Planetballs are circle-Drawn (though no circle tool) characters in the Polandball universe, similar to countryballs. They represent Planets, Moons, Dwarf Planets, Stars, Star Remants, and even Parallel Universes. Space Probes and Astroids also appear but they aren't drawn in ball shape. They have been a little-known staple of Polandball since its founding on Krautchan, they can be crossed over with polandball comics (e.g: Sunball making a cameo in a comic about Germany & Sweden).
The most Poland-like planet is Plutoball, which cannot into planet like Poland cannot into space. Earth is similar to USAball
(because USA thinks it owns all of Earth anyway). Jupiter is occasionally drawn as a cube because Jewpiter. Classic jokes include how the Sun will 'betray' the solar system by becoming a red giant, and how Uranus is made fun of, and how Mars hates Earth for throwing probes/having life.
Major communities of Planetball include /r/Planetball which was founded in 2013, and here. Popularity of Planetball ranges from time to time.