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fragments of different sizes collided with one another as they swirled around the sun. The gravitational pull of the sun meant that heavier material would orbit closer to it, while lighter particles and gas orbited farther away. To some extent, this state of affairs remains in effect today, with the solar system composed ofrocky innerplanets,Mercury,Venus, Earth, andMars, and gaseous outer ones,Jupiter,Saturn,Uranus, andNeptune.
    Whether the object of a search is Easter eggs, fossil bones, or a new kind of solar system, one discovery typically leads to the next. What once was rare turns up everywhere, often right under our noses. The years since the recognition of thedust surrounding Beta Pictoris have witnessed the launch of new satellites, the construction of ever-bigger telescopes, and the use of powerfulcomputers to crunch all the data returning to Earth. This technology has changed our view of the heavens. Far from being a lonely solar system, ours is only one of many in thegalaxy. The sky is filled with other worlds at different stages of their development surrounded by planets of almost every description.
    Powerful technology and great ideas have transformed our notions of the heavens. But do not discount the impact of pure luck.
    In the wee hours of the morning on February 8,1969, a massive fireball woke residents of the Mexican state of Chihuahua. A visitor from space had arrived: a large meteorite that broke apart in the atmosphere. After learning of the event, scientists and collectors poured into the area in droves. Given the size of the boom, the collectors had expected a bonanza, but they had no idea of the extent until they looked carefully inside the rock. Tiny white patches interrupted the dull gray body of the rock itself. Meteorites with these specks were known before, but they were incredibly rare. Laboratory work on the few other meteorites with inclusions like these revealed grains that hint at the chemical signature of primordialrocks of the solar system.
    The meteorite exploded into fragments that spread over about twenty-five square miles of desert. Two to three tons of fragments have been collected in the years since the impact. Even today, more than forty years later, pieces are occasionally found.
    The impact could not have occurred at a more opportune time. In 1969,ProjectApollo was in high gear. With
Apollo 8
having circled themoon just two months before the meteor strike and another as-yet-undeterminedApollo mission set to land on it, labs across the country were gearing up to investigate the chemistry ofmoon rocks. Now, at no expense to the taxpayer, special rocks from space had arrived right on our doorstep. Not only that, but the meteor was so huge prior to breaking up that there were a large number of fragments to share among the different chemistry laboratories capable of making sense of them.
    Scientists performed the routine analysis of the atoms inside the rocks. Some of the mineral grains are so similar to those of Earth rocks that they point to a shared history of the bodies of our solar system, just asSwedenborg,Kant, andLaplace predicted. Other minerals can bedated using thedecay of the atoms inside as a kind of clock. When a mineral forms, the atoms come together as a crystal structure. Once born as a crystal, some of the atoms, such as uranium and lead, change at a regular pace as defined by the laws of physics and chemistry. If you know the relative abundances of the different forms of the atom inside the mineral, and the rates at which they convert to one another, then you can calculate the time since the mineral formed (see Further Reading and Notes for more details).Uranium 238 converts intolead 206 very slowly; it takes 4.47 billion years for half the original amount to decay in this way. This slow rate of atomic change makes uranium and lead ideal atoms to measure theage of veryancient crystals. The uranium and lead concentrations of the Mexican meteorite point to an age for