“If it looks like a duck, swims like a duck, and quacks like a duck, then it’s probably a duck,” jokes Alan Stern, principal investigator for the New Horizons mission. In his view, the same applies to planets.

On July 14, New Horizons will make its closest approach to Pluto. The spacecraft will take the first ever close-range photos and scientific readings of the former planet, possibly revealing rings, subterranean oceans, or even more moons. And it could provide the evidence needed to redefine what we consider a planet.

In January 2006, the New Horizons probe launched from Cape Canaveral to perform a reconnaissance of Pluto, the only one of the nine then-planets that had yet to be observed by a close-encounter flyby. But just seven months later, Pluto was officially demoted to the status of dwarf planet. Stern has been fighting that decision ever since—and he’s not the only one.

The meaning of the word “planet” has been clear, if not strictly defined, since the 19th century. The drama kicked off in August 2006, when the General Assembly of the International Astronomical Union (IAU) held a meeting in Prague and released an official definition of a planet for the very first time. It stated: “A ‘planet’ is a celestial body that”:

  1. “is in orbit around the Sun…” Check.
  2. “has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape…” Check.
  3. “has cleared the neighborhood around its orbit.” Negative.

Sorry, Pluto—planet test failed.

The Planet, Defined

By that point, seven months after launch, New Horizons had only traveled about one-sixth of the distance to its target. Suddenly, it was speeding toward one of many new dwarf planets instead of the last unvisited planet in the solar system.

“Astronomers who are non-experts in planetary science basically passed a bunch of B.S. off on the public back in 2006,” Stern says. “A week later, hundreds of planetary scientists, more people than at this meeting in Prague, signed a petition that rejects the new definition. If you go to planetary science meetings and hear technical talks on Pluto, you will hear experts calling it a planet every day.”

That’s partly because, oddly enough, no one had clearly defined the term “planet” before. In the mid-1800s, there were 13 planets—the eight that we have today, plus Ceres and four other objects in the asteroid belt. Shortly after the discovery of Neptune in 1846, astronomers determined that there were actually thousands of objects orbiting the sun between Mars and Jupiter. Ceres and its four companions were reclassified as asteroids, meaning “star-like,” for their resemblance to distant stars when observed through contemporary telescopes.

Then in 1930, Clyde W. Tombaugh discovered Pluto, and for the next 76 years the planets happily remained at nine—the number that most of us remember from grade school.

Dawn of the Asteroids

That is, until 2005, when a team of astronomers discovered Eris. This little celestial body in the Kuiper Belt, aptly named for the Greek goddess of discord, started one of the largest debates in the history of astronomy.

Scientists originally thought that Eris was significantly larger than Pluto (now they believe that it is only slightly bigger, or nearly the same size) and therefore proposed that the new object should become the tenth planet in our solar system. But around this time, astronomers started to realize that the solar system is probably full of these small planets. As with the discovery of the asteroid belt a century and a half before, concern arose over the number of planets that would have to be added to the growing list.

“This lousy definition from non-specialists rules out everything.”

“The reason astronomers gave [for changing the classification of Pluto and similar objects] is that they don’t want to have to manage a large number of names,” said Stern. “I think that’s pretty twentieth century. There are a ridiculous number of stars, and we don’t rule them out just to be convenient about their names. We don’t rule out galaxies or asteroids, or for that matter, mountains or rivers on Earth.”

Stern is well positioned to make such statements. In a 1991 paper published in the journal Icarus, Stern predicted the existence of many Pluto-sized objects among the disc of debris at the outer edges of the solar system. The confirmation of this theory was a major factor in the IAU’s decision to create a new classification of planet. Stern even used the term “dwarf planet” back in the 1990s, but contrary to the current definition, he meant for the phrase to merely represent a subcategory of planet.

Up for Debate

We now know that the Kuiper Belt is about three times the surface area of everything inside it, and it likely contains hundreds of Pluto-like objects. As Stern points out, it’s ridiculous to assume that any object in the Kuiper Belt could clear its orbital zone—the third criterion for a planet—given the size of the region and its abundance of objects. “If you put Earth out in the Kuiper Belt, it couldn’t clear it either,” he said. “Does that mean Earth’s not a planet?”

What exactly does it mean for an object to “clear its zone”? Stern says it’s ambiguous: “The intended meaning is that its gravity is enough to sweep everything else where it’s orbiting away. But in reality, every planet in the solar system has other small bodies next to it that it has not cleared. So, this lousy definition from non-specialists rules out everything. It rules out Jupiter, because Jupiter has Trojan asteroids. It rules out Neptune, because Pluto crosses Neptune’s orbit. It’s just so poorly thought-out that it’s laughable.”

“They won’t know what else to call Pluto but a planet, and a pretty exciting one.”

In the coming months, all of this could change. In July, when the Long Range Reconnaissance Imager (LORRI) on New Horizons snaps thousands of close-range photos of Pluto, it will bring the ostracized little planet into stark relief for the first time in history. The infrared imager known as Ralph will create new thermal maps of Pluto’s surface, and the ultraviolet spectrometer Alice will take accurate measurements of its atmospheric composition. Discoveries made by New Horizons could lead to a new definition of “planet”—one that would bring not only Pluto, but numerous other objects, including Ceres and Eris into the planetary family.

“I think that one of the things that will come out of the New Horizons mission,” Stern opined, “is that the public will take a look, and they won’t know what else to call Pluto but a planet, and a pretty exciting one.”

Now read planetary astronomer Mike Brown on why he still loves Pluto even though he helped demote it.