What's So Great About Being a Planet?

The definition is as much cultural as it is scientific—but that doesn’t mean it doesn’t matter.

In this artist's rendering, Pluto's largest moon, Charon, rises over the frozen South Pole surface of the dwarf planet. (JHUAPL / SWRI)

Being a planet is not really a state of being. It is instead a human construct, a categorical designation, and a slippery one at that. Which means that whatever Pluto actually is, its essential Plutoness, is not reliant on whether some humans who live billions of miles away from it decide to call it a planet.

Does it even matter what we call Pluto? Well, yes and no.

Pluto’s planetary status has prompted much debate in the past decade. In 2006, the International Astronomical Union reclassified it as a dwarf planet—a move that lots of people described as a demotion. Pluto had left the ranks of then-nine classical planets and joined the peanut gallery of dwarf planets, only a handful of which have names.

But what’s so great about being a planet anyway? In our solar system, planetary status is pretty elite. There are eight planets (Mars, Venus, Earth, Mercury, Jupiter, Saturn, Uranus, and Neptune) and hundreds or maybe thousands of dwarf planets (Pluto, Eris, Haumea, Makemake, Ceres, &c).

An artist’s rendering of Pluto and its largest moon as they would appear if placed slightly above Earth and viewed from a great distance. Pluto’s diameter is about one-fifth that of Earth's. (NASA)

Pluto is tiny compared with Earth, but it's not just its size that makes it a dwarf. Here's how Vox explained it:

What finally led the International Astronomical Union to reconsider Pluto's status was [the Caltech astronomer Mike] Brown's discovery of... Eris, [a dwarf planet] that was actually a bit more massive than Pluto. Aware that this would probably just be the first of many, the IAU voted to approve a new definition that would eliminate all of these objects from the list of planets — rather than continue to add more and more planets in future years.

Gail Oxton, who built the software for New Horizons and has been on the mission since 2002, says she and her colleagues were “devastated” when Pluto got demoted. “And it still stings a little,” she told me, “especially when we hear yet another joke about poor Pluto.” (Oxton says she did find some of those jokes to be funny, however, and she still keeps this comic taped to her wall.)

“While there are other exciting objects in the solar system, this was the last unexplored planet, those things everyone had to memorize when they were in grade school,” she told me. “We were making history. When Pluto was demoted 7 months after launch, it immediately robbed us of some of that excitement.”

A planetary designation may be largely cultural, but that doesn't mean it isn’t meaningful. Funding for research relies on popular understanding. Surely it helps when an object of interest—be it a brontosaurus or a celestial body—is widely known. Alan Stern, the astronomer and aeronautical engineer who fought hard to get financial backing for New Horizons, says the designation is fundamentally about scientific understanding. “The core is about science,” he told me in an email. Stern, who has called the International Astronomical Union's new definition of a planet “sloppy,” has been a vocal proponent for reinstating Pluto as a planet. “It's about leaning forward or retrenching as planetary science comes of age,” he said.

On the other hand, there’s something pleasing about how planetary status is ultimately arbitrary. Pluto’s indifference to the quibbles on Earth is, like so much of what makes space exploration marvelous, a reminder that human existence is a blip. And it can liberating to be reminded of one’s insignificance. Until recently, too, Pluto appeared trifling: an icy and dimly lit rock, distant beyond comprehension, not even a planet, and only visible to humans as a collection of rough pixels.

That has changed.

“For me personally, it doesn’t matter whether or not Pluto is classified as a planet or not,” Alice Bowman, the missions operation manager for New Horizons, told me. “It’s a place unexplored that we now have the technology to visit. We have the opportunity to increase mankind’s knowledge about this system... How very cool it is to change this small faint point of light into a world of color, surface features, and atmosphere.”