The ongoing argument over whether Pluto is an actual planet or just a dwarf on the outskirts of the Solar System has heated up again – with a new proposal to reapply planetary status to the distant iceball.
In a vote in 2006, the International Astronomical Union (IAU) decided to reduce the status of Pluto to that of a dwarf planet, after astronomers argued that its unusual orbit, which was still littered with space junk, made it unqualified. The fact that Pluto is smaller than our Moon was also taken into account.
This stripping of status caused some distress among planetary scientists, most vociferously from Alan Stern, head of NASA's New Horizons space probe to Pluto. "The definition stinks," he said at the time of Pluto's humiliating downgrade.
Now he and other NASA scientists have petitioned the IAU for a new planetary definition that would include Pluto. "We propose the following geophysical definition of a planet for use by educators, scientists, students, and the public," the proposal [PDF] reads.
"A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."
That's pretty much word-for-word the definition the IAU agreed to in 2006, minus two requirements: one, that the planet must have cleared out its orbit of wandering space rocks by impacting or deflecting the debris; and two, that the planet orbit the Sun.
Stern and his colleagues argue no planet in our solar system has actually cleared out its orbit. Earth still gets hit by space rocks, and it happens more often than you might think. Other planets are in the same situation. They also argue that the 2006 definition doesn't cover rogue worlds outside of the Sun's orbit, which they argue should also count as planets. Basically, whatever it takes to include Pluto.
"With the above definition of a planet, we count at least 110 known planets in our Solar System. This number continues to grow as astronomers discover more planets in the Kuiper Belt," Stern and co said. "Certainly 110 planets is more than students should be expected to memorize, and indeed they ought not. Instead, students should learn only a few (9? 12? 25?) planets of interest."
Quite how happy the world's schoolchildren will be with that is debatable. The classroom mnemonic for planetary names – "my very educated mother just served us nine pizzas" – would have to be extended to a small paragraph or two, if Stern gets his way.
Michael Brown, professor of planetary astronomy at the California Institute of Technology and author of How I Killed Pluto and Why It Had It Coming, told The Register that Stern and his crew were just trying it on in the wake of popular feeling for Pluto following the 2015 New Horizons flyby. For one thing, this latest suggested definition will label our Moon as a planet, said Brown.
"It's hard to imagine that there is much of any support for a proposal that suddenly calls the Moon a planet after 500 years of the realization that it is not a planet," he told us.
"I am saddened that this small but loud group continues to try to confuse the public into thinking that there is no substantial difference between the real planets and the innumerable tiny bodies like Pluto. Pluto – like many other objects in our solar system – is a fascinating little world, but that doesn't mean we should stick our heads in the sand and ignore where it fits in the hierarchy of our solar system just to make ourselves feel better." ®