Pluto: Is It Still Considered a Planet?

In 2006, much to the surprise of both the public and some astronomers, the International Astronomical Union reclassified Pluto, demoting it from its position as the farthest planet in our solar system. It was instead placed in the newly formed category of dwarf planets, a classification that also includes Ceres, the largest object in the asteroid belt between Mars and Jupiter.

According to the IAU's Resolution B6, the rationale behind Pluto's reclassification was that Pluto fulfilled only two of the three criteria required to be a planet: it orbits the sun, and its mass is sufficient to form a nearly spherical shape. However, Pluto failed the third criterion, as it shares its orbit with numerous smaller icy bodies in the Kuiper Belt, a distant region of space lying 2.5 to 4.5 billion miles (4.5 to 7.4 billion kilometers) from the sun.

The IAU's decision, made by a small percentage of the world's astronomers and planetary scientists, sparked controversy. Following a 2014 debate hosted by the Harvard-Smithsonian Center for Astrophysics, the majority of the nonexpert audience supported a simpler definition of a planet — essentially, it must be spherical and orbit a star or its remnants — a definition that would include Pluto, according to an article on the center's website.

Now, the debate may reignite due to a paper released 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 mission to Pluto and the Kuiper Belt), and Johns Hopkins University planetary geomorphologist Kirby Runyon, the paper analyzes more than 200 years of scientific research. They discovered that, aside from an 1802 paper by British astronomer Sir William Herschel, the non-sharing of an orbit was never considered a criterion for distinguishing planets from asteroids. In fact, scientists routinely classified asteroids as planets until the 1950s, based on new data highlighting the geophysical differences between asteroids and larger, gravitationally rounded planets.

"We therefore conclude that the argument made during the IAU planet definition controversy, that planet-sized Kuiper Belt Objects should be excluded as planets due to their shared orbits, is arbitrary and lacks historical foundation," the authors wrote.

"If you were to distill a planet definition" from scientific literature, it would likely be something like, 'Planets are objects large enough to be spherical,' with no regard for their location or what they orbit," Sykes explains via email.

Demotion Largely Disregarded by Scientists

The IAU's decision to demote Pluto has largely been dismissed by planetary scientists, according to Metzger in an email. "In science, we classify objects in ways that are scientifically meaningful," he states. "The definition that excludes Pluto as a planet is not helpful because it's not being used in scientific publications. However, the definition that's been in use since Galileo's time — the one most planetary scientists still rely on — is highly useful and appears in our research frequently. This definition from Galileo describes a planet as a geologically complex body, similar to Earth. Pluto undoubtedly fits this description, making it fully deserving of the 'planet' designation as understood by Galileo and planetary scientists for the past 500 years."

Additionally, Metzger argues that the IAU's definition of a planet is actually a step backward, resembling a pre-scientific understanding of nature. "Historically, people thought planets were a small number of gods in their orbits," he explains. "Then, scientists realized the solar system was much messier — planets don't all orbit the sun, and they interact with each other and share their orbits with other objects. The IAU's definition attempts to focus on the solar system's organization, claiming that planets are the few objects that dominate their orbits. This misleadingly suggests that organization is the defining characteristic of solar systems. In reality, the process of a planet clearing its orbit is contingent, incomplete, and often temporary."

Discarding the IAU's definition wouldn't just reinstate Pluto as a planet, but it would also open the door to recognizing other objects, such as 2003 UB313, also known as Eris, a Kuiper Belt object discovered by Caltech astronomer Mike Brown that is 25% larger than Pluto.

"One issue with the 2006 definition is that it's led to a waning interest in the discovery of planets," says Metzger. "Few people realize there are over 150 planets in our solar system. The public often dismisses them as leftover debris like asteroids, which makes them seem unimportant. As a result, the excitement isn't conveyed in classrooms, and the public remains indifferent. However, these are truly remarkable planets, such as Pluto and Charon, and there are more than 150 of them!"

Pluto is a fascinating world, and Metzger calls it the second most complex planet in the solar system after Earth. "Pluto has glaciers flowing down from towering mountains. Its multi-layered atmosphere experiences climate cycles," Runyon explained. "There are mountains as tall as the Rockies that are still growing. It features an ancient ice-lake with a paleo shoreline. Sublimation pits with intricate patterns suggest convection is taking place beneath the ice. Organic molecules cover its surface. There's evidence of an underground ocean, and a heat source must exist to keep it liquid. There’s even a chance that life could exist in that ocean."

In an email, co-author Runyon mentions that, based on New Horizons' 2015 fly-by, there is still much to discover about Pluto, partly because the planet's southern hemisphere was in complete winter darkness at the time, and other regions were captured in low resolution. "We also don't know whether Pluto has, or ever had, a subsurface liquid ocean. Searching for a magnetic field, potentially influenced by the sun's weak magnetic field at that distance, could answer this. However, we'd need to fly a magnetometer on the next spacecraft that visits Pluto," he explains.

Moreover, it's unclear if Pluto's features are unique or typical of other small planets. "For example, are most Kuiper Belt planets simple, marked only by craters and fractures like Uranus' moons or Charon?" Runyon wonders. "Or are they dynamic planets with active features like convecting glaciers, photolysis deposition, sublimation-driven geology, snow, valleys from glaciers and rivers, and complex tectonics (not necessarily plate tectonics)? Triton, which we think was once a Kuiper Belt planet and now orbits Neptune as a satellite, also exhibits rich, active geology like geysers, but its nature differs from Pluto's."

However, Metzger isn't optimistic that the IAU will revisit its decision, saying, "Many members have become too entrenched in their stance. This is why voting isn't appropriate in science. Voting introduces biases. Taxonomical classification is a scientific process, and biases should not influence it. That's why it was a mistake to vote on the definition of a planet. It should never have happened."