“The outermost planet of the solar system has a mass 10 times smaller than hitherto supposed, according to measurements made by Gerard P. Kuiper of Yerkes Observatory, using the 200-inch telescope on Palomar Mountain. On the basis of deviations in the path of the planet Neptune, supposedly caused by Pluto’s gravitational attraction, it used to be estimated that Pluto’s mass was approximately that of the earth. Kuiper was the first human being to see the planet as anything more than a pinpoint of light. He calculated that Pluto’s diameter is 3,600 miles, and its mass is one tenth of the earth’s. It leaves unsolved the mystery of Neptune’s perturbations, which are too great to be accounted for by so small a planet as Pluto.”

Scientific American, July 1950

More gems from Scientific American’s first 175 years can be found on our anniversary archive page.