Thursday, August 27, 2020

faint little object

Never mind why this came up, but I was wondering why it took over 60 years after the discovery of Pluto to find any of the other, now known to be numerous, trans-neptunian objects in our solar system. It wasn't because they weren't being looked for. Clyde Tombaugh found Pluto by chance, but it was in the course of a search for additional planets, which he kept up for another dozen years without finding anything else out in that zone. (Please note also that these things are damned hard to notice. Pluto was first photographed over 20 years before being discovered, but nobody noticed it wasn't just a faint star until Tombaugh used his blink comparator.)

The obvious answers are 1) because he was looking in the ecliptic, and Pluto has the least eccentric orbit of the bunch, or so I have read it does; and 2) though very faint, it's by far the brightest such object. But how much brighter is it? A search for a list of the apparent magnitudes of these objects didn't produce anything, though perhaps you can find one. What I did find was a list of the absolute magnitudes, which is not the same thing, and doesn't tell us how visible they are from Earth. I can get the apparent magnitude from Wikipedia articles for at least some of the objects (not all of them have Wikipedia pages), but I don't want to have to look them all up.

The two brightest in absolute terms are Pluto and Eris, which are both between -0.7 and -1.2, depending on which sources you consult. Pluto's apparent magnitude varies from 13.65 to 16.3, depending on where it is in relation to us. Eris has a much more eccentric orbit, but is mostly much further out, and its apparent magnitude is given as 18.7, though that's got to vary. At any rate it's certainly a lot harder to see, though I note it was first photographed fifty years before it was discovered. (The term for this is "precovery.")

(The best precovery story I've found is this: when Galileo was studying the moons of Jupiter, he noticed a star behind Jupiter which seemed to move, but he ignored it. It was Neptune. Which wouldn't be officially discovered for well over another 200 years.)

Anyway, I'm disinclined to look up a bunch more objects, but that's enough to give me a guess as to what I'd find.

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