yesssssssssss! #LongLivePluto
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No, Titan is number IX. Iupiter, Saturn, Uranus, Neptune, Terra, Uenus, Mars, Ganymede, Titan, Mercury, Callisto, Io, Lunaā¦
Screw this āmoons donāt countā nonsense!
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Isnāt categorization of inherently non-categorizable objects fun?
Though Iām curious about why your spelling is the way it is.
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Iād say āancient Latinā but Iād expect thatād be Vranvs and Venvs, not Uranus and Uenus.
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Different forms of the same letters. But people keep pronouncing Latin V like intermedial Greek B. I think people are less likely to pronounce U that way.
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True, but then gravitational effects were how they knew to look for Uranus, Neptune, Vulcan, and Pluto. Two worked out to be real, two turned out to be mathematical artifacts. So you can probably give this one a 50-50 chance, right?
Why would we order by diameter rather than mass? The former can be potentially tricky to define for irregular objects, or even things with atmospheres. The latter is more straightforward, and if something orbits the object, you donāt even need to look at it to figure it out.
Of course, if you do order by mass, you might notice thereās a surprisingly clean cut-off between the heavier things with high neighborhood clearing and the lighter things with almost none. Theyāre only non-categorizable if you donāt like what that tells you. 
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It seems to me that thereās more of a cut-off between Earth/Venus and Mars/Mercury than between Mars/Mercury and Ganymede/Titan/Callisto.
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Indeed, mass should be used, since gravity is the main thing affecting other objects. Diameter is variable since density can vary quite a bit (e.g., neutron stars).
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Because Iād already posted before figuring I could reorder by mass. I donāt know why the site defaults to order by diameter.
I vaguely remember a theory that Mercury was a larger planet, was destroyed in a collision, and only re-aggregated from the iron core. So there might be a better boundary between the small terrestrial planets and the large moons after all.
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And even more between Jupiter/Saturn. But what stands out to me is not just the size of the Mercury/Ganymede gap, but how it correlates with other things ā a 100% switch from worlds that dominate their region of the solar system above, to moons and members of belts below. I am impressed by that; however you name things, I think thatās as real a distinction as nature is ever likely to provide.
But I have talked enough about this before, and didnāt really mean to subject you to a full repeat here.
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Hereās an interesting calculation:
Letās assume that, by āseven times larger than the earthā, they mean that āplanet nineā (henceforth PIX) would be seven times more massive, and that the lower estimate that they give for its current orbital distance (500AU) is its semi-major axis.
Within our own solar system, the discriminant used to filter planets from dwarf planets, Ī , has two variables: mass and semi-major axis, measured in Earth masses and AU, so PIXās Ī would be Earthās, multiplied by 7, divided by 5009/8.
The divide between Mars and Ceres is fairly uncontroversial because there are three orders of magnitude between their values of Ī (54 and 0.04, respectively), and at least one in each direction from the dividing line at 1.
Our hypothetical PIX would have a Ī of 5.2, almost right on the line dividing ādwarf planetā from āplanet.ā And thatās before taking into account the rather large error bars for both its mass and semi-major axis.
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Thatās an interesting thought. Considering that Ī doesnāt actually measure whether objects are there or not, that still works out to a pretty good description of PIXās proposed role, somewhere between the region-definers and belt members we know.
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I find the results of that equation peculiar. It seems to me that an object 7 times more massive than Earth should be a planet, not a dwarf planet, regardless of the debris in its orbit. Perhaps a cutoff in the definition should be included.
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Thatās pretty much the point Iām trying to make.
Pluto isnāt a planet because it hasnāt ācleared its orbit,ā and, fine, sure, that might not be controversial now. But if PIX is what and where they say it is, we may need to rethink what a āplanetā is all over again.
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Might have to look at the definition of āclearing the orbitā. If Nine exists, it will be at least a partial cause of the eccentricities of the Sednoidsā orbits through orbital resonance. Thatās not too dissimilar to what happens with Neptune - Pluto and the āPlutoninosā have eccentric enough orbits to have their perihelia inside Neptuneās orbit, but they will never hit Neptune as theyāre locked into a 2:3 orbital resonance.
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Let Pluto be a planet. Can we have too many planets?
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Well, the IAU certainly could. They keep track of over a hundred thousand objects, and it becomes hard to use only a flat list of proper names. So they had turned to designations, with years and letters for comets, and Roman numerals for satellites, and Arabic numerals for belt objects. And these had never included the handful of āplanetsā, since they were thought their own category with just a few members.
So to them figuring out the limits of that category wasnāt just an exercise in capturing what we were learning about the solar system ā though I will say I personally had been so excited to see that part unfold, watching over decades as the Kuiper belt went from a prediction to a known thing full of many different worlds, and similar belts were found around other stars. Let alone what is frustratingly always brought up instead, whether the one Americans already knew is getting its respectful title.
It was a question of keeping a working system of nomenclature, one that can handle many new discoveries without problems, and astronomers can hopefully still use for centuries to come. Apparently thankless work, as most taxonomy seems to be, but no less important for that.
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All the astronomic discoveries over the last decades have been absolutely mind-boggling for an interested layman like me.
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Vulcan was one of the artifacts which was the other one?
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I meant Pluto. I was using a confusing shorthand here, I guess, because that is a real object. But it is not what they were searching for, a planet X thought to be responsible for perturbations of Uranusās orbit. Looking out into the distance, relatively bright Pluto was in about the expected place so hailed as the predicted planet.
But early on it was doubtful if it were really massive enough for that, though still overestimated by an order of magnitude; apparently Kuiper thought it would be big enough to clear his eponymous belt. The perturbing mass turns out not to exist, and now we know Pluto is small enough to be the first of many objects there.
I was also misremembering my older history. Neptune was predicted based on Uranus, but Uranus itself seems to have been discovered with no such expectation. So really, we are one for three.
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