Nav: Home

Scientists make the case to restore Pluto's planet status

March 17, 2017

Johns Hopkins University scientist Kirby Runyon wants to make one thing clear: Regardless of what one prestigious scientific organization says to the contrary, Pluto is a planet.

So, he says, is Europa, commonly known as a moon of Jupiter, and so is the Earth's moon, and so are more than 100 other celestial bodies in our solar system that are denied this status under the prevailing definition of "planet."

The definition approved by the International Astronomical Union in 2006 demoted Pluto to "non-planet," thus dropping the consensus number of planets in our solar system from nine to eight. The change - a subject of much scientific debate at the time and since - made no sense, says Runyon, lead author of a short paper making the pro-Pluto argument that will be presented next week at a scientific conference in Texas.

Icy, rocky Pluto had been the smallest of the nine planets, its diameter under three-quarters that of the moon and nearly a fifth of Earth. Still, says, Runyon, who is finishing his doctorate this spring in the Department of Earth and Planetary Sciences, Pluto "has everything going on on its surface that you associate with a planet. ... There's nothing non-planet about it."

Runyon, whose doctoral dissertation focuses on changing landscapes on the moon and Mars, led a group of six authors from five institutions in drafting a proposed new definition of "planet," and a justification for that definition. Both will be presented at the Lunar and Planetary Science Conference's poster session. The poster will be on view for a full day on March 21 at the conference sponsored by the Lunar and Planetary Institute, and Runyon will be on hand for at least three hours to answer questions about it.

The other authors are S. Alan Stern and Kelsi Singer of the Southwest Research Institute in Boulder, Colorado; Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona; Will Grundy of the Lowell Observatory in Flagstaff, Arizona; Michael Summers of George Mason University in Fairfax, Virginia. All the authors are science team members on the New Horizons mission to Pluto, operated for NASA by the Johns Hopkins University Applied Physics Laboratory. In the summer of 2015, the New Horizons spacecraft became the first to fly by Pluto, some 4.67 billion miles from Earth, passing within 8,000 miles and sending back the first close-up images ever made of Pluto.

Runyon and his co-authors argue for a definition of "planet" that focuses on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it. They define a planet as "a sub-stellar mass body that has never undergone nuclear fusion" and that has enough gravitational heft to maintain a roughly round shape. (Even if it bulges at the equator because of a three-way squeeze of forces created by its own gravity and the influence of both a star and a nearby larger planet.)

This definition differs from the three-element IAU definition in that it makes no reference to the celestial body's surroundings. That portion of IAU's 2006 formula - which required that a planet and its satellites move alone through their orbit - excluded Pluto. Otherwise, Pluto fit the IAU definition: It orbits the sun and it is massive enough that the forces of gravity have made it round.

Stern, the principal investigator for the New Horizons mission, has argued in the past that the IAU definition also excludes Earth, Mars, Jupiter and Neptune, which share their orbits with asteroids.

The proposed new geophysical definition omits stars, black holes, asteroids and meteorites, but includes much of everything else in our solar system. It would expand the number of planets from eight to approximately 110.

That expansion is part of the appeal of the new definition, Runyon says. He says he would like to see the public more engaged in solar system exploration. As the very word "planet" seems to carry a "psychological weight," he figures that more planets could encourage that public interest.

The new definition, which does not require approval from a central governing body, is also more useful to planetary scientists. Most of them are closely affiliated with geology and other geosciences, thus making the new geophysical definition more useful than the IAU's astronomical definition.

He has some reason to be optimistic, as the new definition has already been adopted by Planet Science Research Discoveries, an educational website founded by scientists at the University of Hawaii at Mānoa.

"I want the public to fall in love with planetary exploration as I have," Runyon said. "It drives home the point of continued exploration."
-end-


Johns Hopkins University

Related Solar System Articles:

From rocks in Colorado, evidence of a 'chaotic solar system'
Plumbing a 90 million-year-old layer cake of sedimentary rock in Colorado, a team of scientists from the University of Wisconsin-Madison and Northwestern University has found evidence confirming a critical theory of how the planets in our solar system behave in their orbits around the sun.
Why are there different 'flavors' of iron around the Solar System?
New work from Carnegie's Stephen Elardo and Anat Shahar shows that interactions between iron and nickel under the extreme pressures and temperatures similar to a planetary interior can help scientists understand the period in our Solar System's youth when planets were forming and their cores were created.
Does our solar system have an undiscovered planet? You can help astronomers find out
ASU's Adam Schneider and colleagues are hunting for runaway worlds in the space between stars, and citizen scientists can join the search with a new NASA-funded website.
Rare meteorites challenge our understanding of the solar system
Researchers have discovered minerals from 43 meteorites that landed on Earth 470 million years ago.
New evidence on the formation of the solar system
International research involving a Monash University scientist is using new computer models and evidence from meteorites to show that a low-mass supernova triggered the formation of our solar system.
Planet Nine could spell doom for solar system
The solar system could be thrown into disaster when the sun dies if the mysterious 'Planet Nine' exists, according to research from the University of Warwick.
Theft behind Planet 9 in our solar system
Through a computer-simulated study, astronomers at Lund University in Sweden show that it is highly likely that the so-called Planet 9 is an exoplanet.
Studying the solar system with NASA's Webb Telescope
NASA's James Webb Space Telescope will look across vast distances to find the earliest stars and galaxies and study the atmospheres of mysterious worlds orbiting other stars.
'This solar system isn't big enough for the both of us.' -- Jupiter
It's like something out of an interplanetary chess game. Astrophysicists at the University of Toronto have found that a close encounter with Jupiter about four billion years ago may have resulted in another planet's ejection from the Solar System altogether.
IBEX sheds new light on solar system boundary
In 14 papers published in the October 2015 Astrophysical Journal Supplement, scientists present findings from NASA's Interstellar Boundary Explorer, or IBEX, mission providing the most definitive analyses, theories and results about local interstellar space to date.

Related Solar System Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Digital Manipulation
Technology has reshaped our lives in amazing ways. But at what cost? This hour, TED speakers reveal how what we see, read, believe — even how we vote — can be manipulated by the technology we use. Guests include journalist Carole Cadwalladr, consumer advocate Finn Myrstad, writer and marketing professor Scott Galloway, behavioral designer Nir Eyal, and computer graphics researcher Doug Roble.
Now Playing: Science for the People

#530 Why Aren't We Dead Yet?
We only notice our immune systems when they aren't working properly, or when they're under attack. How does our immune system understand what bits of us are us, and what bits are invading germs and viruses? How different are human immune systems from the immune systems of other creatures? And is the immune system so often the target of sketchy medical advice? Those questions and more, this week in our conversation with author Idan Ben-Barak about his book "Why Aren't We Dead Yet?: The Survivor’s Guide to the Immune System".