Lowell had predicted that Pluto's mass would be 6.6 times that of the Earth, while Pickering believed it to be twice the mass of the Earth. As more observations were made of the diminutive planet it became evident that Pluto had to have a mass much less than Lowell's or Pickering's predictions. This was a dilemma: Pluto's mass could not account for the gravitational disturbances in Uranus's motion, yet those very disturbances had been used to predict Pluto's position. Had Tombaugh really found Lowell's Planet X?
Little was discovered about Pluto over the next forty years. Telescopes improved, but the planet remained a mystery. In 1955, Merle Walker and Robert Hardie at the Lowell Observatory measured Pluto's brightness, which varied regularly, to determine that Pluto's period of rotation was 6.4 Earth days. But what of its size?
In 1950, Gerard Kupier used the 200-inch telescope at Mount Palomar and for the first time was able to see Pluto as a globe. He concluded that Pluto's diameter was less than 5,900 kilometers (the Earth's diameter is 12,756 kilometers). Then, in 1965, Pluto was observed as its orbit passed near a star. If Pluto was large enough, it would cover the star -- an occultation. It missed. This near miss confirmed that Pluto's diameter had to be less than 6,800 kilometers, independently verifying Kupier's measurements. Pluto was even smaller than Mars.
Nothing was known of Pluto's composition until 1976, when astronomers at the Kitt Peak Observatory made the first near-infrared photometric measurements of the planet. The spectroscopic analysis showed Pluto to be covered with frozen methane, which is highly reflective. But with so reflective a surface, Pluto should appear brighter to us, if it was of a certain size. To appear so dim, Pluto had to be smaller still than previously thought.
In 1988 Pluto passed in front of a star, the first true occultation observed since Pluto had been discovered. But rather than winking out suddenly, the star gradually disappeared and reappeared. Pluto had an atmosphere. This was assumed to be a thin layer of methane gas, but observations in 1992 revealed it to contain mostly nitrogen, with trace amounts of methane and carbon monoxide.