But the reclassification took place in two separate contexts; one was the astronomical community, and your comment addressed the thinking of those in that group. The other was a large segment of the general public who (to the surprise of a lot of astronomers) had a sentimental attachment to the idea that Pluto is a planet and who fought against its reclassification rather stridently.

People took the re-classification of a celestial body personally and that’s kind of interesting from a sociological perspective (if not an astronomical one).

Do you have a rough order of magnitude estimate of how much hydrogen fusion occurs within a mid-range brown dwarf?  I was led to understand that it was none or negligible, but I’d be happy to stand corrected. 

Also, is the boundary really so thin?  Again, I thought that the mass-gap between non-fusing bodies and hydrogen-fusing bodies was at least 50 jupiter-masses?

1) a sociologist claims that some scientific concept is “socially constructed”
2) scientists point out that this view has absurd implications
3) the sociologist retreats, insisting that what was meant by “X is socially constructed” was so mild and obvious a claim as to be practically tautological.

So what do you mean by “social construction”, and what distinguishes social constructions from those things which are not?  Writing that “it means what differences we decide by consensus are significant enough to warrant redefinition”, if it is to be interpreted non-trivially, seems to suggest that the decision to distinguish protons from electrons (say) is on a par with the decision to distinguish salad forks from fish forks.  Surely you do not believe that to be the case, do you?  Surely the decision to overhaul scientific vocabulary in light of new advances has something to do with the world itself, and the relative fitness of the theories in which those terms take part? 

I offer Paul Boghossian’s gloss:

To say of something that it is socially constructed is to emphasize its dependence on contingent aspects of our social selves. It is to say: This thing could not have existed had we not built it; and we need not have built it at all, at least not in its present form. Had we been a different kind of society, had we had
different needs, values, or interests, we might well have built a different kind of thing, or
built this one differently. The inevitable contrast is with a naturally existing object, something that exists independently of us and which we did not have a hand in shaping.

Do you have substantive disagreements with this characterization?

On this reading, Pluto’s declassification does not imply that planets are “socially constructed”; in fact, quite the opposite.  The definition of planet was revised in light of new discoveries about the properties possessed by star-orbiting bodies, roughly: that Pluto is to the Kuiper belt as Ceres is to the asteroid belt. 

If, in light of these discoveries, we had continued to call Pluto a planet, then I would be in complete agreement that the term “planet” is a folk conception that more or less arbitrarily picks out eight celestial bodies for reasons of cultural/historical significance.  Instead, the declassification brought the term “planet” in line with the facts of the matter about the solar system and the sorts of bodies that tend to form within them.

Consider the following example, due I believe to Quine: “a whale is a great fish.”  If “fish” just means “marine animal”, then the statement is a definitional truth.  If, on the other hand, “fish” is construed in its modern sense, i.e. a member of Osteichthyes, then it’s clear that whales and fish are very different things indeed.  The phylogenetic remoteness of whales from fish is a fact that we learned about the world that prompted us to revise the definition of “fish”.  I say that the revision captures a culturally-independent truth about the world, and hence not socially constructed. 

Do you disagree about that?  If not, why then about Pluto? 

 It was recategorized so that the term “planet” would play a more useful conceptual role in astronomy.  

The term “planet” is only “socially constructed” in the utterly trivial sense that there is a society (of sorts) of astronomers that decides what the word means.  But the criteria for deciding what the word means are pretty much set by objective features of reality.  (I am glossing over a lot of philosophy of science in the interests of being brief.)

If you want a good example of a social construction, consider concepts like “baseball” or “the president”.  Baseball is baseball because of a shared understanding of the rules of the game, and the president is the president because we decide to act accordingly.  The concept of a planet is not socially constructed in this sense.