Back in October I had the chance to give a talk in conjunction with Dr. Bruce Greenway, a fellow Michigander and an actual professional astronomer.  He made the most convincing case I’ve seen for Pluto’s reclassification — demonstrating beyond the shadow of a doubt that Pluto has far more in common with objects like Eris, Ceres, and Pallas than bodies like Mercury, Mars, Earth, or other canonical planets.  The difference in mass is crucial — and the degree to which this massiveness has allowed the principal bodies in question to clear the neighborhood around their orbits.  If you’re wondering whether Pluto’s more like Eris or Mercury, this graph ought to make the case plain:

I’m cribbing this slide from Bruce’s PowerPoint.  It points out exactly how drastically the difference in mass affects the amount of other “crap” in the neighborhood.  And seems to suggest that — all legacy considerations aside — Pluto ought to be categorized with its fellow dwarves.

We ought also to be struck by how extremely unlikely it was that Clyde would have seen such a tiny little thing.


Trans-Neptunian Objects isn’t a term used much any longer. “Dwarf planets” is the preferred term, if you’re being PC. (“Little planets”, if you’re being really PC.)

Anyway, these are the biggies. They’re all out beyond Neptune’s orbit. They’re all large enough to be kinda planetlike in some ways — they’re in hydrostatic equilibrium (meaning they get mostly round and stay mostly round).  They’re not themselves satellites.  But they aren’t large enough to have “cleared their neighboring region of planetesimals,” i.e., to have slurped up all other orbiting bodies unto themselves.  Which means, according to the International Astronomical Union’s Resolution 5A of 2006, they aren’t planets.

And what would Clyde Tombaugh have thought? My impression of Clyde in his later life is of a man at peace with himself.  Every picture I’ve seen of him shows him beaming, and surely he was much beloved.  Though he defended Pluto’s planetary status when it first was questioned in the 1990s, I believe he also recognized his discovery of Pluto as being something of a happy accident.  And today, I can imagine him arguing that we welcome the planetary status of such substantial bodies as Eris, Haumea, Makemake, Orcus, Quaoar, and so on.  The universe had made room for him in such an unlikely fashion, after all – I believe he’d have wanted us to return the favor.  So on Clyde’s behalf, let the IAU know we want Pluto back!

(Thanks once again to the great Laurel Kornfeld for her ongoing, informed contributions to this discussion – and for filling in some details on the IAU kerfuffle.)

If the astronomers at Lowell Observatory suspected they had something not-quite-planetlike on their hands in 1930, they had ample reason to keep their suspicions to themselves.  The long-delayed search for Planet X had finally resumed in early 1929.  Thousands of dollars of new facilities and equipment had been brought in.  Expectations were high.  By February 1930, Clyde Tombaugh had been examining high-resolution photographic images for ten months.  And Lowell Observatory had its reputation to rescue. 

In 2006, Mike Brown finally called their bluff.  His blog is here.  And here’s his thoughtful, funny (scientists are usually funny, except when they’re really not funny at all) piece on the reasoning behind Pluto’s demotion.

“I would have been happy if we’d tried to come up with some new words to describe the eight large objects that orbit the sun, but the word “planet” is cultural, so we’re not touching it. But the IAU felt the pressure to come up with a scientific definition. As scientific definitions go, I think saying there are only eight planets works better, but it’s an aesthetic choice. It’s a religious choice. There’s no science there…

So Pluto is not a planet now, it’s a dwarf planet. A dwarf planet, for those of you who were wondering, is something that looks like a planet because it’s round, but is not a planet. We currently know of about 35 objects that fit the bill of dwarf planet, but we’ll find hundreds.”

Other planet killers include The Lexx, The Foreshadow, The Eye of Ra, The Krenim Temporal Inclusion Ship, Red Matter, The Death Star (I and II), Arkon Bombs, and Ice-Nine.