Mr. XoFFoX, welcome to the board. You're welcome to post your views here. But please, there's no need to be antagonistic. Just state your case politely. We'll attempt to answer it politely in turn.

As far as I know, Mr. Plait has never claimed that a tenth planet or brown dwarf categorically doesn't exist. He's only stated that it doesn't exist as claimed by the Nibiru promoters. There's a difference there.

Another thing is that good scientists are very reluctant to jump to conclusions. They don't like to state that something is true until they are reasonably sure that it exists as claimed. This is not closed-mindedness, it's just caution.

This is why most of science doesn't embrace the idea of anti-gravity. There has been no reliable evidence (or even a viable theory) presented yet that it exists, therefore it's not accepted. When and if such evidence is presented, and when it can be confirmed beyond a reasonable doubt, science will accept it's existance.

Recently, however, the idea of dark energy has become accepted for debate in cosmological circles. New measurements of the expansion of the universe have shown that it is accelerating. Therefore, there seems to be some force counteracting gravity at very large scales. It's still under debate though, and not accepted, but there's enough evidence to give it a closer look.

These are my views. It's up to the BA to answer in more detail if he wishes.

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...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere