Short answers, pulled from the story.
A terrestrial planet is a solid body composed primarily of silicate rocks and metals, possessing a surface you could theoretically stand upon. This definition distinguishes these worlds from floating gas clouds or distant ice balls. The International Astronomical Union recognizes four such planets orbiting closest to the Sun: Mercury, Venus, Earth, and Mars.
The first confirmed terrestrial exoplanet, Kepler-10b, was found in 2011 by the Kepler space telescope. This discovery utilized the transit method to identify Earth-size planets around other stars. The find opened the floodgates for the identification of hundreds of planets ranging from Moon-sized to super-Earths.
Terrestrial planets possess secondary atmospheres generated by volcanic out-gassing or from comet impact debris. Unlike giant planets whose atmospheres were captured directly from the original solar nebula, these worlds create their air through volcanic fury and cosmic impact. This process has shaped surface structures by creating canyons, craters, mountains, and volcanoes.
A carbon planet, also known as a diamond planet, is a theoretical class composed of a metal core surrounded by primarily carbon-based minerals. While the Solar System contains no carbon planets, it does have carbonaceous asteroids such as Ceres and Hygiea. These theoretical worlds challenge our understanding of planetary chemistry.
In September 2020, astronomers using microlensing techniques reported the detection of an Earth-mass rogue planet named OGLE-2016-BLG-1928. This planet is unbounded by any star and free-floating in the Milky Way galaxy. The discovery highlights the diversity of terrestrial bodies that can exist without the warmth of a parent star.
The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure. A greater uncompressed density indicates a greater metal content within the planet. This differs from the true average density, also often called bulk density, because compression within planet cores increases their density.