Is it really possible for something as big as Mars or Earth to be out that far and we not know about it?
Absolutely. We've only charted something like 2 percent of the Kuiper Belt, and only a smattering of scattered-disk objects from farther out. And the Oort Cloud is so far out that we have yet to directly observe any member of it, except possibly
Sedna. We're talking about objects that are much, much farther away than any of the eight known planets. There's a huge, huge volume of space to search, and it's very dark out there, so it will take a whole lot of careful looking to find what's out there.
This. In astronomy classes, we knew this from way back: it was just a matter of time before it would be extended to formal classification. Because, let's remind it here, classification means nothing except for our ease of use. For its own characteristics, Pluto is better classified as a dwarf planet (even if I think the label is unfortunate) than as a full planet. I really can't understand why some people feel butthurt about it, except for a knee-jerk reaction against "ivory tower academicians" who dared to change a classification that was created by, oh wait, older ivory tower academicians. It's just too silly.
Right. There's no sense in being nostalgic for something that was a bad idea in the first place.
And I don't think there's anything wrong with the label "dwarf planet." If we can have dwarf stars and dwarf galaxies, why not dwarf planets? The only problem with it is the bizarre and self-contradictory assertion that a dwarf planet is not a planet at all. A dwarf star is still a star, and a dwarf galaxy is still a galaxy. I say we should just consider dwarf planets to be a particular subset of planets, just as giant planets are.
Then we'll change the definition again. It's not the end of the world. What's up with this resistance to change? This is science, people, not dogma: change is intrinsic to the scientific method. Leaps are made by improving the theory, or discarding it altogether for a better one. We should always strive for the theory that better fit with current data, without letting nostalgia come in the way of accuracy.
Exactly. Change is good, because it means we understand more than we did before.
Heck, it's not even like this hasn't happened before. Ceres and the other large Main Belt asteroids were labeled "planets" for over 60 years before it was realized that they were too small and that "asteroid" was a better label. And that change was so completely accepted that now, hardly anybody knows that Ceres was ever considered a planet. So it's nothing to get hot and bothered about. It's just an adaptation of our labels to reflect our increasing knowledge.
(For that matter, the label "Main Belt" for the asteroid belt between Mars and Jupiter is obsolete too, since we now know that it's probably a lot sparser than the Kuiper Belt. For that matter, I've heard it suggested that the Trojan asteroids at Jupiter's Lagrange points may collectively outnumber or outmass those in the "Main" Belt. In the future, we'll probably call it something like "the Inner Belt" or "the Cis-Jovian Belt.")
I think it would be kinda spooky to find a planet the size of the Earth out that far with like a moon and atmosphere.
Well, obviously an object that far out from the Sun would be extremely cold and would not be at all Earthlike in anything other than size. It certainly could have one or more satellites; many asteroids and TNOs do. Heck, there's so much more clutter out there than there is here in the inner system that finding moons around large TNOs would not be remotely surprising.
As for atmosphere, it is believed that Pluto does occasionally have a tenuous atmosphere of nitrogen, methane, and carbon monoxide. Meanwhile, Eris is bright enough in color that it's believed its surface is covered in methane frost, suggesting that it occasionally has a thin atmosphere of sublimated methane which forms when it gets close enough to the Sun. There are lots of different kinds of atmosphere which, again, are nothing like the atmosphere we have on Earth.