Let me set the scene for you: after a fifteen month hiatus, Doctor Who returned to the telly in Autumn 1986 starring the multi-talented (and multi-coloured!) Colin Baker in an epic fourteen part adventure, the longest in the programme's history, called "The Trial of a Time Lord". The Doctor is summoned through time and space to a courtroom set on a Time Lord space station (possibly the location of the Matrix – no, not the Keano version) where he faces a black clad prosecutor, the Valeyard, who intends to try him for his actions (and probably have him executed!).
Because the season long show was made in three recording blocks, and because they are clearly distinguished by the changing setting for the evidence, these are often given separate "story titles" (based in part on the Target novelisations): "The Mysterious Planet", "Mindwarp", "Terror of the Vervoids" and "The Ultimate Foe" (or sometimes "Time Inc.")
For the first four episodes then, "The Mysterious Planet", the Valeyard's evidence comes from the Doctor's adventure on Ravolox, obviously the planet of the title.
Guess what: it's EARTH!
It's two million years into the future and someone has incinerated the surface of the planet with a firestorm, though it has recovered to the extent that it is covered in forest (and also fur-clad villagers, when the plot demands). The villagers are actually escapees from an underground survival chamber in Marb Station (yes, it used to be Marble Arch) run by a robot called Drathro that's having a bit of a personality crisis because it may just have discovered that it's sentient.
There's an interesting (though soon to be much less interesting) villain called Glitz who is there to steel something that the Doctor doesn't find out about until much later but that aside it's all very much the usual capture-escape-capture-escape finish with a big bang hijinx undermined by some rather poor costumes and jokes, not least the glooping of the companions gag at the end.
So, anyway, why is Ravolox "mysterious"?
The Doctor's decision to go there in the first place is based on the fact that – unique in the universe – Ravolox has the same period of rotation, revolution and angle of inclination as the Earth. The problem with that is that none of those things will be the same in two million years time: tidal effects of gravity will slow the Earth's rotation (the time it takes to spin on its axis, i.e. the length of a day) and its revolution (the time it takes to go around the sun, i.e. the length of a year) and the angle of the axis itself will also wobble about, again affected by gravity (not to mention anything actually hitting the planet which tends to knock these things off kilter too).
Well, you might say, the Doctor means that all of these things are the same for Ravolox as they are for Earth two million years in the future – except, for rather obvious reasons, he ought not to be able to compare with Earth, i.e. because it's NOT THERE!
In fact, it's actually MORE mysterious that the Doctor could KNOW about Ravolox – and it's spooky similarity to his favourite planet – and NOT know that in the same time zone the Earth itself is mysteriously not actually there.
Could it be that the Earth was REPLACED? Leaving aside the sheer waste of effort involved in moving one planet and then moving ANOTHER to sub for it while it's off somewhere else, you would need – surely – yet another planet identical to Earth. And there isn't one, because Ravolox is the only one which is why it's so mysterious.
The thing that ought to be rather more noticeable about Ravolox is that it is the same SIZE as the Earth: its volume and mass. And consequently it has the same surface gravity. (Mind you, so does everywhere else in the Doctor Who universe – maybe the Time Lords got rid of all the planets that weren't the same size as Gallifrey!) Obviously the planet's mass increases somewhat over time as well, as space dust and meteorites and solar wind all deposit more matter on the planet. But it's not going to be nearly so dramatic an increase over the time span.
Equally, its mineral composition (how much carbon, oxygen, silicon, iron and other elements and compounds make up the bulk of the planet) ought to be a pretty good fingerprint.
If you want to be a REAL fanboy, you could say that (unlikely as it may seem) Ravolox is the only planet other than Earth where Silicon Dioxide in the form of QUARTZ can be found. (Just because that's a reference to "The Pirate Planet" doesn't mean anyone actually needs to know that for it to make plot – if not scientific – sense.)
The OTHER big GIVE AWAY is, of course, THE MOON. To the best of our knowledge, the Moon is unusually large as planetary satellites go, almost one sixth the size of the Earth. In fact this gave rise to lots of ideas that it was REALLY a captured PLANETOID and in the Doctor's universe it IS (see "Doctor Who and the Silurians" for this sort of thing). Analysis of moon rock from OUR moon brought back by the Apollo missions has since shown that in fact the Moon is more likely to have been originally a large chunk of the Earth that was blasted off by a collision with a huge asteroid while the planet was still forming. But wherever it came from, it's still a really JOLLY HUGE landmark.
Of course, it's possible that when Earth was moved to become Ravolox, they left the Moon behind – but that brings us back to the whole rotation, revolution thing. The Earth and Moon are A DYNAMIC system, take the Moon away and it'll do very funny things to the way the Earth itself spins.
And anyway, that brings us to the deeper mystery of where IS Ravolox, or where was Earth actually moved to?
Now I do know that writers of Doctor Who tend to be a bit poor when it comes to the ol' COSMIC SCIENCE. This is the series that was always confused about the terms Solar System/Galaxy/Universe even without Terry Nation's ideas about evolution having a fixed path or radiation sickness or icecanos (yes, alright, I know about this), not to mention David Whittaker and the Magic of Mercury.
Nevertheless, this one is particularly egregious and it drives me NUTS every time I see it quoted without qualification in fan guide book.
Earth is moved "a couple of light years" later described as "only two light years".
Now, the most ELEMENTARY "Big Book of Space" will tell you that the VERY NEAREST star system to our own, PROXIMA CENTAURI, is 4.2 light years from the sun. Or TWICE as far away as Earth is allegedly moved.
It's fairly obvious, from the way that there is recognisable green vegetation and indeed rain – and not, for example, an airless frozen wasteland – that the Earth IS about a hundred and fifty million kilometres from a medium sized yellow dwarf star.
But, and let's be completely blunt about this, there aren't any STARS there for Ravolox to be orbiting.
In fact, someone seems to have spotted this problem, because it is later said that it's not JUST the Earth, but it's ENTIRE SOLAR SYSTEM that was moved. Hang on, what? The ENTIRE Solar System – wouldn't the fact that Ravolox ENTIRE stellar system is a perfect match for the Earth's Solar System be just the TINIEST bit of a give away. Not to mention more of a clue than that business with rotation and axial tilt?
It turns out that the PURPOSE behind moving the Earth is so that a rescue mission coming from ANDROMEDA (about two-and-a-half million light years away!) will go flying past and miss – what with the Earth not being at the expected co-ordinates.
(Although if you are going to the trouble of moving the planet and don't care that the population survives, why limit yourself to just two light years: why not two-hundred or two thousand? In fact, wouldn't dropping it down a black hole be an altogether more expedient way of making sure no one ever found your secrets? If you are trying to HIDE the planet, why leave it looking so similar to Earth – rotation yadda, yadda, yadda – that it is remarked upon as odd? I mean if you can move the planet you can at least tip the thing's axis over a couple of degrees, can't you?)
Anyway, two light years out of two-and-a-half million is actually well within the kind of navigation error that you ought to be able to cope with if you are planning on conducting inter-galactic space missions: it's an error of less than one one-hundred-thousandth of a degree in your bearing. Nevertheless, that might work with a robot spaceship – and Drathro shows that the Andromedan's DO use robots for this sort of thing – if the robots are particularly dumb and bound by their programming.
But WHY would it fool the Doctor? Surely he would NOTICE that Ravolox's co-ordinates are right up close to the Earth's!
Of all the "mysterious" things that might make him investigate Ravolox, surely it's the fact that it's RIGHT NEXT DOOR to the Earth, that abruptly there is a new nearest planetary system to Earth, that in fact it's habitable. Oh, and the Solar System seems to be a bit MISSING.
So the REAL mystery must be why would the Doctor THINK that this planet that looks just like Earth in a Solar System that is just like the Earth's that is right next door to where the Earth ought to be but isn't, why would he think this ISN'T the Earth?