In preparation for the flight, the Hubble space telescope has been monitoring Pluto's neighborhood to make sure the New Horizons spacecraft does not smack into any unknown objects. This turned out to be a pretty good idea since Hubble has already discovered two new moons over the course of about a year.
In July 2011, NASA announced the discovery of a fourth moon, labeled P4. This new moon has a diameter of only 8 to 21 miles, making it small enough to escape detection all of these years but large enough to cause problems for a NASA mission. And this past July, Hubble located Pluto's fifth moon, P5 (yes, this naming is quite creative). P5 is even smaller than P4. Oddly, NASA's New Horizons website has yet to announce this discovery. The graphic below provides some size comparisons.
Remember, Hubble located the second and third moon, Nix and Hydra, back in 2005. The largest moon, Charon, was discovered in 1978 by the U.S. Naval Observatory. Pluto and its moons reside inside the Kuiper Belt about 3 billion miles from the Sun.
If you are looking for more on why Pluto is no longer a "real" planet, you can always read Mike Brown's book How I Killed Pluto and Why It Had It Coming. Mike Brown is the Richard and Barbara Rosenberg Professor of Planetary Astronomy at the California Institute of Technology, As he explains, not everyone was happy with his discovery of 10th planet, Eris:
My daughter Lilah, now five years old, is mad at me for killing Pluto. When I began a project 13 years ago to chart the slowly-moving objects of the distant outer solar system, my goal was never to pull Pluto off of its cherished planetary pedestal. I wanted to be a planet discoverer, like William Herschel or Clyde Tombaugh before me. I had a strong feeling that somewhere out there something bigger than Pluto was lurking, and I knew that whoever found it would get to claim the mantle as the only living planet discoverer.
I was right. Something bigger than Pluto was out there (or at least something more massive than Pluto; sizes are a little harder to pin down precisely) and one January morning in 2005, my small team of astronomers and I found it. We announced the discovery of the 10th planet to an unsuspecting world late on the afternoon of Lilah’s 22nd day of life. A little after her first birthday, though, the doors to the planetary club were locked and Pluto and my own discovery were kicked out on the curb. The solar system was down to only eight planets.