First, ask them about who they think first figured out that Nibiru was a planet. Then show them this:

https://web.archive.org/web/20031005233219/http://www.zetatalk.com/thub.htm

These were the people who invented the idea of Nibiru cataclysm impacting Earth. Notice the dates given toward the bottom.

Aside from that, it really depends on which variant of the story they were told.

Planet X was a theory put forth by astronomers to account for gravitational perturbances in the orbits of the outer planets. The idea was that another planet was pulling them from a long distance. Make sure everyone realizes that these were small effects and there was a LOT of math involved. Later, when we sent Voyager to Neptune, better measurements showed we had the mass of the planet slightly wrong, and the corrected number explained everything. Unless you can look at the numbers yourself and mathematically show if there still needs to be another planet, I would suggest you trust the word of the people who can.

Comet Elenin is a popular one. There was a whole swath of madness unleashed when someone noticed that there was an alignment between Earth, the Sun, and Elenin on the day of a major earthquake. A number of things wrong with this:

• There are tons of comets out there, and tons of earthquakes that happen every year. There was nothing particularly special about these two. Sometimes unrelated events happen on the same day.

• There's really no way a comet that small can cause an earthquake. The gravity of a comet, what little there is, would be a force that pulls the entire Earth, not something that breaks loose a thin piece of crust. We get far more powerful gravitational effects from the Sun and Moon, and that does is mess with the water level.

• Some people are saying it is/was a brown dwarf, which makes no sense at all -- brown dwarfs are bigger than Jupiter, which is much further away and clearly visible. Brown dwarfs only get their reputation of being hard to find because we look for them in deep space, far from any other source of light. If Jupiter were that distant, we wouldn't detect it either.

• Astronomy doesn't generally care about alignments. Take the tides -- when the moon and sun are aligned, the tides are strongest, but if they're not quite aligned, the tides are pretty strong too. There's nothing that "switches on" when the moon and sun are at their closest in the sky. There's no reason why a comet would cause an earthquake on the exact day of its alignment, and have no effect any other day.

Sadly, once this started in fringe theory blogs and message boards, it was impossible to stop. Fact-checkers generally don't hang around the abovetopsecret forums. You even had shit like this, implying that Elenin was a code word. By the way "Nibiru in November" is referring to November 2011.

Fortunately, since it's 2015, your friends probably won't say a word to you about the Mayans. But that was a really popular one for a while. The thing to understand is that predictions about Nibiru get it wrong, every time they're made, but the theory itself mutates and takes a new form, and everyone forgets the failed prediction.