Six New Horizons scientists propose geophysical planet definition

Laurel Kornfeld

Six scientists who work on NASA’s New Horizons mission propose a geophysical planet definition in an abstract from a presentation to be made at the Lunar and Planetary Science Conference.

Their proposal addresses several issues they view as problematic regarding the 2006 International Astronomical Union (IAU) planet definition, which is considered a dynamical one because it places primacy on an object’s location.

Among its problems are the fact that it recognizes only objects orbiting the Sun as planets, precluding the nearly 2,000 exoplanets orbiting other stars discovered over the last 20 years, as well as rogue planets, which float freely in space without orbiting any star.

Definition of planet

The IAU’s third criterion for planet status – that an object has cleared its orbit – also precludes all Solar System planets because all have small objects frequently enter their paths, the authors note. Also, the further an object is from the Sun, the larger an orbit it has to clear, meaning objects would have to get larger and larger to be considered planets the further from the Sun they orbit.

“[E]ven an Earth-sized object in the Kuiper Belt would not clear its zone,” the scientists note in the article.

As an alternative, the New Horizons scientists outline a definition centered on the physics of individual worlds and their intrinsic properties.

“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,” they state. For teachers of elementary school children, this can be paraphrased as “round objects in space that are smaller than stars”.

“Our geophysical definition is directly based on the physics of the world itself rather than the physics of its interactions with external objects,” the scientists note.

Classification of brown dwarfs – intermediate objects bigger than the largest planets but less massive than the smallest stars – is left to the future, when more will be known about these enigmatic objects.

The authors maintain that to much of the public, the term “planet” conveys a historic importance that inherently makes an object more interesting.

Additionally, the geophysical definition already predominates in common usage, not just among the public but also in the planetary science community.

To support this notion, the authors cite peer-reviewed articles that use the term “planet” for Saturn’s moon Titan and dwarf planets Pluto, Haumea, Makemake, and Eris.

A new suggestion within the proposal calls for identifying spherical moons of planets as “moon planets”.

Noting the eight planets recognized by the IAU are all modified by adjectives, such as “terrestrial”, “gas giant”, and “ice giant”, they propose the same recognition for dwarf planets and moon planets.

Interestingly, spherical moons were considered “secondary planets” by many 19th century astronomers. Stern, who coined the term “dwarf planet”, intended it to designate a subclass of planets.

While their proposal results in the Solar System having a minimum of 110 planets, this should not be viewed as a problem because teaching students should center on giving them an understanding of the Solar System’s natural organization rather than asking them to memorize a list of objects, the authors state.

In their many discussions of the issue with members of the public, the New Horizons’ scientists point out that most people, including policymakers, resonate intuitively with the geophysical definition.

Planetary scientists, who often deal with the geology of other worlds, are also more aligned with the geophysical definition, they added.

No statement was made as to whether the group intends to present its definition to the IAU at a future General Assembly.

The writers include mission Geology and Geophysics Imaging team member Kirby Runyon of Johns Hopkins University in Baltimore, Maryland; Principal Investigator Alan Stern of the Southwest Research Institute of Boulder, Colorado; Geology and Geophysics Imaging team member Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona; Co-investigator Will Grundy of the Lowell Observatory in Flagstaff, Arizona; Co-investigator Michael Summers of George Mason University in Fairfax, Virginia, and Co-investigator Kelsi Singer, also of the Southwest Research Institute.