A mega-Earth is a massive, dense type of terrestrial exoplanet, with the term coined in 2014 and formally defined in a 2026 study as a planet with a radius between 2.1 and 5 Earth radii and a density exceeding Earth's 5.5 g/cm³. These predominantly rocky, metallic, or ice-rich worlds challenge conventional planetary formation theories, which generally expect such massive bodies to accumulate thick gas envelopes rather than remaining solid. While the first candidate, Kepler-10c, was later demoted to a typical mini-Neptune in 2017, strong examples include K2-56b (16 Earth masses) and the massive Kepler-145b (37.1 Earth masses), which may represent a sub-category of "supermassive terrestrial planets." An even more extreme specimen is the pulsar planet PSR J1719−1438 b, weighing about 330 Earth masses and likely composed of crystalline carbon, thought to be the remnant core of a former white dwarf. Finally, the 2026 study cataloged 13 confirmed mega-Earths—including K2-263 b, TOI-332 b, and Kepler-411 b—establishing this dense, rocky category as a distinct and significant class of exoplanets.