These are objects in space that are too small to be planets but too big to be asteroids. Like full-sized planets, dwarf planets have enough gravity to be round in shape. They orbit the sun or another star. But unlike planets, dwarf planets are too small for their gravity to clear other objects out its path. A planet is big enough that all of the asteroids and comets in its orbit have been pulled in or flung away by the planet’s gravity. But a dwarf planet doesn’t have enough gravitational power. Its orbit remains full of asteroids and other debris.
In a sentence
Being a dwarf planet doesn’t mean Pluto is just a chunk of rock; its heart shape could come from outside impacts or even internal tectonic plate activity.