There is no official name for a half planet. The term "planet" refers to a celestial body that meets specific criteria, such as being in orbit around a star, having sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and having cleared the neighborhood around its orbit.

If a celestial body doesn't meet all these criteria, it's usually classified as something else, such as a dwarf planet, asteroid, or moon.

Here are some terms that might be used to describe a hypothetical half-planet:

* Planetary fragment: This term could be used to describe a piece of a planet that broke apart.

* Protoplanet: This term refers to a body that is forming into a planet but has not yet reached the necessary mass and shape.

* Planetesimal: This term refers to a small, rocky body that is part of a protoplanetary disk.

It's important to note that these terms don't necessarily imply that the object is half the size of a planet. They simply refer to bodies that are in different stages of formation or are not fully formed planets.