As I have commented before on several occasions, I have since my youth been a partisan of the minor planet Pluto . When Pluto was first discovered by astronomers some nine decades ago or so, the planet seemed particularly eccentric. Later on, as our knowledge of the solar system became more complete, it was discovered that Pluto was not the only sort of planet that had a strange elliptical orbit far different from that of either the smaller inner planets or the gas giants, but that its orbit and its inclination at an angle from the plane for the inner and outer planets (as they are called) were typical of an entire type of planet that had hitherto been unknown, of which Pluto was merely the first (and not even the largest) example. The experience of finding odd minor planets in our own neighborhood, as it were, ought to have clued us in that perhaps we did not know all of the types of planets that could exist in our solar system, and that perhaps we were unlike the sort of subdivision where one finds only one or two types of houses repeated endlessly throughout the neighborhood that we are a far different sort of neighborhood with far greater variety. We ought to have considered this given the difference between the rocky inner planets from Mercury to Mars, the asteroid belt, and the gas giants, the comets whose paths we have sometimes charted, and the various plutonoids and other minor planets so far discovered in the Kuiper Belt.
But we still find ourselves surprised by the discoveries we make when we look a bit closer at the spaces beyond Pluto. The New Horizons mission, whose job it has been to explore some of the more notable Kuiper Belt objects that we have been able to notice to date, has not only made some major discoveries in places that we have already known, like the small double planet of Pluto and Charon whose center of gravity is between them, but with the bowling pin-shaped minor planet known at present temporarily as Ultima Thule. And therein is a story of the lack of imagination that has often taken place when planets have been viewed from the light of what we know so far. Planets, especially minor planets far away, are a lot stranger than we have assumed based on the fairly ordinary ones that are close to us.
What makes Ultima Thule so unusual are a few qualities. For one, the object had not even been properly named, but had been given a name that only an astronomer could love, 2014 MU69, showing that it had only been discovered at all somewhat recently, since it is about a billion miles further from the sun than Pluto is. It is additionally shaped somewhat like a bowling pin according to photos taken from the New Horizons flyby, and scientists thought that the planet may have in fact been two minor planets circling each other in the manner of Pluto and Charon. Apparently, though, there are two cores, one of them larger and the other smaller, suggesting that it is possible that there were once two separate bodies that became so close that there was a collision that fused them together into one composite body, but the cores were not of such disparate size that the other core was entirely eliminated. Your guess is as good as mine as to when that happened or if some other reason is responsible for the odd appearance of the planet.
Regardless of its origins, though, Ultima Thule is yet more evidence that the solar system is stranger than we thought. And that is a good thing. It is worthwhile from time to time to have entirely new classes of planet discovered, lest we think that we have seen everything and thus know everything there is to know about planets. If our own neighborhood can provide such surprises like finding the equivalents of tiny bungalows or quirky trailer parks in a subdivision that we thought only contained mini-mansions and ordinary 3 bedroom/2 bath single family dwellings, then we will likely be far more humble when it comes to judging what could possibly be contained in other neighborhoods that are more distant and thus harder to uncover. Perhaps there are entire classes of planet that are beyond our imagination at present that await future discovery. At the very least those who posit cosmologies about the formation of planets can have their imagination fired so that they can better conceive of at least some of the possibilities that we may yet find in the future if we look hard enough. At the very least, this sort of discovery can help justify the expenditure of sending missions like New Horizons out into the great unknown to report on what they find and thus improve our knowledge about our neighborhood.
In the uncertain world of ancient and medieval cartography, Ultima Thule was a land beyond exploration in the far northern reaches, variously identified with Iceland, Greenland, Norway, and more obscure areas. During the long period of arctic exploration, various islands were discovered and thinly populated, areas like Jan Mayer and Svalbard and many others which are among the most distant inhabited places on our planet. But as the blank spots were filled and our knowledge increased, areas received their own name and if not very widely known, were at least no longer so mysterious that people worried that there might be dragons there, even if there still might be polar bears or massive packs of deadly sea ice or other oddities. And perhaps it shall be the same in the future when it comes to space, that the area near to us will be better and better known, tempting us to go and explore remote and challenging rocks in the solar system that can be inhabited only with considerable difficulty and ingenuity, as we seek to expand beyond the regions of our birth and to banish further and further away areas that are unknown and unexplored by us.
 See, for example: