• U.S.

Science: The Star-Planet

4 minute read
TIME

Astronomers have long believed that the universe is teeming with planetary systems, some of which may contain worlds inhabited by intelligent life. Yet they have been hard-pressed to prove their case. Interstellar distances are so vast that even the most powerful telescopes on earth could not spot a planet orbiting the sun’s nearest stellar neighbor, Proxima Centauri, which is a relatively scant 4.3 light-years (or about 26 trillion miles) away.

That statistic did not deter Astronomer Peter van de Kamp of Swarthmore (Pa.) College’s Sproul Observatory. In the late 1960s, after years of patient observation, he provided what seems to be the first evidence of planets beyond the solar system: two large Saturn-size bodies circling Barnard’s Star, which is 5.9 light-years from earth. Now van de Kamp has announced a discovery that may be still more significant. At a meeting of the American Astronomical Society in Las Cruces, N. Mex., he reported finding another unseen body orbiting the star Epsilon Eridani, 10.7 light-years away. The mysterious object does not fit any conventional category. Too large to be considered an ordinary planet, it is also too small to be regarded as a true star.

Tiny Wobbles. How could van de Kamp find-let alone describe-objects so distant that they cannot be seen through optical telescopes or detected by radiotelescopes? He owes his success to a branch of astronomy called astrometry, which includes the measuring of tiny perturbations, or wobbles, in the paths of some stars as they move almost imperceptibly against the background of much more distant “fixed” stars. Astronomers are convinced that those periodic disturbances in what should be a smooth line of motion as the stars wheel about the center of the Milky Way Galaxy can mean only one thing: that the stars are being tugged by the gravitational attraction of planets or small companion stars orbiting around them.

Van de Kamp began his search in 1937. He used Sprout’s 24-in. refracting telescope to photograph at regular intervals the several hundred stars in the sun’s immediate neighborhood in hopes of detecting any odd movements in their paths. In addition to his interest in Barnard’s Star, he was particularly intrigued by Epsilon Eridani. Though most nearby stars are small, relatively faint “red dwarfs,” Epsilon Eridani is a bright yellow-orange star somewhat like the sun with about seven-tenths of its mass and 30% of its luminosity. Thus, if there were any planets in orbit around Epsilon Eridani, at least one might be at the right distance from the parent star to receive enough light and heat to sustain the evolution of life. In fact, the similarities between Epsilon Eridani and the sun prompted radio astronomers in 1960 to aim their big antennas at the star in a brief-and unsuccessful -effort to detect radio emissions that might be emanating from a civilization on a planet in orbit around it.

Van de Kamp had better luck. By 1967, he noticed that Epsilon Eridani was not following “the straight and narrow path of a true single star.” But its wobbles seemed too small to be the resuit of the large gravitational pull of a dark companion star. Cautiously, the astronomer and his associates waited until they had accumulated more than 800 photographic plates of Epsilon Eridani before they felt ready to measure the perturbations and calculate what might be causing them. Now they believe that an unseen object is orbiting Epsilon Eridani at a distance of about eight times the distance between the earth and sun -probably too far off to receive enough heat to support earthlike life. One complete revolution of the object around the central star takes 25 years. Most astonishing of all, the mysterious body is at least six times as massive as the giant planet Jupiter.

Even so, van de Kamp explains, the mass of Epsilon Eridani’s companion is less than 1% of the sun’s; thus the body is probably incapable of sustaining the nuclear reactions that fire all stars. “What we have is not a star in the ordinary sense, nor is it a conventional planet,” he says. “Perhaps, we should call it a ‘star-planet.’ Certainly, it is something that challenges theorists for an explanation.”

More Must-Reads from TIME

Contact us at letters@time.com