Wednesday, July 22, 2015

Alan Stern On Why Pluto Is Still A Bona Fide Planet


View Of Pluto from the Long Range Reconnaissance Imager  Aboard New Horizons spacecraft then half way to its minimum distance of 7700 miles.

Alan Stern, New Horizons principal investigator,  gave a terrific analogy to the demotion of Pluto to a 'dwarf planet' in the NOVA special  'Chasing Pluto'.  He compared the ridiculous IAU vote to a similar vote by canine specialists to vote all small dogs, i.e. pugs, Chihuahuas etc. to 'dwarf dogs' while all larger dogs are 'real dogs'.

Of course, to make the analogy much better,  one would postulate a scenario in which people go crazy about breeding and owning small dogs which soon outnumber large breeds by 10:1 or more. Worrying about a proliferation of small dogs, the specialist canine community makes the decision to vote on redefining them as 'dwarf dogs' to try to discourage ownership and breeding.

In Stern's Wall Street Journal interview (July 18-19, p. A9) he is even more specific, after referencing what led to the IAU vote, i.e. when scientists began finding worlds even  farther away and one (Eris) nearly the same size as Pluto. This led certain investigators (e.g. Michael Brown) to propose a special class for the proliferating small planets, calling them all 'dwarf planets'.

So rather than have public confusion about hundreds of planets, it made sense to them to separate eight regular planets from the multitude of dwarf planets.

As one IAU voter put it (ibid.):

"We're not so keen to have Pluto and all his friends in the club because it gets crowded. By the end of the decade we would have had 100 planets.."

But Stern objected:

"That's a very 20th century view. Our technology used to be very limited, so we could only see a few planets, and we could memorize the list because we thought we knew them all. Arbitrarily limiting the number of planets to a familiar few is like deciding to name only seven mountains on Earth,"

Of course, the response of the pro-dwarfers would be that gazillions more people are interested in space and the planets rather than all the terrestrial mountains. But in fact that planet prejudice is immaterial to Stern's basic point. In another parallel universe, many more might be interested in mountains... but planets -  not so much.

Stern also dissents on more technical grounds given the IAU insists a bone fide planet must have gravitationally swept its neighborhood clear of other objects which Pluto hasn't done. But as Stern observes, a body big enough to clear a small orbit and qualify as a planet might fail to make the cut for a larger orbit (as Pluto has).

In other words, when is a planet not a planet? Well, when it is somewhere else!

Stern goes on to maintain Pluto is a planet simply because it is massive enough that gravity has forced it into a spherical shape. (A sphere - for technical reasons - is the maximum volume shape arrived at from g-forces) By this definition, Ceres also qualifies as a planet.

In one of the most incisive points, Stern acknowledges that planetary scientists regularly define objects this way. Which is why it was wrong to bring the planetary definitional vote before the entire International Astronomical Union - because you're mixing planetary astronomers, with stellar astrophysicists, solar physicists, cosmologists, space physicists, astro-geologists, astro-biologists, astrometry specialists (astrometrists), celestial mehanicians and so forth.

As Stern aptly puts it:

"I know the public thinks we're all the same because we study things in space. So my analogy is doctors. Would you let a podiatrist do brain surgery on you. Wrong specialty! So don't let an astrometrist classify planets."

A point well taken which is why many of us ignore the  supercilious IAU vote and continue to regard Pluto as a planet.

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