Astronomers recently announced they had discovered a new celestial body larger than Pluto, prompting discussion over how to define a “planet” and whether Pluto should still be considered one:

In late July, NASA announced the discovery of a “10th planet”–a lump of rock and ice believed to be bigger than Pluto and the farthest known object in the solar system–but some call the announcement more of a “publicity blitz.” While the agency considers the new celestial body a planet, others are not so sure.

The debate centers around the definition of a “planet” and whether Pluto still deserves to be classified as such.

The recently discovered object–temporarily dubbed 2003 UB313–is now 9 billion miles away from the Sun, or 97 times Earth’s distance and about three times Pluto’s current distance from the Sun.

Right now, the International Astronomical Union (IAU), which unites national astronomical societies from around the world, is working on a new definition for a planet. But until they arrive at an agreement, however, they state that “all objects discovered at a distance from the Sun greater than 40 AU will continue to be regarded as part of the Trans-Neptunian population.”

That means that by the current IAU definition, the new object is not a planet but merely a “trans-Neptunian” object since it’s 97 AU distant (an astronomical unit is the mean distance between Earth and the Sun). Meanwhile, Pluto orbits at an average distance of 39.44 AU, just within the 40-AU boundary and, therefore, still technically a planet.

If, however, the definition that the IAU eventually issues is based on size, then 2003 UB313 should earn that designation since it’s bigger than Pluto, and Pluto is a planet.

“Draw the line at Pluto and say only things bigger are planets.” That’s what astronomer Mike Brown of the California Institute of Technology says. Brown along with Chad Trujillo of Gemini Observatory in Mauna Kea, Hawaii and David Rabinowitz of Yale University, discovered the so-called “10th planet.”

But things are not that easy. Astronomers are discovering “planets” at a very rapid clip. These objects (which seem to be essentially large comets) lie in the Kuiper Belt–a ring of rocky ice balls that starts just beyond Neptune’s orbit. Jillions of Kuiper Belt objects orbit the Sun, and it’s estimated that at least 70,000 of these balls have diameters greater than 60 miles.

That means that if size were the determinant, the solar system could eventually have many more planets. As USA Today science journalist April Holladay writes in this recent column, “The solar system is still under construction, even after five billion years. Over the eons, smaller balls in the belt collided, coalesced, and built up into the larger ones. The dirty-ball ‘planets’ out there are building still. And Pluto is one of them.”

On the other hand, if the IAU astronomers decide to redefine “planet” to exclude all Kuiper Belt objects, the solar system would have one less planet, as Pluto is the largest and first Kuiper Belt object discovered. But many more are expected to be found.

According to this New York Times article, “many (astronomers) have said that if Pluto had been discovered today, it would not have been called a planet.”