Vesta

Since, or before, the IAU demoted Pluto from planet status, astronomers have been trying out different designations for certain bodies in the Solar System. One of those is Vesta, known for a couple centuries as the second largest asteroid. It does orbit, after all, in the Main Belt of asteroids between Mars and Jupiter. At 320 miles in diameter, Vesta is much smaller than several moons, and much smaller than Pluto.

On the other hand, Vesta is more than a big rock. It has evolved, developing an internal structure complete with a radioactive core and mantle. Some scientists want to call it a protoplanet-- a planetary core that never attracted enough additional mass to reach true planethood.

NASA's Dawn spacecraft will arrive at Vesta in July, going into orbit around it. The plan is to spend a year studying Vesta. Maybe after that we'll know what to call it.