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Six New Horizons scientists propose geophysical planet definition

Figure 1. Every discovered 'planet' in our Solar System under 10,000 km in diameter, to scale.

Every discovered ‘planet’ in the Solar System under 10,000 km in diameter, to scale. The geophysical definition of a ‘planet’ includes ~110 known ‘planets’ in the Solar System. Image Credit: Emily Lakdawalla / The Planetary Society

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.

Pluto-Charon false color comparison

Pluto-Charon (false color) size comparison. Image Credit: NASA / JHUAPL / SwRI

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.

 

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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.

Reader Comments

FYI: Lunar and Planetary Science is not a journal. The link is to a conference abstract for the upcoming Lunar and Planetary Science Conference which will be held the week of March 20, 2017.

“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” – That’s clearly NOT the case though. It’s like beating a dead horse – the whole topic was explained dozens of times, even Wikipedia by now has an in-depth coverage for why it’s not the case – https://en.wikipedia.org/wiki/Clearing_the_neighbourhood

Yes, there is no solid line between the dwarf planet and a planet, the specific criteria of where the cut should be made can be argued about (and I’m certain it’ll be made somewhere in the enormous gap between Pluto/Ceres and Mars), but that’s about it.

“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.” – With rouge planets it’s not a problem what so ever. With exoplanets I can agree, it might become a problem once we colonise another solar system, but it’s nothing Stern or his grandchildren will be concerned about during their lifetime, and even then the “fix” is very obvious: you replace “the Sun” with “star” and “solar system” with “star system”.

“While their proposal results in the Solar System having a minimum of 110 planets” – what a disaster… I doubt it will be discarded with more votes in favour than Boss’es definition.

I understand their desire for making all these round worlds be planets: it looks like they all, such as Pluto, Titan, Europa, Enceladus, and Ceres, have complex internal geophysics.

Still I prefer to have a planet orbit a star not a planet.

Bob Clark

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