How I killed Pluto and why it had it coming | Hacker News Reader

How I killed Pluto and why it had it coming

theatlantic.com

60 pointshugh322 comments

I grew up knowing that Pluto didn’t “fit” the pattern, thanks to my youthful obsession with outer space. I think the trouble arises when the only thing children really remember about planets is “My Very Eager Mother …” or whatever mnemonic regular kids use to learn the order. If you actually knew anything about Pluto, you’d know about its irregular, overly elliptical off-plane orbit; its large moon, almost a binary planet; its pathetic size; its pattern-breaking proximity to the sun; and the fact it was an outer planet that wasn’t a gas giant.

So scientists breaking your mnemonic are stealing the only thing you ever knew about the planets.

In German the mnemonic was nicely self-referencing: Mein Vater erklärt mir jeden Sonntag unsere neun Planeten. (Translated: Every Sunday my father tells me of the nine planets.)

A friend of mine who's a planetologist points out that the IAU's definition is really not a good one from a planetologist's point of view. It's centered around how the planet moves (which is what an astronomer can see) instead of around how it's composed (for which we need to send a probe).

It's not even that good from the astronomy point of view. A planet has to have cleared its orbit of other bodies—so what about Jupiter's Trojan asteroids? Are they just handwaved out of the way, or is Jupiter no longer a planet?

It's centered around how the planet moves (which is what an astronomer can see) instead of around how it's composed (for which we need to send a probe).

What does its composition matter? That won't help you distinguish anything. Indeed, to first approximation there are only three types of stuff in the solar system: "gas" (meaning hydrogen+helium), "ice" (meaning water, methane and ammonia) and "rock" (meaning... well, rock and metals). And there's planets made out of each; Jupiter and Saturn are gas, Neptune and Uranus are ice, and the inner planets are rock.

As for the "cleared the orbit" bit, check out wikipedia's article on "Dwarf planet"; talk of clearing the orbit is just shorthand for a genuine mathematically-defined criterion which Jupiter fits and Pluto doesn't.

>What does its composition matter?

I said she was a planetologist. To planetologists, composition is everything. Composition is what the planet is, not where it is.

>Indeed, to first approximation there are only three types of stuff in the solar system:

Yeah, but planetologists don't stop at the first approximation.

>As for the "cleared the orbit" bit, check out wikipedia's article on "Dwarf planet";

Let's see...ah. OK, here's the clause that I didn't know about: "and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence" (emphasis added). The Trojans are clearly under the gravitational influence of Jupiter.

Thanks.

While being the guy who discovered Xena would certainly be worth some notoriety, isn't being the guy who killed Pluto also notable?

It is, but thanks to his portrayal in certain sci-fi shows I generally associate Neil deGrasse Tyson with the death of Pluto. I wasn't aware Mike Brown even existed until I read this article.

Sounds like bike-shedding to me.

The argument is very much inflicted with the bike-shed disease, largely because memorizing the planets is a common school activity so everyone feels they have a stake regardless of their scientific understanding of the subject.

The basic problem is that the definition of a planet has transitioned over time, a lot of time, from a colloquial definition to a scientific one. For the longest time "planet" was just a name for any large, permanent part of the solar system that was worth memorizing. Initially even the Sun and Earth's moon were planets, but the heliocentric revolution placed the Sun in a different category and inadvertently relegated the moon to a minor body along with all the other moons (notable only due to its proximity to human civilization). For a few decades the first 4 discivered asteroids were planets, but the discovery of hundreds and then thousands of similar asteroids brought the realization that these 4 bodies were not quite as notable as once thought. Pluto lasted much longer as the only known KBO for the better part of a century and thus snuck into planet hood.

When Pluto was alone it was much easier to ignore its differences and just chalk it up as the quirkiest member of the planets despite its tiny size, eccentric orbit tilted to the ecliptic, etc.

As we've discovered more planets in other systems and as we've discovered more objects in our outer solar sytem a firmer definition of planet has become more necessary. The new criteria for planet hood are straightforward and useful in understanding the nature and evolution of planetary systems. However, Pluto doesn't satisfy them. In scientific terms this isn't a big deal, but for the public and for the colloquial definition of planet there is still a lot of resistance to change.

P.S. Here's an artists rendering of various known KBOs: http://en.wikipedia.org/wiki/File:EightTNOs.png

Drawing the "planet" line on the other side of all of these certainly seems a lot easier than figuring out where to draw the line within this group.

OT but can anyone explain to me why the planets all have the same orbit plane? I've never read a succesful explanation.

Because they formed (conglomerated) from an accretion disk around the Sun. Being a disk, it is in one plane.

Why a disk? The sphere of dust surrounding the Sun is drawn inwards and conservation of angular momentum flattens it out to eventually form a disk.

Simple explanation: Accretion disk: http://www.daviddarling.info/encyclopedia/A/accretiond.html Planetary formation: http://www.daviddarling.info/encyclopedia/P/plansysform.html

Wikipedia (detailed maths): http://en.wikipedia.org/wiki/Accretion_disc

Take a proto-stellar nebula, it's a big mass of gas and dust thousands of AU in diameter. Each particular bit will generally be moving very slowly in a random direction relative to the whole but the whole thing has some particular net angular momentum. As the nebula collapses to form a star this angular momentum gets concentrated (insert standard ice-skater pulling in their arms description here), which is why stars tend to have some particular spin. The same mechanism of concentration works to help form a protoplanetary disc. Objects in eccentric orbits or orbits tilted to the average plane of the forming disc will be more prone to collisions, causing them to be absorbed or captured by larger bodies or to form a debris cloud of generally lower eccentricity and lower inclination.

Over time these effects add up to cause most of the mass in the inner Solar System to be concentrated in a small number of planets in circular orbits within a narrow range of inclinations.

Note that in the far outer Solar System of the kuiper belt and oort cloud these effects are much diminished (orbital periods are much longer, distances are much larger, overall volumetric mass densities are lower, etc.) so objects are more likely to have eccentric and inclined orbits relative to the inner solar system consensus.

I get most of that, just not how it ends up in a flat disc. as opposed to an array of discs or something similar.

Is the idea that if you took the nebula, and measured across in 1 deg increments, the plane that had the largest percentage of largest objects is the highest probability of becoming the eventual plane?

This seems like the sort of concept that could do with a nicely done animation to explain.

We, the pluto activist, do not take kindly on actions like this. We demand that pluto regains planetary status if your demands don't get met we will take agressiv action. :)

The "Save Pluto" crowd do a disservice to science in general, instead of using this as a way to highlight the scientific method and the importance of always reassessing ideas based on new data. I applaud Pluto's death as a contrast to inflexible memorization of facts and demonstrating that facts are only valid as far as they are backed by observation and nothing is sacrosanct. While it may be difficult for some, I can't think of a better example of the impartiality of science - Technology may someday remove a planet, but for now it's only science.

Yes and no. The classification of Pluto really isn't a matter of science, it's just a matter of words. We could, if we felt like it, declare that Pluto is a planet. We could arbitrarily decide that anything more than 100km in radius is a planet. Or we could declare that nothing smaller than Uranus is a planet (goodbye, planet Earth!)

While the discovery of a bunch of dwarf planets filling in the size gap between Pluto and the smallest asteroids is certainly suggestive that the definition of "planet" needs to be revised, we could quite easily have gone with another definition. From a scientist's point of view I don't care much one way or another, but from a layman's point of view I can see how it'd be nicer to live in a richer solar system with twelve official "planets" rather than our new impoverished solar system with just eight.

Except that it would end up as 40+ planets, most of which are clearly asteroids, in order to pretend that science is constant.

> Except that it would end up as 40+ planets

I think that would be fantastic personally. The semantics of "planet" vs. "asteroid" is clearly far more motivational in terms of sending spacecraft to study them, plot stories around them, etc.

A planet is a place (that one might go), an asteroid is just a marginally interesting hunk of rock.

(it's really just a matter of marketing to be honest)

? I'm not sure I understand. Are talking about marketing the term for the general public, or in order to convince scientists to study them?

As for the scientists, I think they already have an interest in studying asteroids, as evidenced by this list of NASA missions involving asteroids: http://nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.ht...

I really think this is more about organization than marketing.

The labels for things have nothing to do with the state of nature, but rather with the way people work. The fact that we call Earth and Jupiter the same thing but consider the sun and Pluto to be different kinds of things is simply a choice that is meant to aid human understanding, not a statement about nature. There is no such thing as "chair" or "star" or "supermarket" in the laws of physics, those are just words we use to make sense of the world. We could say that the solar systems are made up of "gassies", and "rockies", and "icies", and "firies", and "clumpies" if we wanted instead of Gas Giants, Rocky Planets, Comets, Stars, and Asteroids, and it wouldn't make a damned bit of difference to the way nature actually works (if you get your descriptions of those kinds of things accurate enough, then you could still model the behavior of the solar system just as well as if you were using "planet" in your descriptions instead!), but, of course, it makes a difference to how people think about these things, which might help or hinder our analysis of them. (Note: as an example, if we were using "clumpies" for asteroids, then we might be arguing about whether Ceres is a "clumpy" or a "rocky", since it's much more regularly shaped than things like 216 Kleopatra... And speaking of classifying things, are dinosaurs lizards or birds? :p)

Of course, those labels aren't purely artificial. Once you define some measurement of similarity, you can identify clusters in a data set, if there are clusters to be found. Those clusters are reasonable things to lump together with labels.

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