Class B planets are generally very small, very rocky worlds located
within a star system's hot zone, such as Mercury. In the harsh
daylight, these planets are scorched by their parent star, often to
the point of rock becoming molten. Because Class B worlds have
little to no atmosphere, this heat quickly radiates away at night,
leaving the dark side of the planet a frigid wasteland. As a result,
these planets are highly unsuitable for humanoid life.

Despite their small size, Class B planets are often extremely dense,
with a large inner core, up to 55% of the planet's volume, that is
made of molten iron.

Class B planets are fairly common in the universe.