[nkrisc]

Well, we've not yet found it. There is just a very good case for it's existence.

Also, it's arguably easier to find planets in other solar systems because from our vantage point we can observe the stars and measures the dips in light as the planet transits the star (from our perspective). We can't do that without our own solar system given we too are orbiting the same star as the planet we would want to detect.

Also, remember this planet has an orbital period of about 10,000 - 20,000 years, according to the article. It would have just barely completed one orbit since humans started farming.