Massive hydrogen exoplanet

I was checking the news today and found an interesting story that some scientist discovered a new planet that's not solid but made of hydrogen. It is apparently the biggest planet in the solar system. Scientists discovered circling a star in the constellation Hercules (which I hope means that it's very far from us and will never attempt to hit us). But what I foud most interesting about this story is the fact that the planet was discovered by the same guy who "discovered" ex-planet Pluto! As soon as I found out about this I started asking myself if this thing is really a planet... how do you "discover a planet" anyway?

Citation: CNN