Ogres
"You can imagine the layering a bit like an onion, except in this case we are considering two separate onions of differing size"
Right, then, it's not a duck, it's conjoined twin ogres.
Findings from the European Space Agency's Rosetta probe solved the riddle of Comet 67P/Churyumov-Gerasimenko's unusual shape. They have proof that the comet is in fact two planetary bodies joined at the hip. ESA's Rosetta probe took ten years to reach its target last July and, when it did, the science team had a bit of a shock …