News Space 12 September 2018

Bring back Pluto!


The decision to declare Pluto not a planet was fundamentally flawed, argue a team of astrophysicists. Andrew Masterson reports.


The IAU's decision to strip Pluto of planethood prompted demonstrations and protests.

The IAU's decision to strip Pluto of planethood prompted demonstrations and protests. 

NMSU / Darren Phillips

Pluto was unfairly robbed of its status as a planet and should be promptly rehabilitated, a team of astrophysicists say.

In a paper published in the journal Icarus, the team, led by Philip Metzger of the Florida Space Institute in the US, say the decision in 2006 by the International Astronomical Union (IAU) to reclassify Pluto as a “dwarf planet” was based on a flawed interpretation of the term “asteroid”.

Many people think Pluto was tripped of planetary status because of its size – diminutive, by planetary standards – but this is incorrect. The decision was made because the body does not meet one of the IAU’s primary definitions of a planet.

To be a planet, the union states, any celestial body must “clear” its orbit – that is, it must be the largest gravitational force within that orbit. Pluto’s orbit is affected by the gravitational pull of neighbouring planet Neptune, as well as nearby frozen gases and Kuiper Belt objects.

To Metzger and his colleagues, however, such a definition is indefensible, and flies in the face of centuries of lexicographical norms.

“It's a sloppy definition,” says Metzger. “They didn't say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”

To make their point, the researchers conducted a literature review covering texts from the nineteenth century to the present, investigating how the terms “planet” and “asteroid” were used.

They found that for 150 years, “asteroid” was used to connote a subset of “planet”. In the 1950s, however, there was a number of papers published that showed that asteroids were geophysically different to very large planets – and from then on, the two words came to imply different material realities.

This is significant, argues Metzger and his colleagues, because it shows that “consensus formed on the basis of geophysical differences between asteroids and planets, not the sharing of orbits”.

This, comments co-author Kirby Runyon of the Johns Hopkins University Applied Physics Laboratory, puts the lie to the IAU’s decision.

“We showed that this is a false historical claim,” he says. “It is therefore fallacious to apply the same reasoning to Pluto.”

Metzger also makes the point that the influences on the orbit of any planetary body are, by definition, dynamic, and change over time – even if that time is measured in hundreds of millions of years.

He suggests instead that a planet should be defined, and distinguished from an asteroid, with reference only to its intrinsic properties. Prime among these, he says, is the ability of a body to exert sufficient gravitational force to become spherical.

“And that's not just an arbitrary definition,” he explains. “It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.

“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research. And it would leave out the second-most complex, interesting planet in our solar system.”

  1. https://www.sciencedirect.com/science/article/pii/S0019103518303063?via%3Dihub#!
  2. https://www.iau.org/news/pressreleases/detail/iau0603/
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