or, the age of innocence from an astronomer's point of view
for 76 years, kids in school have learned that our solar system has 9 planets: mercury, venus, the earth, mars, jupiter, saturn, uranus, neptune and pluto. first the four "rocky" ones, small and made out of solid material. then the four "giant" ones, much more massive and made out of a dense mixture of gaseous elements. and then, there was pluto.
pluto was the last one to be discovered, in 1930, and has always had the charm of an outsider. it is the farthest of the sun's court, but not always: its orbit is in fact very eccentric, and crosses the one of neptune, so that the two switch regularly in being the most distant from the sun. another weird fact about pluto is that it is rocky and tiny, like the earth-like planets, and unlike its closer, giant neighbours.
when pluto was still a planet, it also had a satellite, charon. charon was identified as a satellite only because it had been discovered more than 40 years later after pluto, but it would have been more appropriate to refer to them as a double planet system: contrarily as for all other satellites, the mass of charon is not significantly smaller than that of pluto.
then, the past decade witnessed new discoveries: quaoar, sedna and eris, all objects with masses and orbits very similar to pluto's. who knows how many other pluto-like bodies future observations will report. not being able to ignore the new discoveries, but also trying to prevent the number of solar system planets to increase dramatically, the international astronomical union called a halt and came out with an official definition for a planet.
the new definition involves not only being a celestial body orbiting the sun with a nearly spherical shape, but also being the object which contains most of the mass in its own orbit.
not being much larger than its fellow charon, pluto did not fulfill the definition, thus ceased being a planet in 2006.
i grew up with the dream of becoming an astronomer, thinking that the practice of such a pure science was the highest step of human knowledge. not that it mattered much, but at the time i was also aware that i was living in a special planet out of a set of nine. then i started moving my first steps in the world of astronomy and astrophysics, still amazed by the idea of research. i was walking along the twilight zone, uncertain about how to cross it, and suddenly pluto was not a planet anymore. reality was there to be faced: science was no more the purest of all things, it is in fact just a human matter. it was not as neat as i thought, but most importantly i learned how it isolates the individuals who practice it from the rest of the world. i started wondering if this price is worth the game. and i'm still wondering. the age of disillusion.
right after the end of pluto's career as a planet, the american dialect society chose "to pluto" as word of the year. it's supposed to mean "to deprive something of its value". along with the former ninth planet, also my dream was plutoed. but i decided to restore its value. i realised science was not what i had expected, but i chose to deal with it, to adjust my dream and take the most i could from it. pluto is now a dwarf planet. let's see what happens to me and science.
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3 comments:
Pluto still IS a planet; it never stopped being one. What happened was that four percent of the International Astronomical Union, in a highly controversial vote on the last day of a two-week conference, voted in favor of a linguistically-nonsensical definition that states that dwarf planets are not planets at all. Most who voted are not planetary scientists; conversely, many planetary scientists are not IAU members and had no say in this vote. The decision was immediately opposed in a petition of 300 professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto and one of the leading scholars in the world on Pluto and the Kuiper Belt.
That petition can be viewed here:
http://www.ipetitions.com/petition/planetprotest/
The IAU definition is not "reality," but one interpretation and a poor one at that. It defines objects solely by where they are while ignoring what they are. If Earth were in Pluto's orbit, according to this definition, it would not be a planet either.
Of all the Kuiper Belt objects discovered, only Eris has a mass and size greater than Pluto, and not by much.
There is no scientific basis to the preference for keeping the number of planets in our solar system from increasing dramatically. If we have 200 planets, then that is what we have. No one tries to limit the number of stars or galaxies, which are in the billions. This artificial attempt to keep the number of planets low is based on nothing other than sentimental preferences.
A better planet definition is one that keeps the term planet broad to encompass any non-self-luminous spheroidal object orbiting a star. This is because when an object becomes round, a state known as hydrostatic equilibrium, meaning it has enough self gravity to pull itself into a round shape, it develops the same geological processes as the larger planets, processes that inert asteroids do not have. Pluto, Eris, and the other dwarf planets are differentiated into core, mantle, and crust, which makes them far more akin to planets than to asteroids.
In astronomy, dwarf galaxies are still viewed as a subclass of galaxies, and dwarf stars are still viewed as a subclass of stars. The same should be true of planets. We can distinguish types of planets through use of subcategories, with dwarf planet being one of several, the others being terrestrial planets, gas giants, ice giants, super Earths, hot Jupiters, etc.
Reality has not changed; there is no need to accept a decree from this tiny group when even professional astronomers have rejected it, as can be seen from the Great Planet Debate that took place this past summer. You can view the proceedings here: http://gpd.jhuapl.edu/
Counting dwarf planets, we are living on a special planet out of 13: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.
Hang on a minute. Yes, it is a semantic argument, but we haven't a clue whether or not Pluto has a separate crust, mantle and core. We'll probably find out in 2014 when the New Horizons probe gets there.
dear Laurel,
thanks for your comment and sorry for the extremely late reply. I have thought about writing for a while, but never actually did it in the end.
first, I am very sorry that you probably missed the whole point of my post. It is probably my fault, I should have given more weight to the main, fundamental issue I wanted to point out. I did not want to support Pluto's "demotion" in any way, I just used the question as a metaphore. What I really wanted to write about is absolutely a different matter: the frustration that arises in most young scientists after some time they have spent actively practising science. I should have probably written more about this issue, and less about Pluto, but my idea was to tell a story, the one of Pluto's "demotion", in a fully consistent way. It is a fact that the IAU has changed the definition of a planet, I don't find this choice particularly interesting, but my point was simply to tell this story with enough details, and then use it to talk about something else.
second, I believe the discussion about Pluto can go on forever, which is why I was always reluctant about replying to the post!
It is, in my opinion, a simple matter of definitions. Names are only a convention, and so are classifications. Classifications are only useful when they are able to identify the meaningful correlation underlying the existence of different classes of objects; otherwise, a classification is just a pointless catalogue from which we have learned nothing.
I am an astronomer, although not involved in planetary science. The observation and study of planets in our solar system is especially important in the framework of stellar and planetary formation, which is still not thoroughly understood. In such a framework, the dynamical state of an object is a key element to constrain different models. Hence, I can understand this is one of the reasons why the dynamics has been chosen as the strength point of the new definition, which I respect.
I have to admit that it saddened me from a personal and cultural point of view (otherwise I would not have been writing this post, and with such a title!) but that is just feelings. Nothing against feelings, just, they are not science.
I am very happy that a person like you, who is not a scientist, has been so passionate about such a topic and has learned so much about it. I am happy also that people all around the world have discussed and protested about the Pluto "demotion" issue. But I am also sad, because such a big debate, involving scientists and the public at the same time, is focussing on a topic (definitions, classification) from which not much can be learned. There are so many more interesting and deep topics in cutting-edge astronomy and astrophysics that are largely ignored by the general public, which hardly manage to make it into the news, while big debates are held about how we should define planets (no offence to planets, my criticism goes towards the issue of definition).
Objects need a definition, evidently, and definitions are widely accepted because they are based on physics. As you mentioned, dwarf stars are a subclass of stars, true. But why are then brown dwarves not stars, although they have some characteristics similar to those of low-mass stars? because they don't experience thermo-nuclear reactions in their cores, and it is this property that fully defines a star.
But I believe that we need to be flexible, because physical evidence changes by the day. Scientific progress evolves continuously. New instruments and technologies enable us to watch the universe with new eyes from year to year. New discoveries point out new properties of objects, which sometimes require the definition of new classes; but further observations can reveal new characteristics of such objects, and the definition has to adapt again to the newly found properties.
As an example, galaxy clusters, which are the most massive bound objects in the universe, were first discovered by visual inspection of optical images, and thus identified as overdensities of galaxies with the same redshift. As time went by, extensive X-ray observations became available and showed that the mass in galaxies of a "galaxy cluster" is only about one tenth of the hot gas responsible of the X-ray emission of these objects. And they contain even more dark matter than hot gas. By now, the community still refers to them as "galaxy clusters" but maybe one day somebody will come out with a new name such as "hot gas big blobs" or "dark matter large objects" and, unless it is not physically wrong, I don't see why it should not be respected and accepted, but no harm comes from keeping the original, historical name. After all, and I am stressing it, it is only a name.
Again, thanks for your comment and all the best!
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