All missions are sent in search of new data; sometimes that data supports current scientific models, but it often causes scientists to formulate new ideas or revise their current models. Planetary missions in search of new data include, but are not limited to:
MESSENGER has made many surprising discoveries on Mercury during its flybys and orbiting mission. Some of the lava flows on this heavily cratered planet are only one to two billion years old, long after scientists thought this little world cooled.
This mission is on its way to study Pluto and its environment in the Kuiper Belt. Pluto was considered a planet when New Horizons launched, but is now classified as a dwarf planet; regardless of its classification, New Horizons will be the first mission to explore this distant world.
The Kepler Mission is surveying a portion our galaxy to discover dozens of Earth-size planets and determine how many of the billions of stars in our galaxy have such planets. It has discovered well over 1,300 candidates and dozens of confirmed planets so far.
This mission studied the asteroid Vesta for about a year and then departed in July 2012 to visit dwarf planet Ceres, the largest object in the asteroid belt. The scientific findings of this mission will help revolutionize our understanding of planet formation.
Hubble Space Telescope
The Hubble has made detailed observations of planets, comets and asteroids within our solar system over the years, particularly of newly forming planetary systems and of distant gas giants -- taking incredible images of the impact of Shoemaker-Levy 9 into Jupiter in 1994. The Hubble Space Telescope was recently used to discover a fourth and fifth moon around Pluto.