To be a planet, the assembly ruled, a world must meet three criteria:

(1) It must have enough mass and gravity to gather itself into a ball.

(2) It must orbit the sun.

(3) It must reign supreme in its own orbit, having "cleared the neighborhood" of other competing bodies.

 Read the IAU's press release: http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html

The Mars Rovers are still Exploring....Their Missions have been extended, again!
http://marsrovers.jpl.nasa.gov/home/index.html

Cassini continues to orbit Saturn and its moons.  Find out more: http://saturn.jpl.nasa.gov/home/index.cfm

New Horizons continues to head for Pluto and other Kuiper Belt Objects.  It will fly past Pluto in 2015.