Xena is supposedly the tenth
Beyond Pluto
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possibly more in the future, pluto as of course sort of recent news this year is not a planet anymore, but a dwarf planet. With the rate they find stars, planets, etc, don't be surprised in the near future if they find more planets orbiting outside pluto, or a planet who might just even appear closer than that who might have a larger orbit of revolution.
I think it would be cool to have an invisible planet in our solar system. There were more inner planets but they all crashed into each other and now are the asteroids.
None are known, but there has been one hypothesised.
This is because of orbits of long-period trans-neptunian objects like Sedna line up in an odd way, which makes people think there might be a Neptune-sized planet pulling them into line with its gravity.
However, they haven't found it yet after 3 years of looking, and the longer they can't find it, the less likely it becomes.
Of course, if you consider Pluto itself a planet, then a consistent definition would mean you'd call lots of things after Pluto planets.
The most popular rival definition with the backing of proper scientists is that a planet is simply an object orbiting a star which is big enough to be graviationallly rounded. As well as including dwarf planets like Pluto, it would also include objects like the Moon, Ganymede, and Pluto's own moon Charon.
A more restrictive definition (which largely is not advocated by scientists but appeals to many laymen) is to simply drop the "orbital clearance" part from the 2006 definition. This makes all dwarf planets considered Planets. This includes closer dwarf planets like Ceres but also further ones like Eris and the aforementioned Sedna.
As an additional point, Pluto crosses Neptune's orbit, so sometimes Neptune is further from the Sun than Pluto is; although Pluto is further on average.
Does anyone know the planets that are beyond Pluto?