The space program changed the planets as a concept. Before that, the planets were astronomical entities, little better known than when the ancients saw them as gods in the heavens. Today all the planets are viewed as distinct worlds, having been visited by spacecraft—and Pluto (and its oversize moon Charon) will get its turn in the limelight in 2015. (Unfortunately Pluto is no longer considered a true planet.)
But within living memory, scientists used to think the most fantastical things about the planets. In the 1960s, popular guides still showed Mercury and Mars crisscrossed with canals, the lines that Giovanni Schiaparelli, squinting through the best telescopes of 1877, had mapped and named canali (channels). (A splendid book about those times is online.)
Today only Pluto remains mysterious in that antique sense. Pay attention, because in less than a decade a spacecraft will bring us clear pictures of Pluto, and with the ancient overlord of the underworld revealed as yet another pockmarked iceball, that former dreamlike sense of the universe will utterly disappear—except in astrology.
Scientists had only the crudest ideas about the structure of the other planets, and essentially none about their history, until 14 July 1965. On that day the Mariner 4 spacecraft sent back the first close-ups of Mars—covered with craters like the Moon's.
Instantly the "canals" were shown to be optical illusions; instantly scientists had something to chew on; instantly Mars, and by extension the other planets, became a fit object for study by geoscientists, and a new Earth science, planetology, could arise. Even as a child, seeing that first photo in the newspaper, I appreciated what had happened.
More flybys were launched. New data let us ask questions of other planets that we could compare with Earth. Landers landed. Actual rocks began to be retrieved, or analyzed at the site. Every few years a new wave of spacecraft raised new vistas. In 1968 the American Geophysical Union created its Planetology Section, led by the late Gene Shoemaker. The journals began to fill with breakthrough papers, putting the strange landforms of Venus on the same footing as the earthly Tibetan plateau, the geology of Mars, the basins of Mercury, the methane lakes of Titan and the volcanoes of Io.
To see where we are today, just visit Views of the Solar System, the best one-stop treatment of our planetary neighborhood on the Web. Compared to the situation a generation ago, it is like a modern globe put next to a medieval T-O map. Scientists can now talk confidently of the interiors and histories of the planets, even the solar system itself, in ever-growing detail.
Indeed, planetology now encompasses planets around other stars, and inexorably we are finding ever-smaller and more Earthlike examples. The Extrasolar Planets Encyclopedia is keeping up to date on that story. Now we know that planets are everywhere—not special at all, more like aphids in the cosmic vegetation. And now Earth science extends to the whole universe.