Why (if anything) is a planet clearing its neighborhood important?

[Bohandas]

So like a couple decades ago the IAU changed their definition of a planet, to include factors that weren't properties of the body in question - specifically the issue of whether its orbit contains any similar sized or larger bodies - and I never understood the reasoning behind it? Can someone explain to me why this is important? I can't see how it would affect any properties of the body in question - its shape, internal geology, temperature, sunlight received, etc - outside of super extremal cases where it was either constantly passing very close to something extremely massive that was exerting tidal forces on it (or else repeatedly colliding with similar sized bodies but even this is more of a series of one off changes to the object's properties than a continuous distortion of them)

Additionally, what does Eris have to do with it? It's my understanding that the discovery of Eris somehow prompted the change in definition, but I have no idea why, from a scientific standpoint, this should be the case.

The idea I keep coming back to is that there was no scientific reason, and they just lazy and didn't want to have to remember any more planets, or WORSE that they may have had some aesthetic opposition to there being potentially dozens of planets, so they came up with an arbitrary definition to limit the number of official planets.

Or, best case scenario, that they'd decided that orbital mechanics is more relevant to their field than the actual study of the planets themselves. Which would be acceptable but does raise the question of why the hell everyone else adopted this definition of a planet as well.

If anyone has a less cynical explanation can you share it with me?

[LibraryOgre]

My (utterly layman's) understanding is, basically, that precision requires some sort of definition, which planets more or less didn't have. It was a "This is a significant celestial body", without having a clear idea of what made it significant.

Eris was discovered in 2005. Because it is bigger than Pluto (about a quarter more massive), the argument went "Well, if Pluto is a planet, then so is Eris". Which led to the discussion of "Well, what IS a planet? If Pluto is a planet based on size, so should Eris be. But then what about Ceres and the larger objects in the mid-system asteroid field?"

So, they sat down and hammered out a definition. That definition excluded Pluto, Ceres, Eris, and other "dwarf planets" more or less because it did. I'd wager "We need something resembling a manageable number of planets" was part of it, but I'm not sure.

[Tyndmyr]

As Mark says, there was likely some level of "the number of planets should mostly only consist of the historical list" motivation.

Eris has a pretty good case for being a planet if Pluto is. They didn't want to add bodies like that, so they were forced to come up with some criteria to exclude them on.

"clearing the neighborhood" is, ultimately, extremely subjective. Pluto most certainly doesn't clear its orbit because it crosses orbits with Neptune. However, it is difficult to rely on that argument without also disqualifying Neptune for the same reason.

Thus you get the size...so the larger of a pairing sharing an orbit ends up as a planet while the smaller does not. This is extremely subjective. How big is the difference before it stops being "similar"? One could reasonably hold that the Earth's moon is in fact a planet. After all, it's about 27% as large as the earth...that's an unusually close size, with all other moons being far smaller than the planets they orbit.

Some proposed definitions for planets would have the moon qualify, making us a double planet system. Ganymede is also quite large...larger than Mercury.

Ultimately, our current definition is kind of arbitrary, and is very fitted to our current solar system, and is unlikely to work well with any other. Exoplanet research is still ongoing, and we're learning a lot, but it doesn't seem likely that other solar systems are going to be exact mirrors of ours.

[jayem]

I think Eris bridged the gap between Planets/Moons and Asteroids and made it impossible to ignore the problem. Before it was obvious that Asteroids were Asteroids and Planets were Planets.

You can't just discount Eris because it's the same size as Pluto (in practice of course...), but if you include it, then it's pretty much a smooth curve onward. And Ixion is 'clearly' in a different category from Venus/Earth/Mars, you want to keep some sense of proportion.

It might be something to do with Moons already being defined by their planetary role. And to some extent having the same problem (you have 3 moons comparable to Mercury).

And yes, as a non astronomer, I think we should have a category list split by what they do in space, and a separate category list split by what they are internally.

[Tvtyrant]

So like a couple decades ago the IAU changed their definition of a planet, to include factors that weren't properties of the body in question - specifically the issue of whether its orbit contains any similar sized or larger bodies - and I never understood the reasoning behind it? Can someone explain to me why this is important? I can't see how it would affect any properties of the body in question - its shape, internal geology, temperature, sunlight received, etc - outside of super extremal cases where it was either constantly passing very close to something extremely massive that was exerting tidal forces on it (or else repeatedly colliding with similar sized bodies but even this is more of a series of one off changes to the object's properties than a continuous distortion of them)

Additionally, what does Eris have to do with it? It's my understanding that the discovery of Eris somehow prompted the change in definition, but I have no idea why, from a scientific standpoint, this should be the case.

The idea I keep coming back to is that there was no scientific reason, and they just lazy and didn't want to have to remember any more planets, or WORSE that they may have had some aesthetic opposition to there being potentially dozens of planets, so they came up with an arbitrary definition to limit the number of official planets.

Or, best case scenario, that they'd decided that orbital mechanics is more relevant to their field than the actual study of the planets themselves. Which would be acceptable but does raise the question of why the hell everyone else adopted this definition of a planet as well.

If anyone has a less cynical explanation can you share it with me?

Planet isn't a meaningful distinction scientifically, but since cultures distinguished them due to their orbits and visibility science in the early days did as well. They had either the option of outraging the public by eliminating planet as a class, or by eliminating some of the planets from the class based on arbitrary lines.

Or in short, the reasons to have planets were cultural and the scientific community didn't feel like admitting they were keeping some and excluding others based entirely (just mostly) on public perception.

[Khedrac]

I would add another point on the layman's understanding:

Changing the definition of a 'planet' to exlude Pluto et al meant changing the textbooks once. At the time we seemed to be finding a new Kuiper Belt Object of sufficient size to be spherical every year or so (2002, 2003, 2003, 3003, 2004, 2005, 2007) so they were projecting having to keep changing the textbooks if they classified Pluto+ sized ones as "planets". It looked more sensible to change one (to remove Pluto) than having an unknwon number of changes keep coming.

In hindsight we can say that if they had just set the limit at 2000km diameter we would have added Eris and stopped, but that wasn't obvious then.

[halfeye]

Thus you get the size...so the larger of a pairing sharing an orbit ends up as a planet while the smaller does not. This is extremely subjective. How big is the difference before it stops being "similar"? One could reasonably hold that the Earth's moon is in fact a planet. After all, it's about 27% as large as the earth...that's an unusually close size, with all other moons being far smaller than the planets they orbit.

The exception being Charon, which only doesn't apply because they made Pluto not a planet. 27% is the Moon's diameter as a percentage of the Earth's diameter, the Moon's mass is 0.6% of the Earths, Charon by comparison has 50% of Pluto's diameter and 12.5% of Pluto's mass.

I more or less agree with the other things Tyndmyr said.

[Algeh]

I think the Wikipedia article List of Former Planets can help put this in perspective. "Planet" is one of those concepts that has been with us since antiquity, and we've had multiple shifts in what "counts" as a planet as we've learned more about space and different distinctions were useful based on how we understood the universe to work at the time. You can see several historical clusters of "de-planet-ing" following various scientific advances that had lead to discoveries of new planets under the then-current definition, so this isn't the first time people have limited the definition of planet to try and keep it at a reasonable number of objects that shared certain characteristics as the set of planets got larger and making distinctions between things in that large set became reasonable since they didn't all behave the same way.

We'll probably get further revisions to the concept as we learn more about exoplanets and how other solar systems function.

[crayzz]

The idea I keep coming back to is that there was no scientific reason, and they just lazy and didn't want to have to remember any more planets, or WORSE that they may have had some aesthetic opposition to there being potentially dozens of planets, so they came up with an arbitrary definition to limit the number of official planets.

The idea that astronomers were too lazy to remember more than 8 planets while being perfectly happy to remember a truly staggering number of stars, galaxies, exo-planets, comets and everything else is really odd.

Some points:

• All definitions are arbitrary, "arbitrary definition" is a distinction without difference.
• "Planet" historically was actually pretty well defined; they were 7 specific celestial bodies you saw that wandered around in the sky. That would exclude Neptune, Uranus, and Pluto, but would include the moon and the sun.
• Excluding the Uranus and Neptune is widely agreed to be silly; including the sun is widely agreed to be silly; including the moon and mostly agreed to be silly, but you'll find defenders on occasion.
• Pluto's initial inclusion as a planet is basically an historical accident. Neptune was discovered through its gravitational effect on Uranus, and a Planet X was theorized to exist beyond Neptune through similar reasoning. Pluto happened to be discovered near where Planet X was predicted to exist in a search for Planet X. It was included in the list of planets on the assumption that it was one, prior to it being properly studied. The more it was studied, the less it seemed like a proper "planet". It certainly didn't live up to the expectations that were placed on Planet X.
• Among those reasons was that it's orbit crossed Neptune's, a behaviour isn't shared by any other planet.
• Pluto's inclusion on the list of planets was contested basically immediately. Pluto was believed to be much larger than it is at the time, and the consensus in some circles was still "that's more like a large asteroid than a planet." It's actual size wasn't properly estimated for decades. Had it's actual size been known, it's likely it would never have been considered a planet to begin with.
• It's inclusion as a planet was also somewhat political, IIRC. The Americans wanted their own planetary discovery.
• Once Pluto was on the list, despite being fairly atypical for a planet, it opens the door up for even less planet-like planets. And so astronomers were looking down the barrel of an annoying problem. If Pluto is a planet, Eris is a planet. If Eris is a planet, Ceres is a planet. If Ceres is a planet, etc etc etc until we're forced to accept Saturn's rings as being made up of trillions of tiny planets.
• So we need a proper, clear definition. We didn't really have one until now because, again, planet is a really old word and we'd been using it against it's original meaning for centuries by this point.
• That definition should obviously include the 8 100% for sure planets.
• Various definitions were bandied about until a consensus was reached, namely: orbits the sun, round shape from gravity, and cleared its orbit.
• Some of the definitions were actually designed specifically to include Pluto, accusing them of trying to get rid of it out of bias is also very odd.
• It's worth noting that the guy who discovered Eris approves of the modern definition; presumably, were he to be the sort to twist definitions to his own preferences, he'd want his name on a planet.
• The discovery of Ceres is similar to that of Pluto, in that astonomers were looking for a planet to find, and wanted Ceres to fit the bill. The difference is that Ceres was booted from being a planet for basically the same reason that Pluto was booted from being a planet (failure to clear it's orbit), but that happened in the 1850s before generations of school children learned about it and before the internet, so no one really cared.

I'm personally not a fan of "dwarf" planet, I would prefer "major" and "minor" planet, or something to that effect.

[TaiLiu]

The idea I keep coming back to is that there was no scientific reason, and they just lazy and didn't want to have to remember any more planets, or WORSE that they may have had some aesthetic opposition to there being potentially dozens of planets, so they came up with an arbitrary definition to limit the number of official planets.

[…]

If anyone has a less cynical explanation can you share it with me?

I don’t know you or your background, so I sincerely apologize if the following is condescending. But I think your “cynicism” is getting at an important realization—that science is fundamentally a human enterprise that is shaped and blessed and burdened by its history and culture. Arguably much of science is guided by “no scientific reason.” Scientific norms and standards shift over time, and science is not magically warded from societal influence. Astronomy is not unique in this regard.

[Mechalich]

While classifications due require judgement calls and divisions based on human reasoning, they are not necessarily completely arbitrary. The issues surrounding Pluto, like Ceres before it, have to do with time lag in the development of our understanding of aspects of the solar system. The idea that Pluto was merely one of many relatively small icy bodies floating around the outer solar system was floated shortly after it was discovered, but it was not until the 1990s that the existence of the Kuiper Belt, and the numerous objects within it was actually confirmed.

We now know that Pluto is part of a group of dozens (and possibly hundreds or even thousands) of objects in the outer solar system that are both in hydrostatic equilibrium (ie. roughly spherical) and yet at the same time do not occupy a distinct orbital space of their own. Pluto itself is actually in 2:3 orbital resonance with Neptune, part of a large group of objects called plutinos that share this state. As such Pluto shares much more in common with the various large Kuiper Belt (and also Scattered Disk and other further out objects) than it does with any of the eight bodies of the inner system that are orbiting the sun in cleared paths. Consequently it makes a great deal of sense to group Pluto there not with the other traditional 'planets.'

It is arguable whether the 'planet' category makes sense as constructed. A strong case can be made for breaking it up into rocky worlds and gas worlds, but there has been obvious hesitation a modifying the ancient language used.

[Chronos]

I still say that we should deprecate the concept of "planet" entirely, and replace it with three categories, "rockballs", "gasballs", and "iceballs". Earth, Luna, and Ceres are all a lot more like each other than any of them is like Jupiter or Neptune.

[Peelee]

The idea I keep coming back to is that there was no scientific reason, and they just lazy and didn't want to have to remember any more planets

I think it is the height of oddity to assume that people who, on the completion of their schooling, opted to take an additional 6-10 years of optional schooling, and then actively choose careers in the same field as their optional extra decade of schooling, chose a definition on something directly related to their field and would be globally publicized, solely or even partly to avoid memorizing a short list. Memorization which is almost certainly not required by anyone past sixth grade anyway.

[warty goblin]

If you're interested in the dynamics of a solar system, clearing the orbital path seems quite reasonable as a definition, since it tells you quite a bit about the goings-on around that celestial body. It tells you pretty much nothing about the celestial body itself, but orbital mechanics don't depend all that much on planetary composition, and this seems like a very orbital mechanics facing definition.

[Excession]

If you're interested in the dynamics of a solar system, clearing the orbital path seems quite reasonable as a definition, since it tells you quite a bit about the goings-on around that celestial body. It tells you pretty much nothing about the celestial body itself, but orbital mechanics don't depend all that much on planetary composition, and this seems like a very orbital mechanics facing definition.

Orbital mechanics is something we can see from Earth. Composition is something we can't. We have some good theories about the interior of Earth, a few ideas about the Moon and Mars from the few seismometers we landed there, and some variably educated guesses about the rest.

[Eldan]

I still say that we should deprecate the concept of "planet" entirely, and replace it with three categories, "rockballs", "gasballs", and "iceballs". Earth, Luna, and Ceres are all a lot more like each other than any of them is like Jupiter or Neptune.

But a distinction in big balls and small balls helps, along with the distinction into cold balls, hard balls and gassy balls.

[Batcathat]

But a distinction in big balls and small balls helps, along with the distinction into cold balls, hard balls and gassy balls.

I must admit, the idea of reading this sentence out of context makes me giggle like a six-year-old.

[Eldan]

I must admit, the idea of reading this sentence out of context makes me giggle like a six-year-old.

Goal achieved. I spent several minutes thinking about how to work "hot balls" (the stars) and "blue ball" (the Earth) into that one.

[Yora]

The real definition of planet is "The gas giants and Mercury, Venus, Earth, and Mars are planets, everything else is not."

Anything else that followed where simply justifications for the above definitions, not meaningfully distinguishing factors. Jupiter hasn't cleared it's orbit, and Saturn is very much not spherical, but whatever.

[hamishspence]

Jupiter's cleared its orbit of everything anywhere near its own size. And even the asteroids that it shares its orbit with, are pushed into certain locations by Jupiter's gravity - the Lagrange points.

Saturn isn't spherical - but if it did not rotate, it would be spherical. It's in the shape that gravity plus rotation speed dictates for such a massive object.

[halfeye]

The real definition of planet is "The gas giants and Mercury, Venus, Earth, and Mars are planets, everything else is not."

I agree that's what it looks like so far.

Anything else that followed where simply justifications for the above definitions, not meaningfully distinguishing factors. Jupiter hasn't cleared it's orbit, and Saturn is very much not spherical, but whatever.

I think you're missing the point on these, it's not at all the case that Saturn isn't in hydrostatic equilibrium, Haumea is in hydrostatic equilibrium and that's nothing like a sphere at all.

The problem with the definition is that the mass required to clear an orbit increases as the distance of that orbit from the star increases. Mercury in Pluto's or Neptune's orbits would not be a planet. That is very silly. There might be a case for Mercury not being a planet, but it strongly seems to me that whether it's a planet or not shouldn't depend on how near to the Sun it orbits.

[Peelee]

Luna

You mean the Moon? There's no body called Luna, unless you're speaking Italian or Spanish or the like, where that is simply their languages word for "moon".

[Bohandas]

Luna

I believe you mean Theia

[halfeye]

I believe you mean Theia

Selene or Phoebe?

[Mechalich]

I think you're missing the point on these, it's not at all the case that Saturn isn't in hydrostatic equilibrium, Haumea is in hydrostatic equilibrium and that's nothing like a sphere at all.

Right. I'd also add that knowing something is in hydrostatic equilibrium provides real information about composition because we know gravitational processes have organized the internal structure of an object in hydrostatic equilibrium in a way that doesn't happen for objects that are not. So it is a real and useful division that's more than simply 'round, not round.'

The problem with the definition is that the mass required to clear an orbit increases as the distance of that orbit from the star increases. Mercury in Pluto's or Neptune's orbits would not be a planet. That is very silly. There might be a case for Mercury not being a planet, but it strongly seems to me that whether it's a planet or not shouldn't depend on how near to the Sun it orbits.

This is a salient point, especially as we continue to detect planets that defy known examples from our solar system such as those in multiple star systems and free-roaming planets not orbiting any stars at all.

To me the heart of the problem seems to be that for literally thousands of years humans understood that there was a collection of objects in the sky that were not stars and also were not transient objects like comets or meteors that appeared to form a natural class. Early observations with telescopes mostly seemed to confirm this. Sure it got a little tricky when we started spotting asteroids, but with the exception of Ceres it quickly became clear that all the other ones weren't round. It wasn't until the 1990s that humans started to discover all sorts of additional round objects both in the outer portions of our own solar system and around a whole bunch of other stars. As a result, things are very unsettled.

Classification is a contentious business, one that leads to lots of arguments among the, usually small, groups of scientists who work on it. For example, in shark taxonomy there's a still ongoing controversy over whether one group of extinct sharks should be Cretalamna or Cretolamna due to a typographical error by a single Russian scientist in 1958 (this is a real thing, I swear). It should not surprise us that the current situation with planets is such a mess, it's just that because the names of the planets in our solar system are well know, it has spilled out into popular culture.

[LibraryOgre]

Jupiter's cleared its orbit of everything anywhere near its own size. And even the asteroids that it shares its orbit with, are pushed into certain locations by Jupiter's gravity - the Lagrange points.

I've read a description of the solar system as "The sun, Jupiter, and some assorted objects", given Jupiter's relative mass.

[Peelee]

I've read a description of the solar system as "The sun, Jupiter, and some assorted objects", given Jupiter's relative mass.

The Sun. IAU recommends capitalizing the initial letter of f all named bodies.

Also, that definition seems fairly accurate.

[halfeye]

I've read a description of the solar system as "The sun, Jupiter, and some assorted objects", given Jupiter's relative mass.

Saturn is over 1/4 of jupiter's mass.

Jupiter is less than 1/1000 the mass of the Sun.

If anything it should be "The Sun and some very small bits".

[Peelee]

Saturn is over 1/4 of jupiter's mass.

Jupiter is less than 1/1000 the mass of the Sun.

If anything it should be "The Sun and some very small bits".

All other planets and moons in the solar system can fit inside Jupiter at the same time. If you want a solar system to include specific bodies, Jupiter is the biggest, baddest one on the block. By far.

[halfeye]

All other planets and moons in the solar system can fit inside Jupiter at the same time. If you want a solar system to include specific bodies, Jupiter is the biggest, baddest one on the block. By far.

It's the biggest planet. It's very light for a star, in fact too light to be a star.

I wasn't initially sure what you meant about the rest fitting inside Jupiter, thinking about it, you probably mean their combined volumes are less than Jupiter's. Which is probably true, but it seems it's well within an order of magnitude.

My first thought was that if you put spheres with the radius of Saturn (58,232), Neptune (24,622) and Uranus (25,362) touching, they would not fit within a sphere with the radius of Jupiter (69,911) (using Wikipedia's mean radii).

All I'm really saying is you can't have a stellar system without at least one star.