For 76 years, Pluto was the beloved ninth planet. No one cared that it was the runt of the solar system, with a moon half its size. No one minded that it had a tilted, oval-shaped orbit. Pluto was a weirdo, but it was our weirdo.
“Children identify with its smallness,” wrote science writer Dava Sobel in her 2005 book The Planets. “Adults relate to its … existence as a misfit.” People felt protective of Pluto.
So it was perhaps not surprising that there was public uproar when Pluto was relabeled a dwarf planet 15 years ago. The International Astronomical Union, or IAU, redefined “planet.” And Pluto no longer fit the bill.
This new definition required a planet to do three things. First, it must orbit the sun. Second, it must have enough mass for its own gravity to mold it into a sphere (or close). Third, it must have cleared the space around its orbit of other objects. Pluto didn’t pass the third test. Hence: dwarf planet.
“I believe that the decision taken was the correct one,” says Catherine Cesarsky. She was president of the IAU in 2006. She’s currently an astronomer at CEA Saclay in France. “Pluto is very different from the eight solar-system planets,” she says. Plus, in the years leading up to Pluto’s reclassification, astronomers had discovered more objects beyond Neptune that were similar to Pluto. Scientists either had to add many new planets to their list, or remove Pluto. It was simpler to just give Pluto the boot.
“The intention was not at all to demote Pluto,” Cesarsky says. Instead, she and others wanted to promote Pluto as one of an important new class of objects — those dwarf planets.
Some planetary scientists agreed with that. Among them was Jean-Luc Margot at the University of California Los Angeles. Making it a dwarf planet was “a triumph of science over emotion. Science is all about recognizing that earlier ideas may have been wrong,” he said at the time. “Pluto is finally where it belongs.”
Others have disagreed. Planets should not have to clear their orbits of other debris, argues Jim Bell. He’s a planetary scientist at Arizona State University in Tempe. An object’s ability to cast out debris does not just depend on the body itself, Bell says. So that shouldn’t disqualify Pluto. Everything with interesting geology should be a planet, he says. That way, “it doesn’t matter where you are, it matters what you are.”
Pluto certainly has interesting geology. Since 2006, we’ve learned that Pluto has an atmosphere and maybe even clouds. It has mountains made of water ice, fields of frozen nitrogen and methane snow-capped peaks. It even sports dunes and volcanos. That fascinating and active geology rivals any rocky world in the inner solar system. To Philip Metzger, this confirmed that Pluto should count as a planet.
“There was an immediate reaction against the dumb [IAU] definition,” says Metzger. He’s a planetary scientist at the University of Central Florida in Orlando. But science runs on evidence, not instinct. So Metzger and colleagues have been gathering evidence for why IAU’s definition of “planet” feels so wrong.
The rise and fall of Pluto
For centuries, the word “planet” was much more inclusive. When Galileo turned his telescope on Jupiter in the 1600s, any large moving body in the sky was considered a planet. That included moons. In the 1800s, when astronomers discovered the rocky bodies now called asteroids, they called those planets, too.
Pluto was seen as a planet from the very beginning. Amateur astronomer Clyde Tombaugh first spotted it in telescope photos taken in January 1930. At the time, he was working at the Lowell Observatory in Flagstaff, Ariz. Upon his discovery, Tombaugh rushed to the observatory director. “I have found your Planet X,” he declared. Tombaugh was referring to a ninth planet that had been predicted to orbit the sun beyond Neptune.
But things got weird when scientists realized Pluto wasn’t alone out there. In 1992, an object about a tenth as wide as Pluto was seen orbiting out beyond it. More than 2,000 icy bodies have since been found hiding in this frigid outskirt of the solar system known as the Kuiper (KY-pur) Belt. And there may be many more still.
Finding that Pluto had so many neighbors raised questions. What did these strange new worlds have in common with more familiar ones? What set them apart? Suddenly, astronomers weren’t sure what truly qualified as a planet.
Mike Brown is a planetary scientist at the California Institute of Technology in Pasadena. In 2005, he spotted the first Kuiper Belt body that appeared larger than Pluto. It was nicknamed Xena, in honor of the TV show Xena: Warrior Princess. This icy body was left over from the formation of the solar system. If Pluto was the ninth planet, Brown argued, then surely Xena should be the 10th. But if Xena didn’t deserve the title of “planet,” Pluto shouldn’t either.
Tensions over how to categorize Pluto and Xena came to a head in 2006. The drama peaked at an IAU meeting held in Prague, the capital of the Czech Republic. On the final day of the August meeting, and after much heated debate, a new definition of “planet” was put to a vote. Pluto and Xena were deemed dwarf planets. Xena was renamed Eris, the Greek goddess of discord. A fitting title, given its role in upsetting our concept of the solar system. On Twitter, Brown goes by @plutokiller, since his research helped knock Pluto off its planetary pedestal.
Right away, textbooks were revised and posters reprinted. But many planetary scientists — especially those who study Pluto — never bothered to change. “Planetary scientists don’t use the IAU’s definition in publishing papers,” Metzger says. “We pretty much just ignore it.”
In part, that might be sass or spite. But Metzger and others think there’s also good reason to reject IAU’s definition of “planet.” They make their case in a pair of papers. One appeared as a 2019 report in Icarus. The other one is due out soon.
For these, the researchers examined hundreds of scientific papers, textbooks and letters. Some of the documents dated back centuries. They show that how scientists and the public have used the word “planet” has changed many times. And why was often not straightforward.
Consider Ceres. This object sits in the asteroid belt between Mars and Jupiter. Like Pluto, Ceres was considered a planet after its 1801 discovery. It’s often said Ceres was lost its planethood after astronomers found other bodies in the asteroid belt. By the end of the 1800s, scientists knew Ceres had hundreds of neighbors. Since Ceres no longer appeared special, the story goes, it lost its planetary title.
In that sense, Ceres and Pluto suffered the same fate. Right?
That’s not the real story actually, Metzger’s team now reports. Ceres and other asteroids were considered planets — albeit “minor” planets — well into the 20th century. A 1951 article in Science News Letter said that “thousands of planets are known to circle our sun.” (Science News Letter later became Science News, our sister publication.) Most of these planets, the magazine noted, were “small fry.” Such “baby planets” could be as small as a city block or as wide as Pennsylvania.
The term “minor planets” only fell out of fashion in the 1960s. That’s when spacecraft got a closer look at them. The largest asteroids still looked like planets. Most small ones, however, turned out to be weird, lumps. This provided evidence that they were fundamentally different than the bigger, rounder planets. The fact that asteroids didn’t clear their orbits had nothing to do with their name change.
And what about moons? Scientists called them “planets” or “secondary planets” until the 1920s. Surprisingly, people didn’t stop calling moons “planets” for scientific reasons. The change was driven by nonscientific publications, such as astrological almanacs. These books use the positions of celestial bodies for horoscopes. Astrologers insisted on the simplicity of a limited number of planets in the sky.
But new data from space travel later brought moons back into the planetary fold. Starting in the 1960s, some scientific papers again used the word “planet” for objects orbiting other solar system bodies — at least for some large round ones, including moons.
In short, the IAU definition of “planet” is just the latest in a long line. The word has changed meanings many times, for many different reasons. So there’s no reason why it couldn’t be changed once more.
Defining “planets” to include certain moons, asteroids and Kuiper Belt objects is useful, Metzger now argues. Planetary science includes places like Mars (a planet), Titan (one of Saturn’s moons) and Pluto (a dwarf planet). All these places have extra complexity that arises when rocky worlds get big enough to become spherical. Examples of that complexity span from mountains and atmospheres to oceans and rivers. It’s scientifically useful to have an umbrella term for such complex worlds, Metzger says.
“We’re not claiming that we have the perfect definition of a planet,” he adds. Nor does Metzger think everyone need adopt his. That’s the mistake the IAU made, he says. “We’re saying this is something that ought to be debated.”
A more inclusive definition of “planet” might also give a more accurate concept of the solar system. Emphasizing eight major planets suggests they dominate the solar system. In fact, the smaller stuff greatly outnumbers those worlds. The major planets don’t even stay in fixed orbits over long time-scales. Gas giants, for instance, have shuffled around in the past. Viewing the solar system as just eight unchanging bodies may not do that complexity justice.
Brown (@plutokiller) disagrees. Having the gravitational oomph to nudge other bodies around is an important feature of a planet, he argues. Plus, the eight planets clearly dominate our solar system. “If you dropped me in the solar system for the first time, and I looked around … nobody would say anything other than, ‘Wow, there are these eight — choose your word — and a lot of other little things.’”
One common argument for the IAU definition is that it keeps the number of planets manageable. Can you imagine if there were hundreds or thousands of planets? How would the average person keep track of them all? What would we print on lunch boxes?
But Metzger thinks counting just eight planets risks turning people off to the rest of space. “Back in the early 2000s, there was a lot of excitement when astronomers were discovering new planets in our solar system,” he says. “All that excitement ended in 2006.”
Yet many of those smaller objects are still interesting. Already, there are at least 150 known dwarf planets. Most people, however, are unaware, Metzger says. Indeed, why do we need to limit the number of planets? People can memorize the names and traits of hundreds of dinosaurs or Pokémon. Why not planets? Why not inspire people to rediscover and explore the space objects that most appeal to them? Maybe, in the end, what makes a planet is in the eye of the beholder.