Cosmos - Carl Sagan [115]
Other methods of detecting planets around the stars are under development, including one where the obscuring light from the star is artificially occulted—with a disk in front of a space telescope, or by using the dark edge of the Moon as such a disk—and the reflected light from the planet, no longer hidden by the brightness of the nearby star, emerges. In the next few decades we should have definite answers to which of the hundred nearest stars have large planetary companions.
In recent years, infrared observations have revealed a number of likely preplanetary disk-shaped clouds of gas and dust around some of the nearby stars. Meanwhile, some provocative theoretical studies have suggested that planetary systems are a galactic commonplace. A set of computer investigations has examined the evolution of a flat, condensing disk of gas and dust of the sort that is thought to lead to stars and planets. Small lumps of matter—the first condensations in the disk—are injected at random times into the cloud. The lumps accrete dust particles as they move. When they become sizable, they also gravitationally attract gas, mainly hydrogen, in the cloud. When two moving lumps collide, the computer program makes them stick. The process continues until all the gas and dust has been in this way used up. The results depend on the initial conditions, particularly on the distribution of gas and dust density with distance from the center of the cloud. But for a range of plausible initial conditions, planetary systems—about ten planets, terrestrials close to the star, Jovians on the exterior—recognizably like ours are generated. Under other circumstances, there are no planets—just a smattering of asteroids; or there may be Jovian planets near the star; or a Jovian planet may accrete so much gas and dust as to become a star, the origin of a binary star system. It is still too early to be sure, but it seems that a splendid variety of planetary systems is to be found throughout the Galaxy, and with high frequency—all stars must come, we think, from such clouds of gas and dust. There may be a hundred billion planetary systems in the Galaxy awaiting exploration.
Not one of those worlds will be identical to Earth. A few will be hospitable; most will appear hostile. Many will be achingly beautiful. In some worlds there will be many suns in the daytime sky, many moons in the heavens at night, or great particle ring systems soaring from horizon to horizon. Some moons will be so close that their planet will loom high in the heavens, covering half the sky. And some worlds will look out onto a vast gaseous nebula, the remains of an ordinary star that once was and is no longer. In all those skies, rich in distant and exotic constellations, there will be a faint yellow star—perhaps barely seen by the naked eye, perhaps visible only through the telescope—the home star of the fleet of interstellar transports exploring this tiny region of the great Milky Way Galaxy.
The themes of space and time are, as we have seen, intertwined. Worlds and stars, like people, are born, live and die. The lifetime of a human being is measured in decades; the lifetime of the Sun is a hundred million times longer. Compared to a star, we are like mayflies, fleeting ephemeral creatures who live out their whole lives in the course of a single day. From the point of view of a mayfly, human beings are stolid, boring, almost entirely immovable, offering hardly a hint that they ever do anything. From the point of view of a star, a human being is a tiny flash, one of billions of brief lives flickering tenuously on the surface of a strangely cold, anomalously solid, exotically remote sphere of silicate and iron.
In all those other worlds in space there are events in progress, occurrences that will determine their futures. And on our small planet, this moment in history is a historical branch point as profound as the confrontation of the Ionian scientists with the mystics 2,500 years ago. What we do with our