New study rejects IAU rationale for demoting Pluto

Pluto’s status as a planet is still a matter of debate – but recent reports suggest its demotion might have been premature. Image Credit: NASA / JHUAPL / SwRI

A new study that has reviewed some 200 years of scientific literature regarding usage of the term “planet” calls into question the  planet definition adopted by the International Astronomical Union (IAU), which requires objects to “clear their orbits” to be considered planets.

Philip Metzger of the Florida Space Institute and lead author of the paper published in the journal Icarus, noted he found just one scientific paper paper among two centuries of studies on planet classification that used orbit clearing as a defining criterion for planet status, and that was published in 1802 by astronomer William Herschel, discoverer of the planet Uranus.

“Since Galileo’s discovery of Jupiter’s four largest moons, these moons and other spherical satellites of planets have been referred to as planets in scientific literature,” Metzger noted.

Even asteroids were considered a subclass of planets after their demotion in the mid-19th century. Only in the 1950s, after Gerard Kuiper published a paper arguing that asteroids and planets underwent different formation processes, were asteroids classified as non-planets.

“We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing so because it’s functionally useful,” he said.

Metzger and study co-authors Mark Sykes of the Planetary Science Institute (PSI) in Tucson, Arizona, Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado, and Kirby Runyon of the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Maryland, argue that planets should be defined by their intrinsic properties rather than by extrinsic properties, such as their orbital dynamics.

In their paper, they advocate a geophysical planet definition, according to which a planet is any celestial object that is not self-luminous (a star) and is large enough to be squeezed into a round or nearly round shape by its own gravity.

Runyon presented this definition at the 2017 and 2018 conferences of the Lunar and Planetary Science Institute (LPI).

“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be defined on the basis of a concept that nobody uses in their research. And it leaves out the second-most complex, interesting planet in our solar system,” Metzger said, referring to Pluto, which was studied by NASA’s New Horizons spacecraft during a July 2015 flyby.

New Horizons’ findings surprised scientists by revealing Pluto to be a geologically active world with organic compounds, evidence of ancient lakes, floating glaciers, a layered atmosphere, methane snow, and even a subsurface ocean. The latter makes it a potential host of microbial life, along with other solar system ocean worlds, including Ceres, Jupiter’s moon Europa, Saturn’s moons Titan and Enceladus, and Neptune’s moon Triton.

“It’s more dynamic and alive than Mars,” Metzger said regarding Pluto. “The only planet that has more complex geology is the Earth.”

From a geological standpoint, the threshold at which an object is rounded by its gravity is crucial because that is when active geology begins, he explained.

The IAU definition was adopted nine years before the New Horizons flyby, when many scientists still believed Pluto to be a dead rock. It is also problematic because it requires objects to orbit the Sun rather than simply a star, which excludes exoplanets, including rogue planets, which do not orbit any star and therefore have no orbits to clear.

Approximately 300 exoplanets had been discovered when the IAU definition was adopted. Now the number stands at close to 4,000.

In response to the article, IAU representative Lars Lindbergh Christensen said the authors, if they want to change the group’s definition, should propose a resolution through the appropriate IAU Working Groups and Divisions.

However, in their paper, Metzger and co-authors specifically reject the notion of voting by a central organization, which they describe as creating an “illusion of consensus,” in favor of an organic, ongoing process in which consensus is reached by discussion in scientific literature and conferences. To that end, they urge the IAU to rescind its 2006 resolution and not adopt any other in its place.

A new treasure trove of Pluto data may become available in the next few decades, as Stern, who is New Horizons principal investigator, and other scientists, are already advocating a return mission, this time with an orbiter.

Tagged: asteroids dwarf planets Johns Hopkins University Applied Physics Laboratory Kuiper New Horizons Planetary Science Institute planets Pluto The Range

Laurel Kornfeld

Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.