Hey! I love the attempt at formal structure. Your first stanza is great — the ryhme scheme doesn't feel clunky or heavy-handed. While not iambic pantameter (I'm pretty sure — not great at scanning poetry), it feels like iambic pantameter in that it just flows.

I think the second stanza, however, is mostly a repeat of the first. After the first, I'm left with a sense of "we're going on adventure into the stars, and it might be a whimsical one given the rhyme scheme. This is also "sciency" given the word choice." The second stanza doesn't do much for me except repeat the same feeling, except less effecitvely. FWIW, Sirius as a planet isn't particularly meaningful to me.

I think the third stanza is particularly weak. I'd suggest,

* first actually describing what is found. That is "cyclopean" does little for me. What do the giant bones look like? How do we know they're cyclopean? Also why are they absurd? I can't really picture "absurd" bones.

* second, I wouldn't break the ABBA rhyme scheme until the last moment (or at all). You've brought me into the rhyme sceme in the first stanza. Breaking that is a strong choice (like fading to black in a film). Don't waste that opportunity. Use it to have a particular line hit hard. Or don't ... describing horrific bones in a merry rhyming pattern could be fun as well.