Back in 2006, to the puzzlement of many in the nonscientific public and some astronomers as well, the International Astronomical Union decided to demote Pluto from its status as a full-fledged planet in our solar system. Instead, the IAU decided, what had been considered the most distant of the nine planets actually belonged in the new category of dwarf planet, a category that also included Ceres, the largest object in the asteroid belt between Mars and Jupiter.
The reasoning, as explained in the IAU's Resolution B6, was that Pluto only had two of what the IAU decided were the three characteristics of a planet — that it is in orbit around the sun, that it has sufficient mass for its self-gravity to overcome rigid body forces and give it a nearly spherical shape, and that it has cleared the neighborhood around its orbit of other objects, meaning that it's either collided with, captured or driven away smaller objects nearby. Pluto flunked the IAU's last test, because it shares its orbit with thousands of smaller icy objects in the Kuiper Belt, a region that's between 2.5 and 4.5 billion miles (4.5 and 7.4 billion kilometers) away from the sun.
The IAU's decision, which was voted on by a very small percentage of the world's astronomers and planetary scientists, was a controversial one. After a debate among scientists sponsored by the Harvard-Smithsonian Center for Astrophysics in 2014, the majority of the nonexpert audience voted for a simpler definition of planet — basically, that it had to be spherical and orbit around a star or the remnants of one — that included Pluto, according to an article on the center's website.
And now, contention may well flare up again, thanks to a paper preprinted online in the February 2019 issue of the scientific journal Icarus, authored by University of Central Florida planetary physicist Philip Metzger, Planetary Science Institute director Mark Sykes, planetary scientist Alan Stern, who led NASA's New Horizons space probe mission to Pluto and the Kuiper Belt, and Johns Hopkins University planetary geomorphologist Kirby Runyon. They analyzed more than two centuries' worth of studies published by scientists, and found that, with the exception of a paper published in 1802 by British astronomer Sir William Herschel, nobody talked about non-sharing of an orbit as a criterion for distinguishing planets from asteroids. To the contrary, the researchers found that scientists routinely described asteroids as planets until the 1950s, "on the basis of new data showing asteroids' geophysical differences from large, gravitationally rounded planets."
"We therefore conclude that the argument made during the IAU planet definition controversy, that planet-sized Kuiper Belt Objects should be classified as non-planets because they share orbits, is arbitrary and not based on historical precedent," they wrote.
"If you were to abstract a definition of planet" from how it is used in the scientific literature it would be something like, 'Planets are objects that are large enough to be round' — with no consideration of where they are or what they orbit," Sykes explains in an email.
Demotion Largely Disregarded by Scientists
The IAU's demotion of Pluto largely has been disregarded by planetary scientists, Metzger notes in an email. "In science, we classify objects in ways that are scientifically useful," he says. "The definition that says Pluto is not a planet is not useful because scientists are not using it in their publications, but the definition that has existed since the time of Galileo, the one that most planetary scientists actually use, is very useful and we use it in our publications all the time. That definition from Galileo says that a planet is a geologically complex body like the Earth is. Pluto is most definitely a geologically complex body, fully worthy of the term 'planet' as Galileo and planetary scientists have used the word for the past 500 years."
Moreover, Metzger argues, the IAU's definition of a planet actually was a step backward, toward a pre-scientific view of nature. "People used to think the planets were a small number of gods ruling in their orbits," he explains. "Then scientists discovered the solar system is messy, that planets don't all orbit the sun, and that they kick each other around and share orbits with other objects. The IAU definition tries to emphasize the organization of a solar system, saying planets are the small number of objects that rule in their orbits. It communicates the wrong idea that organization is the central truth about solar systems. In fact, for a planet to clear its orbit, the process is contingent, incomplete, and often temporary."
Junking the IAU's definition wouldn't just reinstate Pluto as a planet. It would lead the way to including other objects — such as 2003 UB313, also known as Eris, a Kuiper Belt object 25 percent larger than Pluto discovered by California Institute of Technology astronomer Mike Brown — as well.
"A problem with the 2006 definition is that people have lost interest in the discovery of planets," Metzger says. "Very few people realize that there are over 150 planets in our solar system. People think, well, they are just left over junk like asteroids, so they are not important. As a result, the excitement is not taught in the classroom, and the public does not pay attention. But they are actually amazing planets like Pluto and Charon, and there are over 150 of them!"
And there's plenty that's interesting about Pluto, which Metzger described as the second most complex planet after Earth in the solar system. "Pluto has glaciers sliding down from the mountains. It has a multi-layered atmosphere with climate cycles," Runyon said. "It has mountains as big as the Rocky Mountains, and they are currently being built up. It has an ancient ice-lake with a paleo shoreline. It has sublimation pits in the ice with fantastical patterns that suggest convection is happening under the ice. There are organic molecules draped across its surface. There is evidence of an underground ocean. There must be a heat source to keep that ocean liquid. There is even a possibility that life could exist in that ocean."
In an email, co-author Runyon says that based on New Horizons' 2015 fly-by, there's still much to be learned about Pluto, in part because most of the planet's southern hemisphere was shrouded in winter darkness at the time, and other regions were in low resolution. "We also don't know if Pluto had or has a subsurface liquid ocean. Looking for a magnetic field, perhaps induced from the sun's weak magnetic field at that distance, could address that question, but we'd need to fly a magnetometer on the next spacecraft to visit Pluto," he says.
Beyond that, it's not known whether Pluto's features are unusual or representative of other small planets. "For instance, are most Kuiper Belt planets simple with just craters and fractures, like the moons of Uranus or Charon?" Runyon asks. "Or are they dynamic planets with convecting glaciers, photolysis deposition, sublimation-driven geology, snow, glacial and river valleys, complex tectonics (not necessarily plate tectonics), etc.? Triton, we think, used to be a Kuiper Belt planet and is now a satellite planet orbiting Neptune. It also has rich and varied active geology, e.g., geysers, but of a different nature than Pluto."
But Metzger isn't hopeful that the IAU will reconsider its decision, "because many of its members have become stubborn about it. This is why we aren't supposed to vote in science. Voting creates biases. Taxonomical classification is a part of science, so we should not allow biases to enter in. That is why it was a mistake to vote on the definition of a planet. It should have never happened."