Far-Out Moon

And maybe a new planet

It revolves around the sun at a mean distance of 5.9 billion km (3.7 billion miles) once every 248 years. Even in powerful telescopes, it is visible only as a fleck of light. Pluto, the solar system's ninth planet, was not discovered until 1930, and little is yet known about it. Now astronomers have learned surprising new things about the faroff planet: it appears to have a moon, seems to be much smaller than previously estimated and may some day be stripped of its title as the outermost member of the sun's family of planets.

While Astronomer James Christy was examining photographic plates of Pluto last June--taken with the U.S. Naval Observatory's 155-cm (61-in.) reflecting telescope at Flagstaff, Ariz.--he noticed an elongation in Pluto's image. Checking back on photographs made in 1965 and 1970, Christy found similar stretching, always in a north-south direction relative to the earth. After further measurements, Christy and his colleague, Dr. Robert Harrington, concluded that what they were seeing was actually a moon hi a 19,300-km (12,000-mile-high) orbit around Pluto. The great distance from the earth had prevented astronomers from resolving the planet and its nearby moon into two separate specks of light.

Harrington reckons that the moon (which Christy has tentatively named Charon after the mythological boatman who ferried the souls of the dead across the River Styx to the underworld ruled over by the god Pluto) is about 800 km (500 miles) in diameter. It lies in Pluto's equatorial plane and circles the planet once every 6 days 9 hr. and 17 min.--an interval identical to Pluto's own period of rotation. Hence, an observer on one side of Pluto would always see the moon in the same position in the sky. On the other side, it would never be visible.

Until the mid-'60s, Pluto was thought to be a planet with roughly earthlike dimensions. Not so, say the Naval Observatory astronomers. Using the presence of the moon and knowledge of its orbital characteristics, they have calculated that Pluto's diameter is about one-fifth that of the earth's, its density perhaps less than one-third and, most significant, its mass only .2%. This means that their shrunken Pluto may not have enough gravitational pull to account for suspected irregularities, previously attributed to it, in the orbits of Uranus and Neptune, the seventh and eighth planets from the sun. Then what could be disturbing the two larger planets? Perhaps, suggests Harrington, it is "a new massive object, possibly even a new planet."

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