Destiny's Children, [Exultant][1] by Stephen Baxter (2004). Pluto isn't called 'not a planet', but it is called a *Kuiper belt object* and a *worldlet*:

> There are many bodies out here, some massive. Pluto is one, Port Sol another...
But they add up to only about a fifth of Earth's mass. There should have been thousands of worldlets the size of Pluto or larger.

It took the IAU a few years to decide Pluto belonged in the [dwarf planet][2] class, but many people, including authors, were aware of the reclassification *before* it was made official.


  [1]: http://en.wikipedia.org/wiki/Exultant_%28book%29
  [2]: http://en.wikipedia.org/wiki/Dwarf_planet