

How I killed Pluto and why it had it coming - hugh3
http://www.theatlantic.com/technology/archive/2010/12/how-i-killed-pluto-and-why-it-had-it-coming/67242/

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alanh
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.

~~~
eru
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.)

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metageek
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?

~~~
hugh3
_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.

~~~
metageek
> _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.

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

~~~
AgentConundrum
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.

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DjDarkman
Sounds like bike-shedding to me.

~~~
InclinedPlane
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.

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

~~~
InclinedPlane
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.

~~~
brc
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.

------
nickik
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. :)

------
J3L2404
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.

~~~
hugh3
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.

~~~
J3L2404
Except that it would end up as 40+ planets, most of which are clearly
asteroids, in order to pretend that science is constant.

~~~
Figs
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)

~~~
pjscott
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.

