Alien planets are a staple of science fiction, the canvas on which authors and directors paint the wildest things their imagination can come up with.
An ice planet? Holy shit. This is even better than your idea for a desert planet!
But because the solar system is full of seven boring uninhabitable planets and one that's getting there quickly, modern science fiction has generally looked to other stars in the galaxy for their stories to take place in. But that wasn't always the case, and once we go back a few decades, to a time when our knowledge of the solar system was based mainly on squinting really hard at the sky, all sorts of crazy adventures took place on improbable planets believed to exist right here in our own solar system.
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Or rather, less "crazy adventures" and more "unsettling adventures with strongly racist overtones."
No, not that Vulcan from the Star Trek universe, which is supposedly orbiting a star next door to ours. No, this Vulcan refers to the planet that was theorized to be orbiting the sun somewhere inside Mercury's orbit, in the star's so-called Toasty Belt.
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Some of the things I'm going to mention in this article are lies, so heads up.
This wasn't as dumb an idea as it might seem, and it was based on the fact that Mercury had a crazy ass orbit that didn't make any sense to anyone. Uranus, one of the cold bags of farts in the outer solar system, had been found to have irregularities in its own orbit, and someone worked out that this was because something big and heavy was pulling it around out there. A short search later, we found another cold bag of farts in the outer solar system, which would soon be known as Neptune.
Seconds later, a great wave smashed into the Cracked offices.
The same theory was applied to Mercury's confusing orbit, ending in speculation that there was a planet called Vulcan somewhere nearby. Inevitably a number of science fiction stories were set on its sweaty surface, many of which seemed to focus on the idea that Vulcan was hollow, like this story from the superbly named Captain Future (later turned into a 1970s anime).
Which made the odd decision to portray him as the human pilot of a spacecraft made entirely out of sideburns.
But as you've probably guessed, no one, massive sideburns or not, ever did lay eyes on Vulcan, and by the start of the 20th century we'd figured out that Mercury's strange orbit was caused by the very fabric of space being all stretched to hell around there.
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Like a sweater you took off by pulling down around your torso.
The asteroid belt between Mars and Jupiter really looks like it should have another planet there, and indeed there have been a few back-of-the-envelope-level calculations that insist that a planet should be there, and the fact that there isn't is actually pretty irritating.
Like its absence messes up the whole feng shui of the solar system.
And the fact that there's a whole bunch of rocks there instead of a planet is very suggestive that a planet might have once been there, which a few people took to calling Phaeton. In Greek mythology, Phaeton was the son of Helios and, as is so often the case with teens in Greek mythology, became a cautionary tale, in this case about driving his dad's solar chariot so poorly that Zeus destroyed him.
Which makes him kind of the Armageddon to Icarus' Deep Impact.
This idea of a destroyed planet is like catnip to a certain type of mind that doesn't get out much, and several stories have been written premised on the fact that at one point there indeed was a planet there that had simply been destroyed somehow. Sometimes it was destroyed by a collision with another planet, sometimes its inhabitants blew it up themselves. Perhaps the most famous work to mention it is Robert Heinlein's classic novel Stranger in a Strange Land, where it's ominously suggested that the Martians blew it up just because they didn't like the looks of the people who lived there.
"OHHHH! YOUR FACE!"
We're now pretty sure that there was never really a chance of that, that no planet ever could have formed between Mars and Jupiter. Jupiter's tidal influences are too strong for anything there to coalesce, bad news for anyone hoping to pen further lighthearted tales of planet-scale genocide.
"Also, your dog kept leaving space-turds on our space-lawn."
#3. The Tenth Planet
We're down to eight planets now, but for a good while there we considered Pluto our ninth planet, and there was an awful lot of speculation about the possibility of there being another one even further out there. This isn't a crazy idea at all; in the grand scheme of things, planets aren't that big, and that far from the sun it seemed entirely possible, even probable, that there would be another one out there that was simply really, really hard to see.
Here's a portrait of Pluto to give you an idea of how tough this is. It's basically the smallest dot here. No, the other one.
This is a pretty tempting setting for a story on the frontier of known space, with all the cool things that frontiers have (space-prostitutes). Joe Haldeman's classic The Forever War had a few scenes set on a tenth planet called Charon (which was later the name given to Pluto's moon). Rendezvous With Rama and a whole bunch of other books mentioned a tenth planet called Persephone. Charlie and the Chocolate Factory, itself known for not being a particularly sane book, had a sequel called Charlie and the Great Glass Elevator, which is just deranged and includes a race of aliens that hail from a tenth planet.
They're called the Vermicious Knids, and they devour a couple dozen hotel employees (sorry, space-hotel employees). Yes, this was a children's book, and no, we still don't know what was wrong with Roald Dahl.
The neat thing about these tenth planet stories is that there actually is a tenth planet out there! A lot of them! A half dozen or so have been discovered so far, with new candidates discovered every couple years. In fact, we discovered so many tenth planets that the scientists in charge of counting planets had to scratch their beards a bit and decide whether the very the idea of "planet" needed to be reconsidered, lest we end up with a solar system with several dozen planets and confused schoolchildren everywhere. (That this reclassification killed off Pluto's status as a planet, leading to enraged schoolchildren everywhere, is just one of those great little moments when the real world is as funny as it should be.)