Is Pluto still a planet nine?

Is Pluto a planet, as generations of schoolchildren learned? Or is it really a dwarf planet, as astronomy's official governing body has determined?

NASA administrator Jim Bridenstine says it's the former. "Just so you know, in my view Pluto is a planet," he told reporters on Aug. 23 while touring an aerospace engineering lab in Boulder, Colorado. "It's the way I learned it and I'm committed to it."

Bridenstine may be committed, but the official planet/ dwarf planet call isn't up to him. That responsibility lies with the International Astronomy Union, which demoted Pluto from planet to dwarf planet 13 years ago.

Since then, scientists and sky-watchers have argued, at times heatedly, over what status should be accorded the the 1500-mile-wide rocky ball that circles the sun once every 248 years at the edge of the solar system.

Longstanding squabble

Bridenstine's comments are unlikely to settle the debate, which started in the 1990s with the discovery of Kuiper belt, a ring of rocky or icy objects circling the sun beyond Neptune's orbit.

In light of the discovery, some astronomers and planetary scientists began arguing that Pluto, whose orbit lies within the Kuiper belt has more in common with these small, distant worlds than it does with the other eight planets. Others felt that Pluto still deserved to be a full-fledged planet.

To settle the squabble, the Paris-based IAU in 2006 issued a new definition of the term "planet." According to that definition, a planet is a celestial body that orbits the sun, has enough gravitation to assume a roughly spherical shape and has managed to clear from its orbital path any smaller bits of debris - as massive objects eventually do as a a result of their gravitational tug.

Pluto meets the first two conditions but not the third - and so the organization's members voted to reclassify it.

In the wake of Pluto's status change, hundreds of planetary scientists petitioned to overturn the definition, saying it was ill-conceived. For example, it could be argued that the Earth, Mars and Jupiter have failed to clear the debris from their own orbits, given the numerous asteroids that still populate the region. Critics also point out that only 424 of the IAU's 9,000 or so members took part in the vote.

What makes a planet?

Among Pluto's fiercest defenders is Alan Stern, principal investigator of NASA's New Horizons mission, which sent a robotic spacecraft to Pluto in 2015. Stern calls the IAU decision "misguided," saying planetary status should be conferred on the basis of a celestial object's geophysical features. By that measure, he says, Pluto is clearly a planet. 

"Pluto has an atmosphere, mountain ranges, a core, an interior ocean and many other properties just like the Earth," he says. Pluto also has its on moon, 750-mile-wide Charon. " Planetary scientists ignore the IAU definition and treat Pluto as a full-fledged planet in their research literature. That's more important than any vote." 

On the other side of the Pluto divide is the the California Institute of Technology astronomer Mike Brown, author of a 2010 book entitled " How I Killed Pluto and Why It Had It Coming." In his view, Pluto is simply one of the countless objects that circle the sun beyond Neptune. It's not even the biggest: Eris, a dwarf planet in the Kuiper belt that Brown discovered in 2005, is about 25 percent more massive than Pluto.