It was the neigbor planet

It didn't throw him onto the world, it altered the planet's relationship to its sun. The Reliant went to the WRONG PLANET. The one they were supposed to go to is the one that blew up. Now how a high tech federation starship could miss the fact that their target planet isn't there any more is another question..the debris it must have left should have been a good clue.

He never said that...

Khan and his cohorts were abandoned on Ceti Alpha V by Captain Kirk and Co. in TOS. In the film, Khan is STILL on Ceti Alpha V. Chekov and Tyrell thought that they had arrived at Ceti Alpha VI. I submit the following dialogue into the record... Chekov - 'You lie - on Ceti Alpha V there was life, a fair chance!' Khan - 'THIS is Ceti Alpha V!! Ceti Alpha VI exploded six months after we were left here - the shock shifted the orbit of this planet and everything was laid waste. Captain Kirk never bothered to check...' By the way, planets explode all the time - if we were to take our solar system as a model for the amount of activity in the galaxy, we'd be wasting billions of dollars on pretty boring space programs. Planets DO explode, almost certainly, there are meteors flying around in space and not every planet has the physical and chemical make up that ours does...basically, you're wrong - it's not even a matter of opinion.

Why not? Praxis did.

Why couldn't a planet explode? It really depends what is on or in it. Praxis comes to mind. It sent out a massive subspace shockwave during a mining accident. Granted, that required Klingon intervention, but there had to be a hell of a lot of stored energy there to be released. So why couldn't a plant have lots of an exotic material go unstable?

all the time?

since when have planets ever exploded? did anyone see one go up? rock and ice don't just explode! asteroids hitting one just make big craters.

Point of view

Could've been hit by another celestial body? Could've been unstable to begin with and broke apart? Could've been detroyed by a race we don't know about? Now whether the debris was small enough to NOT impact Ceti Alpha V and be detected by Starfleet, could be true. Changing the orbit so it's closer to the sun and laying waste to plant life and water production, etc. could be true. Now, the subject of the missing planet? No way! Starfleets computer's would've told them the planet was now gone or at least displayed a map of the system and they could've figured it out. Now, whether Ceti Alpha V and VI were the same mass and the current condition of Ceti Alpha V now matched the previous condition of Ceti Aplha VI is another nitpick. Either way, a planet's missing!

It could have gone unnoticed by starfleet

The change in the Ceti Alpha system could have gone unnoticed by starfleet. It seems likely that Ceti-Alpha was in a remote part of the galaxy that was far from Starfleet's area of influence. Kirk's agreement with Khan in the original episode, Space Seed, was to give Khan the oppurtunity to build his civilisation without starfleet interference. And in the movie, Reliant had been searching for lifeless planets for some time now (based on Chekovs's dialoge with Captain Terrell). Presumably they would have searched the records of planets that had already been catalogued by starfleet before traveling around manualy searching for a planet. It is also logical to presume that they would have started close to home and worked their way out towards more remote locations.