Pluto’s place in the Solar System is under debate once again. It’s thanks, in part, to comments from new NASA administrator Jared Isaacman during a recent congressional hearing.
“I am very much in the camp of ‘Make Pluto a Planet Again’,” he said, referencing a growing movement among some planetary scientists to overturn Pluto’s controversial demotion.
Ever since astronomers voted to strip Pluto of its planetary status in 2006, many researchers – and plenty of members of the public – have argued the decision should be reversed.
It’s easy to see why. Pluto is round, has complex geology, and is home to mountains, glaciers and even a thin atmosphere. It certainly feels more planet-like than some anonymous lump of space rock.
Despite Isaacman’s backing, many astronomers are reluctant to reinstate it, warning it could open up a whole can of interplanetary worms.
Because, if Pluto is a planet, then dozens of other worlds across the Solar System may qualify too – perhaps even our own Moon.
A planet divided
The Pluto problem dates back to the early 2000s. Thanks to increasingly powerful telescopes and thorough surveys of the outer Solar System, astronomers discovered a wave of icy worlds beyond Neptune – many of which were uncomfortably similar to Pluto. For instance, Eris, discovered in 2005, is estimated to have around a third more mass than Pluto.
Astronomers faced an awkward question: if Pluto was a planet, shouldn’t Eris be one too? And if Eris was included, where exactly would the list end?
That was the question posed to the International Astronomical Union (IAU), the group responsible for naming celestial bodies. On 24 August 2006, the group voted in a new definition of a planet as a celestial body that:
- Orbits the Sun
- Has enough mass that gravity pulls it into a round shape
- Has cleared the neighbourhood around its orbit.
It was this last point where Pluto fell foul, resulting in its demotion to a ‘dwarf planet’. It’s also the point planetary experts have been arguing over for 20 years.
The problem is that to “clear their neighbourhood”, planets need to be gravitationally dominant enough to kick out or hoover up any other sizeable space rocks from their orbit.
The further you move away from the Sun, though, the larger that orbit becomes and the more space there is to clear. In the outer Solar System, a planet would therefore need to have far more mass to truly clear its orbit.
That’s why critics argue the definition is unfairly stacked against Pluto.
“With the IAU definition, the Earth qualifies as a planet in its present orbit, but if you put it in the outer Solar System, it would no longer be a planet by that definition, which is crazy,” says Dr Alan Stern, a leading planetary scientist from the Southwest Research Institute in Texas, US.
“Identical objects are sometimes planets, sometimes not planets, just based upon their ZIP code.”
Stern isn’t alone in his thinking. Just five days after the announcement, over 300 professional planetary scientists signed a petition protesting the decision – a striking number considering only 411 people voted on the definition in the first place.
The controversy hasn’t faded with time. A paper released in 2018 found that even after 12 years, planetary scientists weren’t using the definition.
And a 2024 paper identified a more fundamental flaw: the first criterion – that a planet must orbit our Sun – automatically excludes exoplanets, those orbiting other stars entirely.
“There is no ‘official’ definition of an exoplanet designated by an official scientific naming body,” says Dr Hannah Wakeford, an astrophysicist from the University of Bristol.
“The IAU definition is not fit for purpose outside our Solar System. If we took this definition to the Proxima system, which has three stars in orbit around each other, technically, the stars themselves would be classed as planets.”
New worlds order
So, what’s the solution? Instead of focusing on where they are, Stern and several other planetary scientists argue planets should be defined based on what they are.
“It’s very simple,” says Stern. “A planet is any object in space which is a) large enough to be rounded by its own self-gravity and b) not massive enough to ignite in nuclear fusion, because those objects are called stars. That’s it.”
This would not only put Pluto back on its planetary pedestal, but it would promote Eris – the body that helped kick it out in the first place. Dozens of dwarf planets discovered in the last 20 years, such as Haumea, Makemake, and Sedna, would be included, as well as Ceres, the only dwarf planet in the inner Solar System.
All in all, Stern’s definition would result in over 100 planets in our Solar System – a number that controversially includes Earth’s own Moon. After all, it’s round under its own gravity, geologically complex and has six times the mass of Pluto.
“It’s wonderful we have discovered so many planets in our Solar System and beyond,” says Stern. “It’s a really quaint old 19th and 20th century notion when we thought there were only a handful of planets.”
For many, though, such a free-for-all is taking things a step too far. One such person is Mike Brown, who led the team that discovered Eris and dozens of other dwarf planets and styles himself as ‘The Man Who Killed Pluto’. He fears such a definition would take the “meaning” out of what it means to be a planet.
“Classification is one of the first steps we take in science in order to understand phenomena,” says Brown. “If you classify things in a poor way it leads you to ask the wrong questions.”
Rather than being an opportunity to explore the diversity of the Solar System, Brown says that he views the push for a new definition as “an excuse to get Pluto back to being a planet. No one ever talked about this definition until after Pluto was demoted.”
The US has particularly strong feelings about the demotion. After all, Pluto was found by an American, specifically astronomer Clyde Tombaugh, in 1930. While speaking with Congress, Isaacman stated he was keen to “make sure that Clyde Tombaugh gets the credit he received once and rightfully deserves to receive again.”
Even Isaacman’s assertion that he wants to “Make Pluto a planet again” has echoes of the MAGA mantra, highlighting that there is as much a political drive as a scientific one.
But while having the backing of the NASA chief certainly puts weight behind the argument, Pluto’s future isn’t up to Isaacman or Stern. The IAU is an independent, international body and shows no signs of wanting to re-evaluate the issue.
Ultimately, whether Pluto is a planet might not even matter. Its demotion hasn’t made it disappear from people’s minds. It’s still being studied by scientists and has even given its name to an entire celestial subclass – Plutoids, referring to any dwarf planet beyond the orbit of Neptune.
“[Pluto] was promoted, in my opinion, to king of the dwarf planets,” says Wakeford. “One which sets an example of its class and exhibits many interesting features – how is that not better than being considered a mediocre planet?”
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