as soon as i saw that all the planets orbit the sun in a disc except pluto which is at like 45 degrees i was happy to consider it a stray that got caught in the suns gravity during its travels - not a planet that coalesced from gas and stuff along with ours 4.5 billion years ago
so big asteroid is my opion
(although what if it formed elsewhere as a legit planet but ended up here hehe who knows)