Pluto is no longer a planet — or is it?

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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.
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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.”
Observations from NASA’s New Horizons mission revealed the surface of Pluto’s Sputnik Planitia region (shown). This area is covered in churning nitrogen ice “cells” (white blocks) These cells constantly bring fresh material up to the surface from below.JHU-APL, NASA, SWRI
Closer views show the rugged water-ice mountains that border some of the nitrogen ice cells.JHU-APL, NASA, SWRI
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.
Amateur astronomer Clyde Tombaugh poses with a homemade telescope. Tombaugh discovered Pluto in 1930 when he was 24 years old.GL Archive/Alamy Stock Photo
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.
On August 24, 2006, members of the International Astronomical Union voted for a new definition of “planet.” This definition reclassified Pluto and its neighbor Eris as dwarf planets — shrinking to eight the number of planets in our solar system.Michal Cizek/AFP/Getty Images
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.
Messy definitions
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.
The dwarf planet Ceres orbits in the asteroid belt. Like Pluto, it was once considered a planet. NASA’s Dawn mission visited the dwarf planet in 2015 and found that it is also a geologically interesting world.JPL-Caltech, NASA, UCLA, MPS, DLR, IDA
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.
Real-world usage
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.”
Pluto — along with hundreds or thousands of other objects similar in size — orbit on the icy outer edge of the solar system. This region is called the Kuiper Belt (white fuzzy ring).NASA
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.’”
Pluto rises above the horizon of its largest moon, Charon, in this artist’s illustration.Mark Garlick/Science Photo Library/GettyImages Plus
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.
Interviews after NASA’s New Horizons spacecraft returned images of Pluto in 2015 show that the dwarf planet continues to charm us all.
Power Words
asteroid: A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
astronomer: A scientist who works in the field of research that deals with celestial objects, space and the physical universe.
atmosphere: The envelope of gases surrounding Earth, another planet or a moon.
average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
celestial: (in astronomy) Of or relating to the sky, or outer space.
Ceres: The largest known asteroid orbiting the sun. It sits 270 million kilometers (nearly 168 million miles) from Earth in the asteroid belt between Mars and Jupiter. At about 1,000 kilometers (600 miles) across, Ceres is so big that it is classified as a dwarf planet. In 2014, astronomers found it spewing water from two places on its surface.
colleague: Someone who works with another; a co-worker or team member.
debris: Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
dwarf planet: One of the solar system’s small celestial objects. Like a true planet, it orbits the sun. However, dwarf planets are too small to qualify as true planets. Prime examples of these objects: Pluto and Ceres.
gas giant: A giant planet that is made mostly of helium and hydrogen, which on Earth are gases. Jupiter and Saturn are gas giants.
geology: The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.
gravity: The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
Jupiter: (in astronomy) The solar system’s largest planet, it has the shortest day length (9 hours, 55 minutes). A gas giant, its low density indicates that this planet is composed mostly of the light elements hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).
Mars: The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.
mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
methane: A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.
moon: The natural satellite of any planet.
Neptune: The farthest giant planet from the sun in our solar system. It is the fourth largest planet in the solar system.
nitrogen: A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn. It comes in two stable forms. Both have 14 protons in the nucleus. But one has 14 neutrons in that nucleus; the other has 15. For that difference, they are known, respectively, as nitrogen-14 and nitrogen-15 (or 14N and 15N).
observatory: (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.
orbit: The curved path of a celestial object or spacecraft around a galaxy, star, planet or moon. One complete circuit around a celestial body.
planet: A large celestial object that orbits a star but unlike a star does not generate any visible light.
planetary science: The science of planets other than Earth.
Pluto: A distant world that is located in the Kuiper Belt, just beyond Neptune. Pluto is the ninth largest object orbiting the sun.
Red Planet: A nickname for Mars.
risk: The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)
solar system: The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
spherical: Adjective for something that is round (as a sphere).
sun: The star at the center of Earth’s solar system. It is about 27,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
telescope: Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
titan: The term for any gigantic being. The term comes from Greek mythology. The six sons and six daughters of the Greek gods Uranus and Gaea were known as titans. Capitalized Titan is a moon of Saturn.
trait: A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.
Twitter: An online social network that allows users to post messages containing no more than 280 characters. (Until November 2017, the limit had been just 140 characters.)
volcano: A place on the crust of Earth (or some other celestial body) that opens, allowing molten rock and gases to spew out from underground reservoirs.
Lisa Grossman is the astronomy writer at Science News. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.
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