts the Asteroid Belt. If the...