They were a cautious bunch, V.M. Slipher especially.  Years of association with Percival Lowell’s brand of lunacy can leave you a little gun-shy.

But the “object” they’ve found is “one apparently fulfilling Lowell’s theoretical findings”.  In other words, this strange, tiny body is just where it’s supposed to be — going by Percival Lowell’s decades of residual calculations.  And it’s definitely “trans-Neptunian” — that is, it’s out past Neptune, and it’s retrograding.  And the only thing that retrogrades is something in orbit around the sun.

So what is it, if not a planet?