Why Pluto Is No Longer Called a Planet
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Pluto was a planet for 76 years and then, on 24 August 2006, it was not. Nothing about Pluto changed that day. What changed was that astronomers had finally written down a definition of the word planet, and the definition had been forced on them by discoveries that made the old, unwritten one impossible to keep.
A planet found by accident
Pluto was discovered in 1930 by Clyde Tombaugh, a 24-year-old assistant at the Lowell Observatory in Arizona, comparing photographic plates for a faint dot that moved. The observatory had been searching for a large ninth planet whose gravity was thought to be disturbing the orbits of Uranus and Neptune. Pluto turned out to be far too small to do that, and the orbital discrepancies were later traced to a measurement error, so the planet that prompted the search never existed. Pluto was found anyway.
For decades nobody could measure it well. Estimates of its size shrank steadily from Earth-sized to something smaller than our Moon. The discovery of its large moon Charon in 1978 finally allowed its mass to be calculated: about 0.2 percent of Earth's. Pluto was odd in other ways too. Its orbit is tilted 17 degrees to the plane of the other planets and so elongated that it spends twenty years of every 248-year orbit closer to the Sun than Neptune.
The neighbourhood fills up
The real trouble began in 1992, when the first object beyond Neptune other than Pluto was found. Over the following decade hundreds more turned up in what is now called the Kuiper Belt, a ring of icy bodies left over from the formation of the solar system. Pluto began to look less like the last planet and more like the largest known member of a crowd.
In 2005 Mike Brown's team at Caltech announced Eris, an object in the same region that was more massive than Pluto. If Pluto was a planet, Eris had to be one too, and so, probably, would several more objects waiting to be found. Astronomers faced a choice between a solar system with an open-ended and growing list of planets and a definition that drew a line somewhere.
The 2006 definition
At its meeting in Prague, the International Astronomical Union voted that a planet must satisfy three conditions. It orbits the Sun. It has enough mass for its own gravity to pull it into a round shape. And it has cleared the neighbourhood around its orbit, meaning it is gravitationally dominant in its region and has swept up, flung away or captured the smaller bodies near its path.
Pluto passes the first two tests and fails the third. It shares its region with thousands of other Kuiper Belt objects and has only a tiny fraction of the total mass in its orbital zone, whereas Earth outweighs everything else near its orbit by more than a million times. The IAU created a new category, dwarf planet, for round bodies that have not cleared their orbits. Pluto joined it alongside Eris, Ceres in the asteroid belt, and later Haumea and Makemake. The eight remaining planets are the ones that pass all three tests:
- •Orbit the Sun directly, not another body
- •Massive enough to be rounded by their own gravity
- •Gravitationally dominant in their orbital zone
What the flyby showed
Ironically, the mission that revealed Pluto as a fascinating world was already on its way when the vote happened. New Horizons launched in January 2006 and flew past Pluto in July 2015. It found a heart-shaped plain of frozen nitrogen, Sputnik Planitia, whose surface is renewed by slow convection and carries almost no craters; mountains of water ice as hard as rock at those temperatures; a thin blue-hazed atmosphere; and evidence of a possible ocean beneath the crust. It also measured Pluto's diameter at 2,377 kilometres, slightly larger than Eris after all, though Eris remains heavier.
None of that bears on the classification, and that is the point supporters of the definition make. Dwarf planet describes Pluto's place in the solar system's architecture, not its interest. Some planetary scientists, including the New Horizons lead Alan Stern, argue the definition is poorly worded and that any round world should count; under that geophysical definition the solar system would have well over a hundred planets, including several moons. The disagreement is really about what a category is for.
The takeaway
Pluto lost its title because astronomers found many objects like it and had to decide where a planet ends and a belt begins. The 2006 definition requires a planet to have cleared its orbit, Pluto shares its with thousands of others, and it now heads the list of dwarf planets, which New Horizons showed to be far more active worlds than anyone expected.