As someone who tends to pay attention at astronomy stories, please allow me to be the first to ask: what the heck are you talking about?!
Pluto was once called Planet X, and it doesn't even hold the rank of planet anymore. Unless you're thinking of the Nibiru scenario? A planet close enough that large would be impossible to miss nowadays, and we'd have noticed it's effect on the gravitational fields long ago.
The moon's diameter is 3476 km, and it barely manages to block out the Sun by being about 385 000 km away. On it's end, the largest planet in the Solar system, Jupiter, has a diameter of 142 984 km. Working the numbers, to completely block out the Sun for a few minutes only, it'd need to be about 15 800 000 km away from us, which is only 10% of the distance to the Sun. Now, you're talking about a block of *three days*, which would require either said planet to move very slowly (read here: we could see it *at day* with naked eyes already), or to be much larger (with about the same result). It simply doesn't add up. We'd be more likely to be fried by a random quasar firing gamma rays at us on December 21st!
You've got a source on any of this, or is this something that you've only heard about? I'm sorry to say it, but it doesn't make sense.
The admin formerly known as Dr. Cossack.
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