Pluto Got Demoted 20 Years Ago. We’re Still Not Sure What a Planet Really Is.
New discoveries have made defining what counts as a planet—and what doesn’t—even more difficult.
Picture a star, about half the sun’s mass shining with a dim red light. Lumbering around it on a 5,000-year orbit is a huge orb called a brown dwarf, which is much heavier than a planet but still not quite massive enough to be a star.
Our own solar system has nothing like a brown dwarf, but here's where it really gets tricky. Revolving around that brown dwarf every 171 days is a gaseous world almost as large as Jupiter. The question is: What do you call this final thing? Is it a moon? A planet? Something else entirely? When this system, whose central star is called CD-35 2722, was reported last month, astronomers didn’t quite know either.
Twenty years ago today, International Astronomical Union (IAU) researchers confronted a similar problem when they voted to define the word planet for the first time. A year prior, an icy body more massive than Pluto had been discovered in the outer fringes of the solar system. Rather than give school children extra worlds to memorize, astronomers chose to downgrade Pluto’s status to a dwarf planet, a move that still has its detractors.
In the intervening decades, the issue has only gotten worse as telescopes uncover ever more oddities floating through the cosmos, like the three-tiered situation around CD-35 2722. Despite such a menagerie, many researchers hold out hope they will eventually produce a straightforward way to tell what is and isn’t a planet.
“In every other field, scientists have defined the terms they need to do their work,” says Jean-Luc Margot, an astronomer at the University of California, Los Angeles. “We know what a triangle is. We know what a virus is. It continues to surprise me that astronomers have not come up with a precise definition of the word planet.”
How Pluto Became a Planet—and Then a Dwarf Planet
For most of human history, planets were simply bright specks in the sky that moved differently than the rest of the stars. Our modern word derives from the ancient Greek planētēs, meaning wanderers. After the telescope was invented in the early 1600s, scientists realized these nomads were in fact celestial bodies orbiting the sun just like our own world. They even found new planets invisible to the naked eye; Uranus in 1781, and Neptune in 1846.
Pluto was a later addition, a distant speck discovered by the American astronomer Clyde Tombaugh in 1930. For almost a century, textbooks explained that the solar system contained nine planets. But by the turn of the millennium, astronomers came to recognize that Pluto wasn’t alone. Thousands of other frozen fragments orbited near it in a region called the Kuiper Belt.
In 2005, a team of astronomers trained their instruments far past Pluto and discovered a glacial orb that was slightly smaller than it in diameter but about 25 percent more massive. Knowing the headache this would cause, the researchers named it Eris after the goddess of strife and discord. The field faced a dilemma: Should they classify Eris as a planet? What did that imply about other frigid worlds in the Kuiper Belt? Where was the cut-off for planetary status?
In an effort to draw some lines, the IAU—which is the arbiter of such things—presented a resolution at its 26th General Assembly in Prague defining a planet according to the following criteria: It must orbit the sun, it must be spherical, and it must be big enough for its gravity to have cleared away other bodies, such as asteroids and similar rocky fragments, near its orbit. The resolution passed, as did a subsequent one creating the new class of “dwarf planet” for Pluto and related bodies.
A public outcry erupted over the decision. Seven-year-olds wrote angry letters to planetariums, and, to this day, even some professional researchers hold a soft spot for the former ninth rock from the sun. “You learn at school that there are nine planets and the last is Pluto,” says Alice Zurlo, an astrophysicist at the Universidad Diego Portales (UDP) in Chile who was an undergraduate at the time of the vote. While she accepts the results, “I’m still sad for that. Pluto will always be a planet to me.”
The definition was problematic in more serious ways. For one, it referenced the sun, leaving out the roughly 6,000 worlds scientists have discovered around other stars. In 2018, the IAU adopted a separate working definition for exoplanets, saying they must orbit a star, brown dwarf, or stellar remnant such as a white dwarf. They must also be lighter than 13 times Jupiter’s mass, and at least 25 times less massive than their parent star. And finally, again, they must have enough gravitational sway to clear away nearby objects.
Astronomers still quibble with the results of both definitions. Some think the upper mass limit of 13 Jupiters is too rigid. Others say the last condition is vague. “Not even Earth or Jupiter have cleared their orbits,” says Ravi Kumar Kopparapu, a planetary scientist at NASA’s Goddard Space Flight Center. After all, asteroid clusters can be found along or near the orbits of both planets.
Observatories have also detected cosmic quirks that defy these definitions. Rogue planets, for example, were born around a star but later ejected by gravitational forces and sent wandering through space. Jupiter-mass binary objects (JuMBOs), are pairs of enormous worlds spinning around one another that may represent the smallest possible brown dwarfs that can be formed. Do these count as planets?
As a whole, the field isn’t sure. “We have the IAU definition people will appeal to, but I don’t think anyone’s really a big fan,” says Kevin Michael Hoy, a Ph.D. student at UDP. “As we find more planets, the borders get fuzzier.”
That’s No Moon—It’s an Exosatellite?
Enter into this mess CD-35 2722. Located about 73 light-years away in the constellation Columba, the star had its brown dwarf companion discovered in 2011. That extremely long orbit means the brown dwarf and red star are sometimes separated by hundreds of times the distance between Earth and the sun. Instruments can make out the tiny speck of the brown dwarf next to its parent body, making it a good test bed to hunt for additional worlds.
Starting in 2023, Zurlo, Hoy, and their collaborators trained the Very Large Telescope in Chile on the brown dwarf and watched it over several years. Slowly, the observations revealed that the object was undergoing a steady and periodic wobble, evidence that the brown dwarf was being tugged gravitationally by a smaller body. “At the very beginning, I couldn't believe it,” Zurlo says.
But the more data they collected, the clearer the signal got, indicating that a Jupiter-scale world was indeed circling the brown dwarf. Then the team had to decide what to call this new world. They began with the term exomoon (analogous to exoplanet), since many researchers have been racing to be the first to discover a moon around a distant planet. But after internal discussions and pushback from colleagues, who balked at the idea of something the size of Jupiter being called a moon, they settled on naming it an “exosatellite.”
“It’s a hand-wavy way of not getting in trouble with people who would get mad if we call it a moon,” says Hoy, the first author of the team’s paper. “And also avoid getting in trouble with people who’d get mad if we called it a planet.”
Yet he admits that the title might be a stopgap measure until the field decides what to name a huge object around a brown dwarf around a star. At a recent exoplanet conference in Portugal, he conducted a poll of just over 100 astronomers: Five percent said the object he found was a moon, 20 percent said it was a planet, and around 60 percent preferred the third category, exosatellite.
Refining the Definition of a Planet
The problem of what counts as a planet or an exoplanet remains unresolved. Margot has tried to address this by delineating what it means for an object to have enough gravitational sway to clear other bodies from its orbit. In 2024, he and two colleagues created a way to quantify this factor using an object’s mass and distance from its central star. Rather than require that every last speck of dust disappear from a world’s orbit, their proposal identifies bodies with the gravitational gravitas to knock away rival objects given enough time.
Applying their calculations to stuff in the solar system shows a sharp distinction between things like Mercury and Jupiter and things like Pluto and Eris, quantitatively sorting the planets from the not-quite-planets. All known exoplanets would also count as planets according to their criteria (as would the exosatellite around CD-35 2722, Margot says). Rogue planets and JuMBOs wouldn’t, though, since they don’t orbit stars, brown dwarfs, or stellar remnants.
Margot and his collaborators proposed that their definition be taken up at the most recent IAU General Assembly in 2024 in Cape Town, South Africa, though officials chose not to, perhaps remembering the Pandora’s box they opened with Pluto (“It’s been 20 years, but there’s probably still trauma within the IAU leadership,” he says). The researchers haven’t decided if they will submit their definition again for the 2027 General Assembly in Rome, Italy.
Science runs on categories, classifying this from that. Perhaps that explains the passions this issue arouses and why it has yet to be closed. Whenever astronomers spot something new, one of the first questions that they—and members of the public—ask is, “Is it a planet?,” says Margot. “If we don’t have a satisfactory definition, we can’t answer that.”
