Pluto is still a planet, according to NASA Administrator Jim Bridenstine. The announcement puts him in good company with noted human astronomer Jerry Smith (above right) who staked out a similar position on our solar system’s smallest and most distant dwarf planet. Smith was unavailable for comment, but Bridenstine was willing to speak out on the controversial topic anyway. While attending a FIRST robotics event in Colorado, Bridenstine stuck up for the diminutive orb.
“Just so you know, in my view, Pluto is a planet. And you can write that the NASA Administrator declared Pluto a planet once again. I’m sticking by that, it’s the way I learned it, and I’m committed to it,” Bridenstine said.
Bridenstine is, of course, wrong. Bridenstine is, of course, right. How you read the situation depends on which sources of authority you credit and how much you care about things like being aligned with the opinions of scientists and astronomers. But before we discuss that epistemological point, let’s talk about planets. Why isn’t Pluto a planet any longer?
The Problem With Planets
Once upon a time, the number of planets considered to be part of the solar system was far higher than it is today. Improvements to telescope design throughout the 1800s soon meant that astronomers were swimming in “planets,” including objects like Vesta, Ceres, and Juno. At the time of their discoveries, all of these asteroids were considered to be planets. The discovery of 10 Hygea was hailed in the 1850 Annual of Scientific Discovery, which declares that the solar system is now comprised of 18 planets — more than double the number we recognize today.
As for Pluto, it has a Kuiper Belt problem. In 1992, scientists discovered 15760 Albion and with it, confirmed the existence of the Kuiper Belt. It was the first trans-Neptunian object (TNO) to be discovered after Pluto and Charon, but it was far from the last. There are now more than 2,000 TNOs known to exist, and while Pluto bears little resemblance to any of the inner planets, its erratic orbit and general characteristics fit the template of a TNO perfectly. (Fun fact: Neptune’s moon Triton is a very similar world to Pluto, as far as its composition and geology and is regarded as a captured TNO that likely wreaked havoc on whatever moon system Neptune possessed before it was captured by the gas giant).
Pluto, as imaged by New Horizons.
Many of these TNOs are close to Pluto in size and shape. To take them into account, we either needed to once again drastically expand the number of planets in the solar system or define the word in a way that would exclude objects like Sedna, Qaoar, Eris, Makemake, and Haumea. In 2006, the International Astronomers’ Union decided to formalize the definition of a planet. It considered a number of proposals, including some that would have recognized the dwarf planet Ceres and even potentially 4 Vesta as a planet.
Ultimately, the IAU decided that a planet had three distinguishing characteristics:
1. It is in orbit around the Sun.
2. It has sufficient mass to be in hydrostatic equilibrium (it must be rounded by the effects of its own gravity).
3. It must have “cleared the neighborhood” around its orbit.
Clearing the neighborhood means that the planet is gravitationally dominant in its own local system. The Moon is much larger than a typical satellite for a planet Earth’s size should be — that’s one of the reasons we think it formed in an unusual way — but Earth still completely dominates the Earth-Moon gravitational system.
Point #2 disqualifies an object like 4 Vesta from being a planet because Vesta is not (quite) in hydrostatic equilibrium. Point #3 knocks out objects like Ceres and Pluto. Ceres is in the midst of the asteroid belt, while Pluto’s barycenter with its moon, Charon, is outside of Pluto itself. Charon does not orbit Pluto — Charon and Pluto orbit a common point in space, above both of their surfaces.
Therefore, according to the IAU, Pluto is not a planet because it does not meet the third qualification.
But Bridenstine’s declaration that Pluto is a planet because that’s what he was taught is a common way for people to understand this situation as well. One common cognitive fallacy that humans fall prey to is anchoring bias. It’s our tendency to recall the first piece of information we learned about a thing, whether that information is true or not, and it “anchors” our perceptions of later information that we are presented with. Just because we don’t classify Pluto as a planet any longer doesn’t mean Pluto doesn’t “feel” like a planet, for lack of a better phrase.
I sometimes wonder if the entire issue would have been less controversial if scientists had communicated that the number of planets was going to have to be changed, no matter what. People who get unhappy about Pluto not being a planet often fixate on the number of planets in Earth’s solar system, as though nine were a better number than eight. One wonders how they would have reacted to discovering that instead of nine, the new appropriate number was well over 20. If Pluto and the TNOs are planets, then objects like Ceres would also have a strong claim to the title as well. One suspects the defenders of Nine Planet Theory would be just as unhappy with that world as they are today, with one critical difference: By declaring there are just 8 planets, astronomers avoided asking everyone to memorize a dozen new names.