Short answers, pulled from the story.
A terrestrial planet is a class of planet composed primarily of silicate rocks or metals, as distinct from the larger gas giant planets made mostly of hydrogen, helium, and water in various physical states. Within the Solar System, the four recognized terrestrial planets are Mercury, Venus, Earth, and Mars.
The International Astronomical Union recognizes Mercury, Venus, Earth, and Mars as the Solar System's terrestrial planets. Under the broader geophysical definition, Earth's Moon, Io, and sometimes Europa may also qualify, as may the large asteroids Pallas and Vesta.
The uncompressed densities of terrestrial bodies in the Solar System decrease with distance from the Sun, consistent with the temperature gradient that existed in the primordial solar nebula. Mercury, at 0.39 AU, has an uncompressed density of 5.3 g per cubic centimeter, while Pallas, at 2.77 AU, measures only 2.9.
Kepler-10b, discovered in 2011 by the Kepler space telescope, was the first confirmed terrestrial exoplanet. The Kepler telescope was specifically designed to find Earth-size planets around other stars using the transit method.
Astronomers reported in 2013, based on Kepler space mission data, that there could be as many as 40 billion Earth- and super-Earth-sized planets orbiting in the habitable zones of sunlike stars and red dwarfs in the Milky Way. Around 11 billion of these estimated planets may orbit sunlike stars.
A terrestrial planet has a central metallic core, typically iron, surrounded by a silicate mantle. A coreless planet is a theoretical type of solid planet consisting of silicate rock with no metallic core, thought to form farther from the star where volatile oxidizing material is more common. No confirmed coreless planets exist in the Solar System.