Counting the number of planets is ... tricky.

The trouble starts with the IAU definition, which requires -- among other things -- that the prospective planet clear its orbit. This famously caused trouble for Pluto, a debate which had mostly died down by the time fixity fields were discovered. It flared up again when the IAU definition would have ruled out Earth.

Antichthon was the first extra planet constructed. Well, the first artificial solar satellite of sufficient size to maintain an atmosphere and livable environment entirely through 'natural' gravity, in a way that would be stable without the support of fixity fields, if required.

But it wasn't a planet, people were quick to say. It was artificial, for one thing. For another, it shared an orbit with Earth. Antichthon took up the empty position of 'counter-Earth', a location chosen to cause minimal orbital disruption to the rest of the system.