Sykes is among those who prefer a simple and inclusive definition of planet status: if an object is big enough for its own gravity to squeeze it into a rounded shape, then call it a planet. That would make a planet of Pluto again, as well as Ceres and a growing number of other bodies (see diagram).
The IAU had originally proposed something similar, but in Prague one faction objected. They pointed out that Pluto is really just a piece of debris, merely one of a swarm of icy objects out beyond Neptune called the Kuiper belt. The objectors complained that under the simple definition, many Kuiper belt objects would achieve planet status and they persuaded the meeting to add the condition that a planet must have either thrown out or sucked up any nearby junk, or in their words, “cleared the neighbourhood around its orbit”. Only the eight planets from Mercury to Neptune meet that condition.
It is also the sticking point. “It is a horrible mistake,” says Alan Stern of the Southwest Research Institute in Boulder, Colorado, who leads NASA’s New Horizons mission to Pluto. “Any definition that allows a planet in one location but not another is unworkable. Take Earth. Move it to Pluto’s orbit, and it will be instantly disqualified as a planet.”
While Earth’s gravity is easily strong enough to have cleared the debris from our relatively small neighbourhood, two factors mean that it would fail to do the job if placed at Pluto’s distance: the outer solar system is vast, and everything moves much more slowly out there. According to Sykes, 4.5 billion years would not be nearly long enough for a small and sluggish Earth to sweep those great expanses clean.
Source: Stephen Battersby, Is Pluto a planet after all?, New Scientist (July 27, 2009).