The phrase "known planets" refers to the planets that have been discovered and confirmed by scientists. These are the planets that we have information about and know exist. Full definition
The tally of known planets outside the solar system now stands at more than 450, of which about 50 have been discovered just this year. (scientificamerican.com)
Until the 1990s, the nine planetary members of our own solar system were the only known planets. (solarviews.com)
I consider myself an agnostic and believe that we don't know our planet's true history to dismiss our holy books. (religion.blogs.cnn.com)