“Dwarf planet” is out.
They are now referred to as Little Planets.
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In San Francisco "dwarf planets" are also out, but at least there are 60 planet pronouns to describe the genders of the dwarf planets, so assigning a correct name shouldn't be a big issue once its location is found.
Of course, since the planet's gender is still unknown, it's best to refer to the lost small planet as a "size-challenged" one.
NOTE: Though I am neither an astronomer nor astrologer, I can say with roulette wheel confidence that dark matter and black holes are the answer to this dwarf planet mystery.
If astronomers can get the public to believe the accuracy of their black hole analyses at the reach-out-and-touch-some-galaxy range of 1,600 light YEARS (distance to the closed black hole.), solving the mystery planet problem should be a piece of cake.
All that's required is to explain the dark forces at work at only 4 light-HOURS away (34 million [average] miles to Neptune). It's as easy as writing a press release and photo-shopping some full-color "photos" of the mysterious (gender-and-location-to-be-determined) planet, plus pulling out of thin air (AKA outer space) an unverifiable theory on why the mini-black hole is causing the mischief.