Scientists make the case to restore Pluto's planet status

March 17, 2017 by Arthur Hirsch
The Long Range Reconnaissance Imager aboard NASA's New Horizons spacecraft captured this photo of Pluto's surface in 2015. Credit: NASA / JHUAPL / Southwest Research Institute

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

The approved by the International Astronomical Union in 2006 demoted Pluto to "non-planet," thus dropping the number of planets in our solar system from nine to eight. The change—a subject of much scientific debate—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 of the moon and nearly a fifth of the Earth. Still, says, Runyon, who is finishing his doctorate this spring in the Krieger School of Arts and Sciences' 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 the definition and its justification that 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.

This processed image is the highest-resolution color look yet at the haze layers in Pluto’s atmosphere. Shown in approximate true color, the picture is constructed from a mosaic of four panchromatic images from the Long Range Reconnaissance Imager splashed with Ralph/Multispectral Visible Imaging Camera four-color filter data, all acquired by NASA’s New Horizons spacecraft on July 14, 2015. The resolution is 0.6 miles per pixel; the sun illuminates the scene from the right. Credit: NASA

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; and 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 closeup 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. In a short paragraph, 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 gravity and the influence of both the sun 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—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.

In the center left of Pluto’s vast heart-shaped feature lies a vast, craterless plain that appears to be no more than 100 million years old and is possibly still being shaped by geologic processes. Credit: NASA / JHUAPL / SOUTHWEST RESEARCH INSTITUTE

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

The new geophysical definition omits stars, black holes, asteroids, and meteorites, but it includes 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, says Runyon. He says he would like to see the public more engaged in exploration. As the very word "planet" seems to carry a "psychological weight," he figures that more 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.

New close-up images taken by NASA's New Horizons spacecraft of a region near Pluto’s equator reveal a range of youthful mountains rising as high as 11,000 feet (3,500 meters) above the planet's surface. Credit: NASA / JHUAPL / SOUTHWEST RESEARCH INSTITUTE

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," said Runyon. "It drives home the point of continued exploration."

Explore further: A geophysical planet definition

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Rockguy
3.1 / 5 (8) Mar 17, 2017
I read the book by Mike Brown "Why I killed Pluto and why it had it coming" (ISBN-13: 978-0385531108). I found his argument compelling and thorough. With all do respect to the New Horizons scientists I believe this is an emotional bias. Seriously do you want 110 planets? Anyways Pluto is a dwarf planet, so it is still a type of planet.
Shakescene21
2.8 / 5 (5) Mar 17, 2017
I found Mike Brown's arguments to be arbitrary and small-minded.

What's the problem with having 110 "planets"? The more we explore, the more we discover.
This issue is really about definitions rather than basic science.
JongDan
4 / 5 (7) Mar 17, 2017
What's the problem with having 110 "planets"? The more we explore, the more we discover.
This issue is really about definitions rather than basic science.


The problem is that the whole thing becomes inexplicably messy when it is obvious that those "planets" don't all share the same degree of importance regarding celestial dynamics. Satellite worlds (like Europa) are directly bound to another planet, and Pluto shares orbit with other similar objects. Even the line between what constitutes a large satellite and what is a true binary planet is thin – if one goes by the barycentre-above-surface definition, then pairs that are further apart require smaller size of secondary component to satisfy that definition? I think the criterion of "clearing the neighbourhood" is a good criterion.
But I agree that "dwarf planet" is a bad term. It also translates bad into other languages. I'd rather go with Asimov's term "mesoplanet" – which can again be expanded to include satellites.
SiaoX
2.3 / 5 (6) Mar 17, 2017
If we can make planet from Moon, why not Pluto? Pluto looks way richer than Moon.
sirdumpalot
3.4 / 5 (5) Mar 17, 2017
Wasn't it the case that the turn out for the vote was only around 5%, because it was held at the end of whatever conference that was happening, and more or less everyone who had the right to vote left beforehand??
nkalanaga
4.2 / 5 (5) Mar 17, 2017
I still say that, if you don't want 110 planets in the Solar System, simply declare anything orbiting a larger planet to be a moon, not a planet. That eliminates most of them right there, without affecting the status of what most people would call a "planet".

Then we'd have giant/jovian planets, terrestrial planets, and dwarf planets, all of which would be classed as "planets". At least in English, it's silly that a "dwarf planet" is NOT a planet.
IronhorseA
2 / 5 (3) Mar 17, 2017
Also, definitions are like scientific theories, and should follow Occam's razor. Mike brown added an arbitrary addition to what the working committee had developed which made the definition more complicated. Also, its subject to change as the other bodies 'sharing' an objects orbit might eventually be swept out given enough time, whereas a definition should be designed to last.
Mark Thomas
2 / 5 (4) Mar 17, 2017
"Seriously do you want 110 planets?"

LOL, the Milky Way probably has more than trillion, even by the current definition.

Using the proposed definition plus excluding moons and setting the minimum radius to be 1,000 kilometers, we have 10 known planets in our solar system, including Pluto and Eris.
arcmetal
2.6 / 5 (5) Mar 18, 2017
Wasn't it the case that the turn out for the vote was only around 5%, because it was held at the end of whatever conference that was happening, and more or less everyone who had the right to vote left beforehand??

That's exactly how it happened. Making the results that followed kind of silly.
arcmetal
2 / 5 (4) Mar 18, 2017
"Seriously do you want 110 planets?"

LOL, the Milky Way probably has more than trillion, even by the current definition.

Using the proposed definition plus excluding moons and setting the minimum radius to be 1,000 kilometers, we have 10 known planets in our solar system, including Pluto and Eris.


I believe that with the current rules there are 8 known planets in the entire universe.

It is as if we decided that there are only 3 rivers in the entire world, the Nile, the Yangtze, and the Amazon.... The rest are dwarf rivers.
arcmetal
2.3 / 5 (6) Mar 18, 2017
I read the book by Mike Brown "Why I killed Pluto and why it had it coming" (ISBN-13: 978-0385531108). I found his argument compelling and thorough. With all do respect to the New Horizons scientists I believe this is an emotional bias. Seriously do you want 110 planets? Anyways Pluto is a dwarf planet, so it is still a type of planet.

Well, by using these current rules (not clearing its orbit, etc.) the Earth is also a dwarf planet. (or not a planet at all since its the ground beneath us and not... whatever).
nkalanaga
3 / 5 (4) Mar 18, 2017
By the original Greek definition of "planet", Earth wasn't one, but the Sun and Moon were.

And, by the current IAU definition, a "dwarf planet" is NOT a "planet", which is the main objection many people have to their definition. It is illogical in most languages! Simply declaring "dwarf planets" to be a subclass of planets would solve the entire issue, but that's what many supporters of the current definition don't want.
FredJose
1.4 / 5 (9) Mar 18, 2017
craterless plain that appears to be no more than 100 million years old and is possibly still being shaped by geologic processes.

I like how the caption on the pluto image tries to hide the conundrum faced by scientists today of having this small, virtually insignificant little rock out there in the middle of nowhere still show seismic activity - in complete defiance of a supposed 4.6 billion years age for the solar system.

Even if one were to be generous and assume a middling age of only 1 billion years for Pluto, it should be pretty much dead by now, with no geological activity to be seen whatsoever.
The idea of even one 100 million years of existence of the surface itself is simply red herring trying to deflect attention from the fact that Pluto is the very antithesis of a nebular planetary formation theory.
arcmetal
2.6 / 5 (5) Mar 18, 2017
I liked Alan Stern's definition for how to categorize an object in space. He called it the "Star Trek" definition.... If you where on the bridge of the Enterprise and you came upon Pluto, would you call it a planet, an asteroid, a comet, or maybe debri?
TheGhostofOtto1923
5 / 5 (9) Mar 18, 2017
Even if one were to be generous and assume a middling age of only 1 billion years for Pluto, it should be pretty much dead by now, with no geological activity to be seen whatsoever.
-And you know this... how? Are you assuming that its true because it simply must be true, that it only makes sense as any reasonable person would agree, and so who needs evidence?

And also because it supports your preconceived notion that the creator just twitched his nose and the universe appeared poof?
Tony Lance
5 / 5 (2) Mar 18, 2017
110 planets proposed would include 8 planets and following,
which alas are two short of the 110. Best I could do.
Mike Brown's Dwarf Planet database 1 to 75 (500km or more)
http://web.gps.ca...ml#table
remaining 25 to total 108 as follows:-
Moon, Charon, Ganymede, Titan, Callisto, Triton,
Europa, Io, Titania, Rhea, Oberon, Iapetus, Umbriel,
Ariel, Dione, Tethys, Ceres, Vesta, Miranda, Proteus,
Enceladus, Hygeia, Mimas, Huya, Pallas.
Mark Thomas
3 / 5 (5) Mar 18, 2017
Planet: "A non-fusing, non-moon, distinct object with a minimum radius of 1,000 kilometers."

Short, sweet and robust. Excludes stars, brown dwarfs, moons and asteroids. Covers only gravitationally-rounded objects as a practical matter. Better still, Includes traditional planets, exoplanets AND rogue planets. Leaves us with ten (10) currently known planets in the solar system, which the public should be able to accept and remember. There is a very significant drop off in radius between Eris (radius ~1,163 km) and the next largest such object, 2007 OR 10 (radius ~750 km).

https://en.wikipe...eter.svg

A little more subtly, with the addition of just Eris to the prior panoply of planets, perhaps we can begin to think about sending a New Horizons-type probe to the only unvisited planet in the solar system (Eris). This definition makes exploration of Eris more likely. Think about it.
Eikka
5 / 5 (3) Mar 18, 2017
Even if one were to be generous and assume a middling age of only 1 billion years for Pluto, it should be pretty much dead by now, with no geological activity to be seen whatsoever.


Pluto's surface geology is 98% nitrogen ice - not rock - on top of a layer of water ice. The surface geology is driven by cryovolcanism.
nkalanaga
4.5 / 5 (4) Mar 18, 2017
Sounds good to me, although you may want to rephrase "non-fusing" as "never had fusion". After all, white dwarfs and black holes are "non-fusing"...
Mark Thomas
3 / 5 (3) Mar 18, 2017
nkalanaga, I agree with the point you are making, and I struggled to get the correct language here. I would define "non-fusing" only slightly more precisely as never having had continuous fusion conditions. Note that the word "never," being an absolute, makes me a little uncomfortable. For example, Jupiter is only about 13 times too small to sustain deuterium fusion. Since fusion requires quantum mechanical tunneling even in our sun, there may be an occasional fusion between deuterium nuclei inside Jupiter. Maybe a very low level of fusion is playing a role in the amount of heat Jupiter is giving off. Until we can get a neutrino detector close to Jupiter, it is hard to be certain.

To be candid, the non-moon term is also a little fuzzy because at some point a moon becomes so large that we are looking at double planet situation. So just how big does a moon have to be before it becomes a double planet? I don't have an answer for that at the moment.
ding
3.8 / 5 (5) Mar 18, 2017
No offence, but I don't really care about semantics. They're all celestial bodies to me.
nkalanaga
5 / 5 (1) Mar 18, 2017
Mark: Yes, I can see your point. In theory, fusion could occur on Earth, one reaction at a time, from quantum effects and cosmic ray impacts. I probably should have said "never had sustained fusion", which would be even more long-winded.

For the planet-moon/double-planet division, that probably comes down to "I'll know it when I see it", which certainly isn't a scientific definition. I can think of two solutions, neither perfect:

Barycentric definition: If the pair's barycenter is inside one of the bodies, it's a planet-moon relationship. Earth and Moon fall in that category. Pluto-Charon's barycenter is between the two, in space, so they would be a double planet.

Mass: Define a mass or radius ratio. 1:8 mass would give a radius ratio of about 2:1, so any time one is more than twice the diameter of the other, it's a planet-moon case. Pluto-Charon just barely qualify as a double planet under this one as well. The Moon is still a moon.

eachus
not rated yet Mar 18, 2017
To be candid, the non-moon term is also a little fuzzy because at some point a moon becomes so large that we are looking at double planet situation. So just how big does a moon have to be before it becomes a double planet? I don't have an answer for that at the moment.


The size of the Moon? Seriously, when you look at the interactions between Earth and the Moon that move them closer together and further apart, it is hard to make them fit any other category than double/twin planets. Especially since the current model for the Moon's formation has a Mars sized object strike the proto-Earth a glancing blow, and the rubble in orbit eventually becoming the Moon. Change the parameters slightly and you can get identically sized planets.

Or to put it more simply, two proto-planets collided, and this is what's left. ;-)
SiaoX
Mar 19, 2017
This comment has been removed by a moderator.
Enthusiastic Fool
1 / 5 (1) Mar 19, 2017
Does the current definition of planet account for close binary systems that have otherwise "cleared" their orbit except of each other? Is there a resonance that would allow two objects to clear their orbit but be on opposite sides of their orbit as a kind of far binary? If an object shared a barycenter with another above the surface of both objects is it a binary?(Sun & Jupiter) Not that my opinion matters but if the current system had dwarves as a class of planet it'd fix up a lot of the drama. I'd also be for an expanded count that was just substellar mass that's self-rounded.
TMcGrath
3 / 5 (2) Mar 19, 2017
One of the things the IAU left out of their definition was the maximum size of a planet. I would revise the definition of a "planet" as follows:

1) The object must orbit its parent star directly;
2) The object must be massive enough to achieve hydrostatic equilibrium; and
3) The object must not so massive that it fuses deuterium.

If the object is not orbiting its parent star directly, then it is a moon. The object must be large enough to achieve a round shape. We can still use whatever adjective that is appropriate, such as "dwarf," or "minor," or "giant," etc., it is still a planet. Lastly, there has to be a maximum mass for a planet, and that should be before the object fuses deuterium because that is the point when the object becomes a brown dwarf.
TMcGrath
1 / 5 (1) Mar 19, 2017
I liked Alan Stern's definition for how to categorize an object in space. He called it the "Star Trek" definition.... If you where on the bridge of the Enterprise and you came upon Pluto, would you call it a planet, an asteroid, a comet, or maybe debri?

I would call it a "collisional family." Something smacked Pluto - hard!

https://en.wikipe...l_family
Daein
2 / 5 (1) Mar 19, 2017
I think the word they are looking for is "world" and not "planet." The current definition of a planet would be difficult to extend to situations outside our solar system. For instance, it's possible to have a solar system were a Mimas sized world is considered a planet while a Saturn sized planet isn't as long as there are many other sized planets in a substantially similar orbital path. I don't think the solution is to call every spherical body a planet. I actually think the solution would simply be to separate the orbital classifications from the physical ones.
Mark Thomas
5 / 5 (1) Mar 19, 2017
nkalanaga, makes sense to me. So either define "non-fusing" as "never had sustained fusion" or simply insert those words into the definition. Regarding the moon definition, you make good points. As you essentially pointed out, either the barycentric or mass ratio definitions could be a starting point, but neither are perfect. Whoever settles on the final definition probably needs to consider a wide variety of situations, some of which we are unaware of right now. For example, if the larger planet under the barycentric definition is impacted hard enough to remove the surface and raise the barycenter into space, it seems odd that a mostly unaffected moon would become a planet. If enough of that material eventually returns to the impacted planet then its partner may return to moon status. Situations involving exchanges of material could potentially cause oscillation between moon and planet status. I think this needs more study.
Mark Thomas
5 / 5 (1) Mar 19, 2017
eachus, the problem with including orbit in the planetary definition is that it excludes billions of rogue planets, but that might be acceptable to many. Because there must be a continuum of objects, arbitrary definitions like 1,000 km minimum radius are the only way we are going to get precision. With more information, it may turn out that the maximum planet size can be set to a particular mass to exclude deuterium fusion, e.g., ~13MJ.

On a different note, it is ironic that commenters here on phys.org are being far more careful with this than the IAU was back in 2006. :-)

nkalanaga
5 / 5 (1) Mar 19, 2017
If enough material is removed from one body to change the definition, the mass loss itself would probably disrupt the orbit. The former moon would now be moving too fast, and would move to a wider, probably eccentric, orbit, making it more likely to be removed by stellar perturbation, where it would become a planet by default. But, yes, it is theoretically possible to change a moon to a planet by that method, and then change it back again.

13 MJ is already the unofficial lower limit for brown dwarfs, which seem to have been accepted as a non-planet, non-stellar deuterium burning category, so that would work fine as an upper limit.

And, just my opinion, but "planet" should include rogues, which don't orbit anything. That's why I left stars out of my definition. Otherwise we need another name for them, which means even more controversy.
Mark Thomas
not rated yet Mar 19, 2017
nkalanaga, at least you and I have come to agreement. While not perfect, this seems very reasonable considering what we know at present. Note that I could see some additional debate at the high end based on whether the object formed more like a star (by gravitational collapse) or a planet (by accretion), e.g, some planetary mass brown dwarfs.

I would also build into the definition that it is subject to better information. We have barely scratched the surface on exoplanets and are still waiting for our first exomoon, so I would not presume even the best possible answer in 2017 would still suffice in say 2117.
EnsignFlandry
not rated yet Mar 19, 2017
This is butterfly collecting or insect-naming. Its not science. Mike Brown gave a good argument based on physical characteristics that differentiates a planet from a dwarf planet or moon.
nkalanaga
5 / 5 (1) Mar 19, 2017
I don't know that how it formed would make a difference, but once we have more data, it might, possibly by affecting composition. At the planet/brown dwarf edge, formation method might affect how much lithium and deuterium the object starts with, and thus whether it can fuse the deuterium. If we can get the data, a questionable object depleted in deuterium or lithium could be classed as a brown dwarf, while one with "natural"? amounts is probably a planet.

So, yes, more debate is likely in the future. And, as you said, we haven't found any exomoons yet, so have absolutely no data on whether our system is "normal" moon-wise.

The biggest problem I can see with adding formation methods would be proving how something formed, millions or billions of years afterwards. We're still debating Jupiter, and we can send probes there.
SiaoX
1 / 5 (1) Mar 20, 2017
Mark Thomas
5 / 5 (3) Mar 20, 2017
"This is butterfly collecting or insect-naming. Its not science."

Some say perception is everything. The public rightfully expects scientists to explain science. Being as clear and consistent as possible within the bounds of science is very important. Who else would you prefer to decide what a "planet" is?

About Mike Brown's arguments, they were predicated on the existence of many Pluto-like objects of roughly the same size and larger. Eris was supposed to be the first of an entire class of such objects, not the last. Mike's prediction has turned out to be false. Even 11 years after redefining Pluto to be a dwarf planet, it is still the largest known object in the solar system beyond Neptune. Eris is a close second, not to mention more massive than Pluto, but there is a considerable size gap that was not predicted between Eris and 2007OR10 as I noted above. Note that I believe Mike Brown's prediction about Planet X will also turn out to be false, but time will tell.

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