Tomorrow morning, less than 100 hours will remain before the first machine fashioned by human hands completes the initial reconnaissance of the last of the Solar System’s “traditional” nine planets. NASA’s New Horizons spacecraft—launched 9.5 years ago, way back in January 2006—is approaching a 14 July rendezvous with the dwarf world Pluto, its binary companion Charon and a system of four tiny moons: Nix, Hydra, Kerberos and Styx. This entire group are named in honor of key figures and locations associated with the ancient Greek and Roman underworld and this nomenclature is entirely fitting, for they reside in one of the darkest, coldest and gloomiest parts of the Sun’s realm. However, this region has drawn new light in the past couple of decades, following the discovery of the long-hypothesized “Kuiper Belt” of objects beyond Neptune, and as discussed in yesterday’s AmericaSpace Pluto history article this has thrown the nature of Pluto as a “classical” planet into question. As the mission enters its final days before Closest Approach, AmericaSpace’s New Horizons Tracker and a series of articles by Mike Killian, Leonidas Papadopoulos, Paul Scott Anderson and myself will cover the discovery and exploration of Pluto and the unfolding developments as the spacecraft seeks to make this unknown world known.
Within weeks of its February 1930 discovery by U.S. astronomer Clyde Tombaugh, the first suspicions surfaced that Pluto was not quite what it seemed. Speculation abounded in the 1930s and 1940s that the new world might be a long-period comet or possibly a displaced asteroid, with the first suggestion of an entirely new breed of “ultra-Neptunian bodies” and a growing recognition that Pluto was likely too small to be properly classified as a major planet. At first, it was generally believed that Pluto was of equivalent equatorial diameter as our Home Planet, but telescopic observations over the following decades steadily progressed this estimated size and mass in a downward direction. In 1949, research by the U.S. astronomer Gerard Kuiper revealed a size approximately midway between Mercury and Mars, with a mass about a tenth of Earth, whilst spectroscopic observations of highly reflective methane on the dwarf world’s surface and the discovery of Charon in the 1970s, together with their known distance from the Sun, pegged Pluto at about 0.01 Earth-masses. This rendered it as much as five times smaller than our Moon and, more recently, 2006 observations by the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) have yielded an approximation of just 0.00218 Earth-masses.
As described yesterday, the discovery of other objects in the outer Solar System, including Chiron—the first of the “centaurs”, which populate a region between the orbits of Saturn and Uranus—gradually opened astronomers’ eyes and minds to the realization that the classical term of “planet” needed better clarification and perhaps even a redefinition. In conjunction with these discoveries, the idea of a “reservoir” of icy and rocky material, beyond Neptune, had steadily grown, with great speculation that a “belt” residing between 38 Astronomical Units (3.5 billion miles or 5.7 billion km) and 50 AU (4.6 billion miles or 7.5 billion km) from the Sun could provide a source for many of the Solar System’s “short-period” comets, with orbits of less than 200 years.
Towards the end of the 1980s, the term “Kuiper Belt” was coined and in August 1992 U.S. astronomers David Jewitt and Jane Luu reported the discovery of the first Kuiper Belt Object (KBO), a reddish-hued world, residing somewhere between 37 AU (3.4 billion miles or 5.5 billion km) and 59 AU (5.5 billion miles or 8.8 billion km) from Earth. Its mean distance was subsequently pegged at 44 AU (4.1 billion miles or 6.6 billion km) and it requires approximately 292 years to circle the Sun. Known as “(15760) 1992 QB1”, it became the first trans-Neptunian object to be identified, after Pluto and Charon, and its discovery literally opened the floodgates, with another KBO, designated “(181708) 1993 FW”, reported by Jewitt and Luu in March 1993. By the end of the decade, more than 60 KBOs had been found, with around ten new objects identified each year, but it was the early part of the present century which saw the most profound discoveries and carried the most profound consequences for Pluto itself.
In June 2002, a KBO about half the size of Pluto was discovered by U.S. astronomers Chad Trujillo and Michael Brown. Named “Quaoar”, in honor of an ancient Tongva creator-deity, it was observed at a magnitude of about 18.5 and most recent estimates give it an equatorial diameter of about 670 miles (1,070 km). It lies about 43.3 AU (4.02 billion miles or 6.5 billion miles) from the Sun, with an orbital period of 284.5 years. At the time of its discovery, these dimensions made it the largest object found in the Solar System since Pluto, although that would not last. Signs of water-ice and small quantities of methane—together with the tantalizing possibility of ancient cryovolcanism—have also been found and, in February 2007, a tiny moon (and probably a collisional fragment), named Weywot, after Quaoar’s son in Tongva mythology, was detected.
Less than 18 months after the discovery of Quaoar, in November 2003, the team of Trujillo, Brown and fellow astronomer David Rabinowitz identified the KBO Sedna, a distinctly reddish object, whose orbital characteristics were later determined to carry it as far as 937 AU (87 billion miles or 140 billion km) and as close as 76 AU (7.1 billion miles or 11.4 billion km) from the Sun. It requires an estimates 11,400 years to circle its parent star. Its nature as what Brown described as “the coldest, most distant place known in the Solar System” made the name Sedna, in homage to the Inuit goddess of the sea, thought to live at the bottom of the frigid Arctic Ocean, an appropriate one. At the time of its discovery, Sedna was about 89.6 AU (8.3 billion miles of 113 billion km) from the Sun, approaching perihelion, which it will eventually attain in mid-2076. Estimates of Sedna’s diameter from NASA’s Spitzer Space Telescope and the European Space Agency’s (ESA) Herschel Space Observatory have reached a ballpark approximation of around 995 miles (1,600 km) and spectroscopic analyses have identified the presence of water, methane and nitrogen ices—together with possible hydrocarbon “sludges”, known as tholins—on a surface which is described as one of the reddest in the Solar System, almost as red as Mars.
These discoveries provided exciting evidence that the “ragged edge” of the Sun’s realm was intensively populated with potentially hundreds of worlds, many of which approached the dimensions of Pluto itself. Then, in January 2005, a team at the Palomar Observatory’s Samuel Oschin telescope, headed by Brown, discovered Eris. Today known as the most massive known dwarf planet in the Solar System, Eris resides 96.4 AU (8.96 billion miles or 114 billion km) from the Sun. Its diameter is believed to be about 1,445 miles (2,326 km) and it is about 27 percent more massive than Pluto. This led it to be described as “the tenth planet” in some areas of the media, but the discovery, size and mass of Eris brought to a head the need for a redefinition of what a “planet” actually was. It was clear that, as more objects beyond Neptune were found at an increasingly high rate, the Solar System would consist not of nine planets, but of thousands, and a highly disputed and bitterly divisive debate began which would ultimately reclassify Pluto, not as a fully fledged planet, but instead as the largest-known member of the Kuiper Belt.
Other discoveries during this period included the dwarf worlds Haumea and Makemake, both of which are somewhat smaller and less massive than Pluto, but both of which appear to share commonalities in terms of their surface constituents. However, the debate which precipitated the controversial International Astronomical Union (IAU) decision in 2006 to demote Pluto to a dwarf planet centered upon the size and mass of Eris itself. Ironically, Eris is named in honor of the ancient Greek goddess of chaos, strife and discord. This proved highly appropriate, for Eris’ existence indeed caused chaos in our understanding of the Solar System, great strife among astronomers and significant discord among the general public, whose echoes continue to resonate to this day.
The decision, it seemed, was stark. Either they all had to be classified as planets, or an entirely new nomenclature had to be developed.
During the course of 2005, a committee chaired by British astronomer Iwan Williams, serving as the IAU’s president of planetary systems science, proposed three possible definitions for a planet. Firstly, and most simply, was the cultural definition, that a “planet” is a “planet” if enough people say it is a “planet”. Secondly, from a structural standpoint, an object was a planet if it was sufficiently large to form a sphere, and thirdly, from a dynamical standpoint, if it was sufficiently massive to “clear its orbital neighborhood” of debris. In August 2006, the IAU announced an initial proposal which described a planet as a celestial body with (a) sufficient mass for its self-gravtiy to overcome rigid body forces so that it assumes a hydrostatic equilibrium (near-round) shape and (b) was in orbit around a star and was neither a star itself, nor the satellite of a planet. This proposal would have immediately increased the Solar System’s population to 12 planets, with Charon—which does not “orbit” Pluto, but is actually its smaller “binary” companion—as well as Eris and the dwarf world Ceres, located between Mars and Jupiter, added to the list.
For the time being, Pluto remained a planet.
However, the neat first proposal created its own problems, with other astronomers arguing that dozens of other objects could also “probably” fit the definition. “The analogy that I always like to use is the word continent,” said Mike Brown in an August 2006 interview. “You know, the word continent has no scientific definition…they’re just cultural definitions, and I think the geologists are wise to leave that one alone and not try to redefine things so that the word continent has a big, strict definition.”
Opponents to the first proposal offered another possibility. Headed by the Uruguayan astronomer Julio Ángel Fernández, they argued that a planet was (a) by far the largest object in its local population, (b) carried sufficient mass for its self-gravity to overcome rigid body forces to that it assumed a hydrostatic equilibrium (nearly-round) shape and (c) did not produce energy by any mechanism of nuclear fusion. Under this proposal, the eight “classical planets”—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune—were the only planets in the Solar System, with Ceres, Pluto “and several other large trans-Neptunian objects” redefined as “dwarf planets”.
On 22 August, the draft proposal was extensively reworked, and, two days later, after “lively” vocal debate, the main sticking point appeared to center upon whether an object’s orbital characteristics should form part of the definition criteria. At length, on 24 August, the IAU issued its final draft definition. A “planet” was henceforth defined as “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly-round) shape and (c) has cleared the neighborhood around its orbit”. At the same time, “dwarf planets”—a group into which Pluto would eventually be slotted—was “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly-round) shape, (c) has not cleared the neighborhood around its orbit and (d) is not a satellite”. It was further resolved by the IAU that Pluto was recognized as a dwarf planet and as the prototype of a new category of Trans-Neptunian Objects (TNOs). Two years later, in June 2008, the IAU formally approved the name “plutoid” to describe dwarf planets beyond Neptune, with Pluto and Eris identified as the initial members of this category. Since then, Haumea and Makemake have been added to the list, with potentially many more in the coming years.
The response from around the world to Pluto’s demotion—to “dwarf planet”, to “Trans-Neptunian Object”, to “plutoid”—was dramatic. Most visibly, New Horizons Principal Investigator (PI), Alan Stern, described it as “an awful definition” and representative of “sloppy science”. He likened the decision to defining “people” as not being “people”, for a purely arbitrary reason, such as because they happened to live in groups, but more significantly noted that Earth, Mars, Jupiter and Neptune have failed to fully clear their neighborhoods, pointing to the existence of 10,000 near-Earth asteroids and around 100,000 Trojans in the Jovian orbital path. “If Neptune had cleared its zone,” he said, “Pluto wouldn’t be there.”
Around the world, car bumper sticks compelled motorists to “Honk if Pluto is still a planet” and the IAU was disparagingly redefined as the “Irrelevant Astronomical Union”. Educational textbooks, still on the presses, were placed on hold, pending the decision, and the verb to pluto—meaning to demote or devalue someone or something—was voted “Word of the Year” in 2006 by the American Dialect Society. Concurrently, the American Name Society recognized Pluto as “Name of the Year”, with ANS President Cleveland Evans noting that “our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name. We may no longer believe in the Roman god Pluto, but we still have a sense of personal connection with the former planet”.
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Although I am in the “Pluto is a planet” camp, the New Horizons mission and those that follow will fundamentally change our understanding of the Pluto/Kuiper Belt region of the solar system. Let’s remember that new discoveries lead to new thinking and understanding.
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