“In my view, Pluto is a planet.” This is a sentence that mostly anyone can utter without having the least repercussion, but when it comes from the mouth of NASA administrator Jim Bridenstine, it’s grist for the headlines, to say the least. “You can write that the NASA Administrator declared Pluto a planet once again. I’m sticking by that. It’s the way I learnt it, and I’m committed to it,” he said in a recent appearance. While the words of the man who runs the world’s first space agency won’t change the status of what is now officially a dwarf planet, the episode reminds us that the controversy has not faded.
Pluto’s official death as a planet occurred on August 24, 2006. After more than three quarters of a century appearing in textbooks as the ninth planet of the Solar System, on that summer Saturday the International Astronomical Union (IAU), meeting in Prague for its 26th general assembly, approved an official definition of planet that left out the small and distant world.
To understand why, one has to head to the past, all the way back to 1801. It was in that year that the discovery of an object between Mars and Jupiter, called Ceres, raised a question for astronomers: is it a planet or not? Initially it was accepted as such, but when the number of similar objects began to grow, it sparked the discussion. In the end it was decided to catalogue Ceres and its companions as asteroids. Similarly, when, almost two centuries later, objects the size of Pluto began to be detected in the confines of the Solar System, such as Eris and Sedna, more than a few astronomers dusted off the old question.
New objects that defy definition
The IAU then felt the need to approve a formal definition of planet, something it lacked until then. After an extensive debate, it was proposed that: “A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.” The assembly voted and approved this definition, which left out Pluto for not fulfilling the third requirement, since it shares the zone of its orbit with other objects.
The reactions were as immediate as they were forceful. Opponents of the decision criticised the procedure: only 424 of the 9,000 IAU members were present at that meeting in Prague, and the organisation only allows voting in person. The critics especially censured the technical aspects. Planetary scientist Alan Stern, principal investigator of the NASA New Horizons mission that flew past Pluto in 2015, has been one of the strongest opponents to the IAU definition. “That definition is highly flawed, so much and on so many levels that essentially no one in the professional planetary science community uses it in their research work,” he sums up to OpenMind.
Right from the start, Stern argued that other planets of the Solar System also share their orbit with a multitude of objects, to which astronomers counter that in these cases the planet’s gravity is clearly dominant. According to Carlos de la Fuente Marcos, an astronomer specialised in orbital dynamics, “if there are more objects similar to the one in reference, at a similar radial distance and that do not orbit around them (that is, they are not satellites like the Earth-Moon system), talking about the planet does not seem correct.” For this astronomer, the IAU definition is “reasonable and correct.”
What makes a planet?
But according to Stern, the heart of the problem lies in defining something not by itself, but by something external: “We don’t classify objects in astronomy by what they are near, we classify them by their properties,” he told Nature in 2006. In fact, after 13 years of controversy, this has persisted as the main argument of those who reject the IAU definition. Leaving aside the sentimental or cultural reasons, of which Bridenstine’s seems to reflect a popular position, many experts insist that it should be the intrinsic parameters of objects, and not the dynamics of their orbits, that define a planet.
As Planetary Science Institute researcher David Grinspoon, co-author with Stern of Chasing New Horizons: Inside the Epic First Mission to Pluto, explains to OpenMind, the IAU definition “is fine if what you are primarily concerned with is orbits and dynamics; if you are concerned with planets as objects, as places, as bodies with properties to be modelled and compared with other bodies, then it is deeply flawed.” “I like to use the IAU planet definition action in public lectures as an example of a science organization engaging in an anti-science act,” adds Mark Sykes, CEO and director of the Planetary Science Institute. A scientific taxonomy, he says, is something that is not voted on: “The IAU has unfortunately promoted the idea that science is a matter of opinion, not good in this day and age!”
This is why planetary scientists claim that it’s them, not astronomers, who should define what is a planet and what is not. “We are the scientists who actually study planets, as opposed to galaxies, stars, black holes, etc.,” planetary geologist Kirby Runyon of Johns Hopkins University tells OpenMind. “For instance, planetary scientists would never presume to vote on the definition of neutron star, which are studied by astrophysicists.”
Those who hold this position also argue that planets have historically been understood by their own traits. “Planets were originally defined dynamically as moving objects in the sky. Then Galileo came along and determined that they should really be considered like the Earth, geophysical,” says Sykes. “Asteroids continued to be referred to in the literature as a type of planet until Kuiper published a paper in 1953 that stated that asteroids were geophysically different from planets,” he adds.
The quest for a new definition
And this geophysical difference is based above all on one criterion: the spherical shape distinguishes a body with active geology from a mere piece of rock. In fact, studies of Pluto undertaken by New Horizons have shown that it is a very complex world, with several moons, an atmosphere, organic compounds, varied landscapes and possible liquid oceans under the icy surface.
Thus, planetary scientists support a geophysical definition, according to which: “A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.” Or more simply, a planet is a round celestial object smaller than a star. For his part, De la Fuente Marcos mentions a frequent objection to this approach, which is that it would force us to consider more than a hundred objects as planets, including numerous moons. “By that logic, there are too many countries in Europe, too many rivers and too many mountains!” responds Sykes. “Doesn’t that make planets more interesting?”
The geophysical definition of planet has already gained so much acceptance among planetary scientists that, “it is becoming the de facto standard,” says Stern. What’s more, although it’s a question mark whether the IAU will react in any way —the IAU presidency didn’t respond to OpenMind‘s questions— this doesn’t seem to worry planetary scientists. “No action by the IAU is necessary for the Geophysical Planet Definition to be acceptable or even official,” notes Runyon.
For Grinspoon, the IAU is not even the appropriate authority on this matter. And deep down, Sykes stresses, although the resolution has had a great impact on the media, its effects on the scientific community have been minimal: “Few people apply the IAU definition in their scientific work. It has little utility,” he says. “Scientists have not changed their behaviour or their language just because the IAU made a holy proclamation. I don’t care if the IAU changes its definition of planet or not,” he concludes.
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