Is the solar system unique? Or are there other planets in orbit around other stars in the Milky Way and other galaxies? Many scientists believe that stars with planetary systems are more the rule than the exception in the universe, but they have yet to prove it. Inter stellar distances are so great that the most powerful telescopes on earth are hopelessly inadequate for sighting small, dark planets that might be in orbit around other stars.
Undaunted by so formidable an obstacle, Astronomer Peter van de Kamp, the director of the Sproul Observatory at Swarthmore (Pa.) College, set out 31 years ago to search for dark companions of nearby stars. His long effort has been well rewarded. Last week the Dutch-born, 67-year-old astronomer an nounced the first "solid evidence" that there is a system of planets other than the solar system. He has detected two planets circling Barnard's star, some 35 trillion miles away from the earth, in the constellation Ophiuchus.
Stubborn Search. Van de Kamp and his assistants found the Barnard plan ets by using a classical astronomical technique: searching for irregularities in the path of a celestial body, a wobble that might be caused by the gravitational pull of a dark, unseen companion. As early as 1844, for example, astronomers concluded from wobbles in the path of Sirius that the bright star was accompanied through space by a star too faint to be seen from earth. The same technique has been used to establish that several other apparently single stars are actually members of a binary sys tem; they have stellar companions that are invisible from the earth.
In 1938, when Van de Kamp started a concentrated search for these unseen companions, he and his assistants began to photograph at regular intervals some 40 of the stars closest to the earth, plotting their paths and looking for wobbles. They devoted most of their attention to Barnard's star because it is the closest star visible in the Northern Hemisphere and moves across the sky ; rapidly in relation to the distant "fixed" stars, making it relatively easy for astronomers to trace its path. "We concentrated and gambled on one object," i says Van de Kamp. "It was one of those crazy, stubborn, all-out efforts that paid off."
Stubborn indeed. It was not until 1956 and thousands of photographic plates later that Van de Kamp was able to distinguish a significant disturbance in the path of Barnard's star. And it was not until 1963 that he had analyzed his results carefully enough to announce that a planet-sized object rather than a dim star was orbiting Barnard. "I wanted to tread slowly," he explains. "The Zeitgeistthe spirit of the timehad to be just right."
Circular Orbits. One characteristic of the unseen Barnard planet disturbed Van de Kamp: its orbit seemed too elliptical in comparison with the nearly circular orbits of most solar-system planets. Patiently continuing his monitoring of the star, he exposed more photographic plates, refined his data, and early this year came to the conclusion that Barnard's wobbles are caused not by a single planet in an unusual orbit but by two planets in nearly circular orbits.