PLUTO'S LANDSCAPE cannot be seen clearly from Earth, but astronomers' best guesses about the planet are illustrated in this artist's rendering. Pluto's surface is believed to be composed of nitrogen, carbon monoxide, methane and water ices. Some of this material apparently sublimates into a rarefied atmosphere when the planet is in the part of its orbit that is closest to the sun. Geysers may occasionally erupt, shooting plumes of gas and ice many kilometers above the surface. Pluto's moon, Charon, looms overhead. Image: EDWIN FAUGHN
Until about 10 years ago, most planetary scientists considered Pluto to be merely an oddity. All the other planets neatly fit into what astronomers knew about the architecture of the solar system--four small, rocky bodies in the inner orbits and four gas giants in the outer orbits, with an asteroid belt in between. But distant Pluto was an icy enigma traveling in a peculiar orbit beyond Neptune. Some researchers, most notably Dutch-American astronomer Gerard Kuiper, had suggested in the 1940s and 1950s that perhaps Pluto was not a world without context but the brightest of a vast ensemble of objects orbiting in the same region. This concept, which came to be known as the Kuiper Belt, rattled around in the scientific literature for decades. But repeated searches for this myriad population of frosty worlds came up empty-handed.
In the late 1980s, however, scientists determined that something like the Kuiper Belt was needed to explain why many short-period comets orbit so close to the plane of the solar system. This circumstantial evidence for a distant belt of bodies orbiting in the same region as Pluto drove observers back to their telescopes in search of faint, undiscovered objects beyond Neptune. By the 1980s telescopes were being equipped with electronic light detectors that made searches far more sensitive than work done previously with photographic plates. As a result, success would come their way.
This article was originally published with the title Journey to the Farthest Planet.