The planets in question are extremely slow moving (taking 200 - 400 years to orbit) which makes them much harder to detect. It took developing an improved technique to detect these slight movements to figure out the planets were there. Even with the new methodology the scientists would not have been able to detect these planets because they move so slow without the use of the old Hubble Telescope data. The details of the new methodology is probably too difficult for anyone to understand without an astrophysics degree (smile) but if you are interested in the nitty gritty details there are articles on the NASA website that really delve into the topic.
The planets are "extra-solar" which means they revolve around a star other than our own star. The new planets orbit a young and massive star that has been designated HR 8799. It is 130 light years away from Earth.
The scientist who discovered the new planet is David Lafreniere of the University of Montreal. His results will be published in the Astrophysical Journal, a peer reviewed journal.
The way in which these new planets were discovered gives us hope that even more exciting discoveries are on their way by the re-analysis of archived Hubble Telescope data.