A very interesting article can be read at Reuters.

In 2004, scientists at NASA received data from Voyager 1 when it reached the edge of our solar system that showed an apparent dent in our heliosphere. They thought it was just an anomaly until it's twin, Voyager 2, confirmed that there is a supermassive alien space craf.... er, magnetic field affecting the heliosphere.

So.. planet, dark matter or aliens? You decide.