we don't sing about what?

To my recent collection of cover versions of "We Don't Talk About Bruno," A. Ducker offered a link to a parody version, "We Don't Talk About Pluto." It's quite amusing and well-written, though better-written than -sung (and they ought to learn how to spell "its"). But: over fifteen years since the IAU reclassed Pluto as a dwarf planet, and people are still that upset about it?

It might help to remember why Pluto was reclassed. Pluto was discovered in 1930. It was supposed to be another gas giant like Uranus or Neptune, but it wasn't, and the more we studied it over the years the less well it fit in with the other planets. But it wasn't until 60 years later that we began to discover other trans-Neptunian objects that were more like Pluto and started to realize that was the class into which it properly fit. And after due consideration the IAU formally created such a category. That's the story.

But what I wondered is, why did it take 60 years? Could some of the others been found earlier if we were looking? If not, how much does Pluto stick out from this new category? The key feature, I figured, was the apparent magnitude. I got a list of important TNOs from Wikipedia, and then looked them up individually, and have here what I haven't seen elsewhere, a list of them, ordered by apparent magnitude:

formal name	nickname	disc.	observatory	apparent magnitude

Pluto				1930	Lowell		13.65-16.3 (mean 15.1)
Makemake	Easterbunny	2005	Palomar		17.0
Haumea		Santa		2004	Palomar		17.3
Eris		Xena		2005	Palomar		18.7
Quaoar		Object X	2002	Palomar		19.1
Orcus				2004	Palomar		19.1
Ixion				2001	Cerro Tololo	19.8
Lempo				1999	Kitt Peak	19.9
Varuna				2000	Kitt Peak	20.3
Sedna		Dutch		2003	Palomar		20.5-20.8
(unnamed)			1996	Mauna Kea	21
Gonggong	Snow White	2007	Palomar		21.4
(unnamed)	Buffy		2004	Mauna Kea	21.8-21.9
(unnamed)	Niku		2011	Mount Lemmon	22
(unnamed)	Drac		2008	Mauna Kea	22.89
Albion		Smiley		1992	Mauna Kea	23.3
(unnamed)	Biden		2012	Cerro Tololo	23.34
(unnamed)	Farout		2018	Mauna Kea	24.6
(unnamed)	FarFarOut	2018	Mauna Kea	25.3
Arrokoth	Ultima Thule	2014	Hubble		26.6

I've inserted the nicknames because many of them hit the news before they were officially named, so the nickname became what they were generally known as. I could have told you that there were TNOs known as Xena and Easterbunny, but would have blanked on what their official names were.

Anyway, from this list it's clear that Pluto is the brightest. But if you know magnitudes, you can see that even Pluto is faint beyond the reach of an ordinary backyard telescope, let alone the naked eye. Tombaugh's telescope was, I think, the 61 cm at Lowell, and a list at Wikipedia's Apparent magnitude article indicates that a 50.8 cm telescope can see up to magnitude 16.4. So he could probably have not quite seen any other TNOs. The discovery of Pluto - which had been photographed before, but nobody had noticed it because it wasn't the big planet they were looking for - was sheer luck. It might not have been discovered till later, with a better understanding of what it was.

Remember that Ceres was also originally considered a planet, and it was only after the discovered asteroids began to multiply in profusion that a new category was created. The same thing happened here.