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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 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.

Contents

Structure

Solar terrestrial planets

Extrasolar terrestrial planets

See also

References

External links

Structure

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.

Solar terrestrial planets

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.
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.
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.
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, the kepler mission, and Overwhelmingly Large Telescope.

See also

★ Jovian planet/Gas giant planet

★ Chthonian planet

★ Dwarf planet

★ Gliese 581 c

★ Planetarium

★ Planetary habitability

References

★ Astronomers Find First Earth-like Planet in Habitable Zone ESO - European Organisation for Astronomical Research in the Southern Hemisphere, 27 April 2007

★ Found: one Earth-like planet Astronomers use gravity lensing to spot homely planets. By Mark Peplow, News @ Nature.com, 25 January 2006.

★ Beaulieu J.P., et al. (2006) ''Nature,'' '439,' 437-440.

★ National Science Foundation press release "Closer to Home."

★ A New Path to New Earths National Science Foundation webcast.

★ Ogling Distant Stars National Science Foundation grant report.

★ Wolszczan's Pulsar Planets.

★ PLANET Homepage.

★ RoboNet Homepage.

★ OGLE Homepage.

★ MOA Homepage.

External links

★ SPACE.com: Q&A: The IAU's Proposed Planet Definition 16 August 2006 2:00 am ET

★ BBC News: Q&A New planets proposal Wednesday, 16 August 2006, 13:36 GMT 14:36 UK