|Pluto and its five moons as observed by the Hubble Space Telescope. Charon is roughly half as large as Pluto; the other moons are much smaller. Image credit: NASA|
Pluto was discovered in 1930, during Lowell Observatory's hunt for a planet beyond Neptune. The observatory, founded in 1894 by wealthy Bostonian Percival Lowell to find proof of intelligent life on Mars, had begun its search for a trans-Neptunian planet in 1906.
The search for Planet X (as Lowell dubbed his hypothetical world) was at least partly motivated by the growing disdain with which Lowell's Mars theories were greeted by professional astronomers. Lowell was eager that his observatory should be seen to be credible; discovery of a new planet would, he felt, restore and cement its eroded credibility.
Lowell employed a bevy of young women as "computers" to attempt to determine the position of Planet X based on the motion of the planet Neptune, which did not orbit the Sun precisely as expected. By assuming that Pluto had a mass six times as great as Earth, Lowell and his assistants narrowed the region of the sky where they expected to find Planet X to a portion of the constellation Gemini.
Percival Lowell did not live to see a trans-Neptunian world found (he died in 1916). Following his death, the search for Planet X stalled while his observatory and his widow feuded over the money he had bequeathed to endow Lowell Observatory in perpetuity. The search did not resume in earnest until 1929. When it did, it was meant to survey the entire ecliptic, the invisible line in the sky along which the planets move. The ecliptic corresponds to the plane of the Earth's orbit about the Sun.
On 18 February 1930, 23-year-old Lowell Observatory astronomer Clyde Tombaugh discovered that a tiny dot of light on photographic plates he had made on 23 January and 29 January 1930 had changed position slightly against the background stars. The small movement signified that the object Tombaugh found was moving slowly and thus was far from the Sun. The tiny dot in Gemini, near Lowell's predicted position for Planet X, was subsequently found on plates dating back to before Lowell's death.
Lowell Observatory revealed Tombaugh's find to the world on 18 March 1930, on what would have been Percival Lowell's 75th birthday. It named the object Pluto, for the god of the cold, dark Roman underworld. The observatory staff selected the name in part because its first two letters matched Percival Lowell's initials. Newspapers around the world hailed the discovery of the Solar System's ninth planet.
Pluto was a puzzler, however. An object six times Earth's mass was expected to show a disk when observed using large telescopes, but Pluto did not. Furthermore, the planet had a bizarre tilted orbit that partly overlapped that of Neptune.
As astronomers continued their observations of Pluto, they revised estimates of its size downward. By 1960, some astronomers thought that it was about the size of Earth; others thought it might be as small as Mercury. This only deepened the mystery surrounding the planet, for if it was to account for the observed discrepancies in Neptune's orbit, then it had to be several times as massive as Earth. Some astronomers proposed the existence of another, larger planet beyond Pluto. One scientist proposed a much more novel explanation.
Dr. Robert Forward, a physicist at Hughes Aircraft Company, drew attention to Pluto's unusual characteristics in an article he published in Missiles & Rockets magazine on 2 April 1962. He did not speculate about what those characteristics might mean. That task he handed off to author George Peterson Field.
Field was in fact Forward's pen name. Safely hidden from professional ridicule behind the protective cloak of his nom de plume, the newly minted Ph.D. physicist could freely speculate in a "science fact" article in the December 1962 issue of Galaxy science fiction magazine that Pluto was a gift from a "Galactic Federation."
He began by calculating that a body about the size of Mercury but with six times the mass of Earth would be so dense that it would have to be made of the collapsed matter found only in certain dwarf stars. Such an object could not exist naturally; unrestrained by the massive gravity of a dwarf star, it should have exploded long ago. Therefore, he asserted, Pluto must be artificial.
He suggested that Pluto was in fact a "gravity catapult." He wrote that "it would have to be whirling in space like a gigantic, fat smoke ring, constantly turning from inside out." A spacecraft that approached the ring's center moving in the direction of its spin would be dragged through "under terrific acceleration" and ejected from the other side.
If the acceleration the ultradense smoke ring gave the spacecraft were about 1000 times the acceleration Earth's gravity imparts to objects (that is, 1000 gravities), then the ring would boost the spacecraft to nearly the speed of light in about one minute. The passengers and crew would, however, feel nothing as their spacecraft accelerated, for the gravitational force from the roiling ring would act on every atom of their bodies and their ship uniformly. The ring would slow by a small amount as it accelerated the spacecraft.
He wrote that a "network of these devices in orbit around interesting stars" would provide "an advanced race" with an "energetically economical" means of star travel. The rings in the network would "cartwheel slowly" so that over time they would point at many possible destination stars.
A spacecraft a ring accelerated could, upon arriving at another star in the network, enter that star's ring moving against the ring's roiling motion. This would decelerate the spacecraft very rapidly and increase the ring's rate of motion by a tiny amount. In effect, the spacecraft would pay back the network for the acceleration it borrowed when it began its journey.
He ended his article by noting that such a device could be shot through space by a larger gravity catapult and braked "by pushing against a massive planet," such as Neptune. This, he added, might account for Pluto's odd orbit with respect to the eighth planet. He speculated that, at some time in the past, the Galactic Federation had noted the rise of humans and had launched Pluto toward Sol as "a coming out present."
|Artist's impression of New Horizons at Pluto, 14 July 2015. Charon is visible at upper right. Details on Pluto and Charon are speculative but colors are accurate. Image: NASA|
Pluto, it turned out, has only about one-quarter of 1% of Earth's mass. Subsequently, it was found to have a diameter of only about 2350 kilometers, making it only two-thirds as large as Earth's moon. Shortly after the turn of the 21st century, Pluto was found to have four more moons, all much smaller than Charon.
Though Pluto did not turn out to be a link in a galactic transportation network, it did turn out to be a link to something big. Pluto was the first member body of the Kuiper Belt to be found. The Kuiper Belt, a part of the Solar System long theorized but only confirmed beginning in 1992, is the "third realm" of bodies orbiting the Sun after the Sun-hugging realm of the rocky planets and the realm of the giant planets. It is far bigger than the first two realms combined.
As New Horizons closes in on Pluto, we know of more than 1000 bodies in trans-Neptunian space. Astronomers estimate that more than 100 times that number might exist. Assuming that New Horizons continues to operate as planned, mission planners expect to direct it past several more Kuiper Belt Objects after the Pluto flyby.
In 2006, representatives of the International Astronomical Union (IAU) voted to adopt a new definition of "planet" and create a new class of Solar System body: the "dwarf planet." Pluto and the largest Main Belt asteroid, Ceres, became dwarf planets. Many saw this as a demotion for Pluto and a promotion for Ceres. The move was controversial, not least because the IAU definitions are ambiguous. It seems likely that at some point they will undergo revision.
If Pluto is so small that it cannot account for the discrepancies in Neptune's orbit, then what does? In August 1989, the Voyager 2 spacecraft flew past Neptune. By carefully tracking the robot spacecraft, celestial dynamicists refined their estimate of Neptune's mass. When they did, the observed discrepancies in its orbital motion vanished. There was thus never a need to find a Planet X. Error had led to coincidence, and the result was early knowledge of mysterious Pluto.
"Pluto - the Gateway to the Stars," Robert L. Forward. Missiles and Rockets, 2 April 1962, pp. 26-28
"Pluto, Doorway to the Stars," George Peterson Field, Galaxy Magazine, December 1962, pp. 78-82
Galileo-style Uranus Tour (2003)