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 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 skywatchers have argued, at times heatedly, over what status should be accorded the 1,500-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 the 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 mostly small, distant worlds than it does with the other eight planets. Others felt that Pluto still deserved to be a full-fledged planet.

 

NASA's New Horizons spacecraft captured this image of Sputnik Planitia -- a glacial expanse rich in nitrogen, carbon monoxide and methane ices -- that forms the left lobe of a heart-shaped feature on Pluto's surface. Source: NASA

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 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 own 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 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.

New views on an old discovery

Pluto was discovered in 1930 by Clyde Tombaugh, a young astronomer working at the Lowell Observatory in Flagstaff, Arizona. Astronomers designated it a planet even though was is unlike the next-farthest planets, the gas giants Jupiter, Saturn, Uranus and Neptune. Pluto was an “oddball at the edge of the solar system” in the words of Brown, who has discovered dozens of small objects beyond the orbit of Neptune.

Stern isn’t swayed. Not only is Pluto a planet, he says, but so too are the big spherical bodies in the Kuiper belt and, closer to home, some of the largest of the objects in the asteroid belt between Mars and Jupiter. If that pushes up the number of planets into the dozens or even the hundreds, so be it.

Caltech astrophysicist Jessie Christiansen shares that point of view. “The argument I don’t agree with is, ‘There would just be too many planets if we let them all be planets,’” she says. “There are 100 billion stars in the galaxy. We didn’t stop calling them stars because the number got too big.”

Will Grundy, an astronomer at the Lowell Observatory, says that IAU astronomers reclassified Pluto because they were “freaked out by the prospect of having new planets being announced every year — which is happening because the technology has advanced to the point where we’re discovering all these small, distant things in the outer solar system. But as a planetary scientist, I think it’s fabulous. The more, the better!”

The debate is likely to heat up once again — and not just because Bridenstine has weighed in. Astronomers have found tantalizing evidence for the existence of a massive “super-Earth" beyond Pluto, and they say its existence could be confirmed within the next few years. Perhaps ironically, some refer to the object with the same term once reserved for Pluto: Planet Nine.

Passionate as they are, scientists on both sides of the debate are aware that the argument is largely a semantic one. As Christiansen says, “Pluto doesn’t care what we call it.”