Because consideres pluto as a icy body rather than a planet
Hyperbole, you can't think of something a billion times a day.
In my opinion, the whole poem is quite ironic - although she is mentioning the exultation and the royal color of death, the poem itself begins with the narrator saying that she cannot breathe - that she doesn't want to die.
So, I would say that the ironic parts are:
Exultation is the going
Of an inland soul to sea, -
Past the houses, past the headlands,
Into deep eternity!
The Whites learn that their son Herbert is dead.
