A terrestrial planet, telluric planet or rocky planet is a planet that is primarily composed of silicate rocks. The term is derived from the Latin word for Earth, "Terra", so an alternate definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from gas giants, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks an iron core. Terrestrial planets have canyons, craters, mountains, and volcanoes. Terrestrial planets possess secondary atmospheres -- atmospheres generated through internal vulcanism or comet impacts, as opposed to the gas giants, which possess primary atmospheres -- atmospheres captured directly from the original solar nebula.

Earth's solar system has four terrestrial planets: Mercury, Venus, Earth and Mars, and one terrestrial dwarf planet, Ceres. Objects like Pluto are similar to terrestrial planets in the fact that they do have a solid surface, but are composed of more icy materials (see Ice dwarf). During the formation of the solar system, there were probably many more (planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the solar nebula. Only one terrestrial planet, Earth, is known to have an active hydrosphere.

Extrasolar terrestrial planets

The majority of planets found outside our solar system to date have been gas giants, simply because gas giants are larger and therefore easier to see or infer from observation. However, a number of extrasolar planets are known or suspected to be terrestrial.

The first terrestrial planets ever detected outside our solar system were detected by Aleksander Wolszczan orbiting the pulsar PSR B1257+12. Their masses being 0.02, 4.3, and 3.9 Earth masses. The planets were observed because their transit caused interruptions in the pulsar's radio emissions. Had they not been orbiting around a pulsar, they would not have been found.

When 51 Pegasi b, the first extrasolar planet found around a fusing star, was discovered, many astronomers assumed it must be a gigantic terrestrial, as it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. However, subsequent measurements of its diameter confirmed it was a gas giant.

In June 2005, the first planet around a fusing star that is almost certainly terrestrial was found orbiting around the red dwarf star Gliese 876, 15 light years away. That planet has a mass between six and nine times that of earth and an orbital period of just two Earth days.

On 10 August, 2005, Probing Lensing Anomalies NETwork/Robotic Telescope Network (PLANET/RoboNet) and Optical Gravitational Lensing Experiment (OGLE) observed the signature of a cold planet designated OGLE-2005-BLG-390Lb, about 5.5 times the mass of Earth, orbiting a star about 21,000 light years away in the constellation Scorpius. The planet revealed its existence through a technique known as gravitational microlensing, currently unique in its capability to detect cool planets with masses down to that of Earth.

Already in spring 2005, a microlensing signal indicating the presence of planet OGLE-2005-BLG-169Lb was observed , which is 13 times the mass of Earth and orbiting a star approximately 9,000 light years away. This planet may be either a gas giant or terrestrial. The newly discovered planet orbits its parent star at a distance similar to that of our solar system's asteroid belt.

Theoretically, there are two types of terrestrial or rocky planets, one dominated by silicon compounds, as Earth is, and another dominated by carbon compounds, like carbonaceous chondrite asteroids. These are the silicate planets and carbon planets (or "diamond planets") respectively.

A number of telescopes capable of directly imaging extrasolar terrestrial planets are on the drawing board. These include the Terrestrial Planet Finder, Darwin, New Worlds Imager, and Overwhelmingly Large Telescope.