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Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
Defining Planethood
HarbingerDawnDate: Wednesday, 08.06.2016, 16:52 | Message # 136
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steeljaw354, I do not understand your obsession with size. Diameter alone says very little about an object. A neutron star is smaller than any planet, yet is insanely more massive and more influential. Likewise, a Mars-sized object with the bulk properties of Mercury and a Mars-sized object with the bulk properties of Pluto will be very different. Size, mass, and location within a system are all important factors in describing an object, yet you are fixated on only one of them. I think you should explain why.




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steeljaw354Date: Wednesday, 08.06.2016, 19:06 | Message # 137
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If you have a round stone 5mm in size and set into a "clear" orbit around the sun it's a planet, but Pluto isn't by this rule, does that make sense? If you have a size limit it eliminates that.

Edited by steeljaw354 - Wednesday, 08.06.2016, 19:08
 
midtskogenDate: Wednesday, 08.06.2016, 19:32 | Message # 138
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Quote steeljaw354 ()
f you have a round stone 5mm in size and set into a "clear" orbit around the sun it's a planet

No, it has to be massive enough to become round by its own gravity. A pebble doesn't qualify.

Again, mass is more important than size.





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steeljaw354Date: Wednesday, 08.06.2016, 19:39 | Message # 139
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What mass limit do you propose then?

- See more at: http://www.space.com/29571-w....TZ.dpuf

Here we have a simpler method for determining planet status plus MORE

"Why Pluto Is a Planet, and Eris Is Too
This artist's view shows Pluto center, and its large moon Charon, as well as two of its four smaller moons (at top left and right). Pluto's four tiny moons are Nix, Hydra, Kerberos and Styx.
Credit: David A. Aguilar (CfA)
Tim DeBenedictis is the lead developer of the SkySafari line of iOS and Android apps at Simulation Curriculum, the makers of Starry Night, SkySafari and the free Pluto Safari app. DeBenedictis has been writing astronomy software since high school, and graduated from MIT in 1993 with a degree in earth, atmospheric and planetary science. Passionate about space, DeBenedictis is self-taught in mechanical and electrical engineering, and has launched his own private microsatellite into space. He contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

The International Astronomical Union (IAU) got it wrong. Our solar system has 10 planets.

As NASA's New Horizons spacecraft glides its way to the cold outer reaches of our solar system to take the first-ever up-close look at Pluto, the time is right to revise the International Astronomical Union (IAU)'s 2006 definition of a planet, which resulted in Pluto's "demotion" from planet to ambiguous dwarf-planet status.

Pluto's place

For those unfamiliar with the issues that led to that highly controversial decision, here's a quick recap: It started with Pluto itself, discovered on Feb. 18, 1930, by Clyde Tombaugh, a young American astronomer working at Lowell Observatory in Flagstaff, Arizona. Pluto turned out to be rather unlike the other eight large objects orbiting the sun. Pluto is much smaller than Mercury, and only two-thirds the size of Earth's moon. Its orbit is tilted and eccentric, crossing Neptune's. No other planet acted like this. In 2000, astronomers found other objects orbiting the sun in the deep outer solar system, with qualities very much like Pluto's. They were given names like Sedna , Quaoar, Ixion, Varuna, Makemake and Haumea . Many were close (but not quite equal) to Pluto in size. All of them had tilted, eccentric orbits; quite a few of those orbits crossed Neptune's.

The tipping point came in 2005. California Institute of Technology astronomer Mike Brown, along with Chad Trujillo of Gemini Observatory and David Rabinowitz of Yale University, discovered a new massive body in the solar system. This new body, which astronomers latter dubbed Eris, was particularly noteworthy: Not only did it possess a moon, but at the time, it was estimated to be larger than Pluto. Subsequent observations revealed that Eris and Pluto are nearly identical in size, though Pluto is likely a few kilometers larger. Initially, Brown had named the newly discovered body Xena (after the protagonist of the eponymous TV show, with a sneaky Planet X reference). Although the name Xena didn't stick, the IAU later officially — and aptly — christened it Eris after the Greek goddess of chaos and discord.

So, it seemed quite clear that if Pluto was our solar system's ninth planet, then Eris should be its 10th. And if Eris and Pluto were planets, why shouldn't Makemake and Haumea be considered planets as well? And what if there were even bigger objects out there to be discovered? Why shouldn't the solar system have 15 planets, or 40? (Can you imagine the mnemonic device that would be required to remember 40 planets in the solar system?!)

For all who were in support of granting planet status to these objects, an equally adamant camp insisted that none of these objects, including Pluto, deserved to be called planets, and that our solar system contained only eight objects worthy of planet status. Neptune would be the last and final.

A worsening problem

With the intention of solving the debate once and for all, members of the IAU met in 2006. They spent days debating how to establish unambiguous definitions for the objects in our solar system. In the end, Resolution 5A was born:

RESOLUTION 5A

The IAU therefore resolves that planets and other bodies in our solar system, except satellites, be defined into three distinct categories in the following way:

(1) A planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and © has cleared the neighborhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], © has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies.

These are, to put it bluntly, terrible definitions. Despite its goal of providing unambiguous definitions, Resolution 5A actually contains the kind of ambiguity that most scientific organizations would protest. It adds confusion, resolves little and makes nobody happy.

So how did that become the official definition of a planet? A very strange vote. If you think low voter turnout is limited to politics, consider this: Only about 4 percent of IAU members were present for the vote on Resolution 5A. But it was travel schedules, not apathy, that caused this abysmal turnout. You see, the vote took place on the last day of the IAU meeting, when many people had to leave to catch flights back home — 424 astronomers were present, even though IAU membership in 2006 was just more than 10,000. As a result, Pluto lost the status it had enjoyed for more than 80 years and became a dwarf planet overnight. [Pluto Demoted: No Longer a Planet in Highly Controversial Definition]

The term "dwarf planet" itself causes confusion. You often hear people say Pluto is still a planet but that it just happens to be a dwarf planet now. But despite the name, the IAU does not consider a dwarf planet a planet — unlike a dwarf star, which is still a star, or a dwarf galaxy, which is still a galaxy. So much for eliminating ambiguity! But just the conflicting use of the word planet isn't the most unclear part of the resolution.

Picking apart Resolution 5A

Let's dissect resolution 5A "Definition of 'planet'", one of six IAU Resolutions that were passed at the Closing Ceremony of the General Assembly in 2006.

*Nearly round shape. There is an element of something good here. We all intuitively feel that a planet should be round, or nearly so. But what is "nearly"? How lumpy and bumpy must an object be to no longer qualify as a planet or a dwarf planet? How smooth must the "ball" be? The Earth, which we all agree is a planet, is nearly round on some scales, but on others, it's not. If you're standing in the bottom of the Grand Canyon, the Earth isn't even close to nearly round.

*Cleared the neighborhood. I've tried to wrap my head around this phrase for years, tried to convince myself that it makes sense — but I just can't swallow it. The IAU is trying to express that, in addition to being round, a planet should be the dominant gravitational force in its local region of the solar system. That's not an unreasonable position. Certainly the Earth and Jupiter are the dominant objects in their local regions. Neptune surely is, too. Even though Pluto's orbit crosses Neptune's, Neptune forces Pluto into something called a 3:2 resonance (for every three times Neptune goes around the sun, Pluto goes around twice), preventing collision. But have any of these planets actually "cleared the neighborhood" around their orbits? No. Pluto is still clearly in Neptune's "neighborhood." For that matter, Jupiter has two well-known groups of asteroids, the Trojans, which lead and follow Jupiter along in its orbit. For that matter, the Earth hasn't quite "cleared the neighborhood" around its orbit, either, to which anyone who saw the near-Earth asteroids that entered Earth's atmosphere near Chelyabinsk, Russia, on Feb. 15, 2013, or Tunguska, Siberia, on June 30, 1908, can attest. So are Earth, Jupiter and Neptune the dominant gravitational objects in their local neighborhoods? Yes, clearly. Have they cleared their neighborhoods? No. Not by a long shot.

Other scientists have weighed in on the matter. Alan Stern, principal investigator for the New Horizons mission to Pluto, made it clear he disagrees with the IAU resolution. "Any definition that allows a planet in one location but not another is unworkable. Take Earth. Move it to Pluto's orbit, and it will be instantly disqualified as a planet," Stern said.

The biggest problem with the IAU's planet definition is that it replaces an already-ambiguous concept ("What is a planet?") with three more ambiguous concepts, ("nearly round," "cleared" and "neighborhood"). Indeed, the only definitive part of the IAU resolution on which everyone can agree is the first part: (1) A planet is in orbit around the sun. It's why the moon is not a planet. My 6-year-old niece intuitively understands this. It's the only part of the IAU definition I would keep.

The way out

Let's look at some other kinds of definitions that are clear and unambiguous.

*International boundaries. It's well understood that the portion of North America north of 49 degrees, between the Canadian provinces of British Columbia, Alberta, Saskatchewan and Manitoba, and the U.S. states of Washington, Idaho, Montana, North Dakota and Minnesota, is called Canada, and the portion below that latitude is called the United States. There's no physical demarcation — no river, no mountain range — along the 49th parallel. There's no subtle change in vegetation or geological structure. But there is a hard, sharp, clearly defined, well-understood boundary that unambiguously answers the question, "What is Canada?" It's the country north of the 49th parallel. It passes the 6-year-old-niece test.

*Constellation boundaries. Back in 1888, the IAU defined an intricate set of boundary lines in the sky, precisely outlining the groups of stars that were commonly referred to as constellations. It also declared that there would be 88 of these constellations. Lines were chosen carefully, to respect traditional choices about which star might lie in which constellation, and in the end, the definitions were clear. There is no ambiguity about which particular point in the sky falls within which constellation. And, you can tell precisely when a moving celestial object (like Pluto) might cross from one constellation to another. It's an arbitrarily agreed upon but well-defined system of definitions that has served the astronomical community well for more than 100 years.

*The Karman line that defines the edge of space. Where does the Earth's atmosphere end and outer space begin? Clearly, there is no physical boundary. There is no bubble holding the atmosphere that one must pierce on one's way to the International Space Station. The air just gets thinner and thinner until you can ignore it. But in reality, air molecules continue to exist, albeit in smaller numbers, out to an altitude of many thousands of kilometers and beyond — indeed, some of these air molecules may have made it as far as Pluto by now! However, since the early days of manned spaceflight, a near-universally accepted definition is that space begins at an altitude of 100 kilometers (62 miles). In fact, this definition is accepted by the Fédération Aéronautique Internationale (FAI), an international standard-setting body for aeronautics and astronautics. Pilots who've flown higher than 100 km have officially earned the title of astronaut. It's another arbitrary, but widely accepted convention that is clear, unambiguous and easily passes the 6-year-old comprehension test.

What the IAU should have done in 2006, and could easily do moving forward, is to crystallize the definition of the word "planet" as unambiguously as it defined the boundaries of the constellations in 1888. Yes, that definition would have been arbitrary, and yes, the actual physical objects themselves would gradually transition from larger to smaller, and don't care in the least what we choose to call them. But the IAU could have chosen a definition that resolves the debate in a far more satisfying manner than it actually did.

The 1,000-km rule

So, what would be a better definition for the objects in the solar system?

(1) A "planet" is a celestial body that (a) is in orbit around the sun, and (b) has a maximum surface radius greater than 1,000 km (620 miles).

(2) All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies."


Edited by steeljaw354 - Wednesday, 08.06.2016, 19:50
 
midtskogenDate: Wednesday, 08.06.2016, 19:49 | Message # 140
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It was IAU's proposal. It must have a mass, size and composition that create hydrostatic equilibrium.




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spacerDate: Wednesday, 08.06.2016, 19:50 | Message # 141
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i am not sure pluto and other big kaiper belt objects is massive enough to have clear orbit and have nothing to disturb them.
pluto disturbed by neptun, the others probably disturbed by planet 9, as you can see thier orbits are not perfect round.
and pluto is smaller than some of the big kaiper objects so why declare pluto as planet becuase of history but not the bigger objects?





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steeljaw354Date: Wednesday, 08.06.2016, 19:53 | Message # 142
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Planet 9 doesn't exist until the images are shown. Mercury's orbit isn't round either, it is tiled and more eccentric.
Make sure you watch this video.
https://www.youtube.com/watch?v=CRrl-PSiF0w


Edited by steeljaw354 - Wednesday, 08.06.2016, 19:54
 
WatsisnameDate: Wednesday, 08.06.2016, 21:13 | Message # 143
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That's actually one thing you've proposed so far that I find fairly reasonable. Let's not assume planet 9 exists until we have even more compelling evidence. That doesn't necessarily need to be in the form of an image, however. We don't need an image of a black hole to have enormous confidence that they exist, for example.

As for determining the lower bound for planetary definition, size alone is a terrible metric. Especially if you're just going by "1000km" or whatever. This is an arbitrary choice in many ways. It's arbitrary because you are imposing a base-10 number on a universe that does not know or care about base 10. You are imposing "kilometers" as a distance measure on a universe that does not know or care about the definition of a meter. There is nothing in nature that makes 1000km a special dividing line in the context of astrophysical bodies. Why use it?

Let us instead be motivated by something physical, allowing nature itself to guide us in defining how to classify things. Observe that objects assume a shape by hydrostatic equilibrium when they get large enough for their composition. Apply physics to determine why that should be and calculate the conditions for which it happens.





 
steeljaw354Date: Wednesday, 08.06.2016, 21:29 | Message # 144
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Or we could do this idea which I have seen a few times, I have taken a liking to it. "Wanderer classification"
All are classified into the same category called Wanderers, we have 2 subset.
Planets: Mercury,Venus,Mars,Jupiter,Saturn
Dwarf Planets: Ceres,Uranus,Neptune,Pluto,Eris

How you classify, Planets are objects that constantly appear in the sky and have a magnitude above magnitude 5 they are rounded objects and not man made. Dwarf Planets are objects that have a magnitude below 5 and are round in shape, not man made. Moons are objects that orbit planets and dwarf planets and not man made, Dwarf moons are objects that orbit asteroids as they are very small, not manmade.

Earth isn't classified into any of these because it isn't visible in the sky.
 
HarbingerDawnDate: Wednesday, 08.06.2016, 23:46 | Message # 145
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Quote steeljaw354 ()
"Wanderer classification"

This is without a doubt the most useless classification system you've proposed yet. It doesn't take the intrinsic or contextual characteristics of any of the bodies into account. It also calls Neptune a "dwarf", but not Mercury. It's arbitrary and insane. It's EXTREMELY unscientific.





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steeljaw354Date: Thursday, 09.06.2016, 00:30 | Message # 146
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Neptune is a dwarf in this classification because it is very dim, size isn't taken into account, only apparent brightness in this classification.
 
FaceDeerDate: Thursday, 09.06.2016, 01:50 | Message # 147
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A classification based on apparent magnitude is, quite frankly, the worst thing I've heard yet. The Moon and the Sun become planets under it as well, along with a whole bunch of artificial satellites in low Earth orbit. And of course it's not at all applicable to exoplanets, and someday if we've got colonies on other worlds they'll have an entirely different set of objects that count as "planets" to them.

I'm getting a little baffled here. Why do you dislike the IAU's definition so much that you'd cast this far afield looking for alternatives?
 
OstariskDate: Thursday, 09.06.2016, 03:36 | Message # 148
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Quote FaceDeer ()
Why do you dislike the IAU's definition so much that you'd cast this far afield looking for alternatives?

I agree. using apparent magnitude isn't much help for a new planet classification system, as that would mean that stars like Betelgeuse will become planets when they go supernova, as (according to many theories) any red supergiants (In my case, red giants the form from blue stars) which should go supernova within our time here in the universe, will light up the sky, outshining the moon and making night become like day, illuminating the sky. This would also mean that meteors that appear bright in the sky, such as the one that lit up New Zealand skies, will also become planets. However, I do believe the IAU's planet classification is wrong.

According to the IAU, a planet is a planet when it's orbit is cleared of debris, which means the planets such as the Earth, Jupiter, and Saturn, are dwarf planets, as Jupiter obviously has its trojan asteroids, meteors can be seen falling through earth's sky constantly, and at times, Saturn can be observed having objects falling into the planet. This means that only Mercury, Venus, Mars, and Uranus will be the only actual planets in the system (If I can recall correctly).






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Edited by Ostarisk - Thursday, 09.06.2016, 03:48
 
spacerDate: Thursday, 09.06.2016, 04:26 | Message # 149
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ohh and i need to fix myself, when i wrote circle orbit i meant to say perfect orbit like earth etc that doesnt have really strange Aphelion
and Perihelion. sorry happy





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FaceDeerDate: Thursday, 09.06.2016, 04:53 | Message # 150
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The debate is starting to loop. As I explained in my post one page earlier, the presence of any particular little bit of detritus in an object's orbital zone is not sufficient to disqualify that object for planethood. That would just be downright silly - the IAU would not propose a definition that excluded all of the major bodies of the solar system. And indeed, the IAU's definition has a footnote that explicitly calls out the eight known planets as being examples of planets under it. So if you read the IAU's definition and think "according to this Jupiter is not a planet" that means you've misunderstood something about the IAU's definition, not that the IAU has misunderstood something about it.

I'll try to summarize it again.

There exist several different formulae that one can use to produce a number that represents either an object's capability to clear other objects from its orbit (the Stern-Levison parameter is one such) or that represents the degree to which the object actually has cleared other objects from its orbit (the Soter planetary discriminant).

This table shows the actual numbers for various large bodies in our solar system. These numbers account for things like the Trojan asteroids, Earth-crossing meteors, and other such bodies. It turns out it doesn't matter too much which specific formula you use, they all produce a strikingly similar pattern.

Compare the degree to which Earth or Jupiter has cleared their neighborhoods to the degree to which Pluto or Eris have cleared their neighborhood. None of them have cleared them perfectly, as you point out. But the planets have cleared them to a degree five or six orders of magnitude greater than the non-planets.

Furthermore, as Watsisname illustrated with this excellent histogram, the numerical degree of orbit-clearing for these two groups of bodies shows a clear pattern of clustering. Planets have a similar amount of orbital cleanliness, and non-planets have a similar amount of cleanliness, and there's a huge region in between those two groups that is unoccupied. A bimodal distribution like this is an excellent basis for a division in a classification system, you see this sort of pattern throughout all scientific disciplines.

The difference between planets and non-planets in this regard is large, clear, meaningful, and easily demonstrated. This is not an ambiguous measure, there're no weird "gotchas" or edge cases or ambiguities, the IAU didn't somehow accidentally declare that there's no such thing as a planet or that Earth is not a planet or anything silly like that.

The eight planets really have effectively cleared their orbits of other debris. The other major solar system objects really haven't done so. The numbers are right there in black and white, the method of calculating them and the theoretical justifications for choosing those methods is available for scrutiny in the referenced papers, you can check the results yourself if you don't believe them. If you wish to criticize the methods by which those formulae operate then by all means explain how they've gone wrong. But please be finished with insisting that they give results that they don't actually give.


Edited by FaceDeer - Thursday, 09.06.2016, 04:55
 
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