Rocky worlds much bigger than Earth may be surprisingly rare
Planet hunters once spoke of super-Earths as if giant rocky worlds were everywhere. Then modelling of how mass relates to size suggested that beyond about 1.6 Earth radii, planets tend to become gassy mini-Neptunes instead. Earth and Venus may sit near the upper limit for solid ground.
Terrestrial, or rocky, planets are made mainly of silicate rock and metal and have solid surfaces, unlike the gas giants of hydrogen, helium and water. The International Astronomical Union counts four in the Solar System: Mercury, Venus, Earth and Mars. By a geophysical definition the Moon, Io and sometimes Europa qualify too, and occasionally the big asteroids Pallas and Vesta. All share a basic layout, an iron-rich core wrapped in a silicate mantle. Their air is secondary, released by volcanoes or delivered by comets, while the giants captured theirs straight from the young solar nebula.
The early Solar System teemed with rocky building blocks, but most were swallowed or thrown out by the four big terrestrials. Pallas and Vesta survived largely intact, battered out of round by impacts, while others were smashed down to bare metal or rock cores, like 16 Psyche and 8 Flora. Farther out, beyond the snow line, round bodies such as Ceres, Pluto, Titan and Triton are icy worlds of ice and rock instead.
Composition shows up in uncompressed density, what a planet's materials would weigh at zero pressure; higher values mean more metal. Among bodies orbiting the Sun it falls with distance, echoing the heat gradient of the primordial nebula, and Jupiter's big moons show the same pattern. Earth alone has an active surface ocean, though Europa likely hides one under its ice.
Beyond the Sun, the first planets found, in the early 1990s, circled the pulsar PSR B1257+12. In 2005 Gliese 876 d, seven to nine Earth masses and orbiting its red dwarf in two days, became one of the first rocky candidates around an ordinary star. Kepler-10b, found in 2011 by the Kepler telescope, was the first confirmed rocky exoplanet. By 2024 the estimated switch from rocky to intermediate planets sat near 4.4 Earth masses.
Source: Terrestrial planet
