A terrestrial planet, telluric planet or rocky planet is a planet that is primarily composed of silicate rocks. Within our solar system the terrestrial planets are the closest planets to the Sun. The terms are derived from Latin words for Earth (Terra and Tellus), so an alternative 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.
Theoretically, there are two types of terrestrial or rocky planets, one dominated by silicon compounds and another dominated by carbon compounds, like carbonaceous chondrite asteroids. These are the silicate planets and carbon planets (or "diamond planets") respectively.
In addition, Earth's moon and Jupiter's satellites Io and Europa can also be regarded as terrestrial worlds, though since they orbit planets they cannot be considered planets themselves. Io and Europa have mainly rocky compositions despite forming beyond the snow line. This may be because the region of the circum-Jovian disc in which they formed was kept too warm by radiation from the proto-Jupiter to contain large quantities of icy material.
|Object||mean density||uncompressed density||semi-major axis|
|Mercury||5.4 g/cm³||5.3 g/cm³||0.39 AU|
|Venus||5.2 g/cm³||4.4 g/cm³||0.72 AU|
|Earth||5.5 g/cm³||4.4 g/cm³||1.0 AU|
|Moon||3.3 g/cm³||3.3 g/cm³||1.0 AU|
|Mars||3.9 g/cm³||3.8 g/cm³||1.5 AU|
|Vesta||3.4 g/cm³||3.4 g/cm³||2.3 AU|
|Pallas||2.8 g/cm³||2.8 g/cm³||2.8 AU|
|Ceres||2.1 g/cm³||2.1 g/cm³||2.8 AU|
The majority of planets found outside our solar system to date have been gas giants, presumably 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.
Aleksander Wolszczan detected the first extrasolar terrestrial planets. The three planets orbit the pulsar PSR B1257+12 with masses of 0.02, 4.3, and 3.9 times that of Earth's. They were discovered by accident: 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 diameter measurements of a similar extrasolar planet (HD 209458 b), which transited its star showed that these objects were indeed gas giants.
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 of 5 to 7 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 newly discovered planet orbits its parent star at a distance similar to that of our solar system's asteroid belt. 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.
In April 2007, a team of 11 European scientists announced the discovery of a planet outside our solar system that is potentially habitable, with Earth-like temperatures. The planet was discovered by the European Southern Observatory's telescope in La Silla, Chile, which has a special instrument that splits light to find wobbles in different wave lengths. Those wobbles can reveal the existence of other worlds. What they revealed is a planet circling the red dwarf star, Gliese 581. The discovery of the new planet, named Gliese 581 c, is sure to fuel studies of planets circling similar dim stars. About 80 percent of the stars near Earth are red dwarfs. The new planet is about five times heavier than Earth, classifying it as a super-earth. Its discoverers aren't certain if it is rocky, like Earth, or if it is a frozen ice ball with liquid water on the surface. If it is rocky like Earth, which is what the prevailing theory proposes, it has a diameter about 1 1/2 times bigger than our planet. If it is an iceball, it would be even bigger.
A number of telescopes capable of directly imaging extrasolar terrestrial planets are on the drawing board. These include the Terrestrial Planet Finder, Space Interferometry Mission, Darwin, New Worlds Mission, Kepler Mission, and Overwhelmingly Large Telescope.
|Closest planet to 1 MEarth||PSR 1257+12 C||PSR 1257+12||3.9 MEarth|
|Closest planet to 1 AU orbital||HD 142 b (gas giant)||HD 142||0.980 AU|
|HD 28185 b (gas giant)||HD 28185||1.031 AU|
|HD 128311 b (gas giant)||HD 128311||1.02 AU|
|Closest planet to 365-day orbit||HD 142 b (gas giant)||HD 142||337 d|
|HD 92788 b (gas giant)||HD 92788||378 d|
|Closest to 300 K||Mu Arae e||Mu Arae||308 K|
|Gliese 581 c||Gliese 581||290 K; A first Earth-like planet in habitable zone, possibility of liquid water.|