Terrestrial planet

The terrestrial planets Mercury, Venus, Earth and Mars in true colors, sizes to scale.

A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun. The terms are derived from Latin words for Earth (Terra and Tellus), as these planets are, in terms of composition, "Earth-like".

Terrestrial planets have a solid planetary surface, making them substantially different from the much larger gas giants, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.

Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) reported in January, 2013, that "at least 17 billion" Earth-sized exoplanets are estimated to reside in the Milky Way Galaxy.

Structure

Terrestrial planets all have approximately the same type of structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but has a much smaller iron core. Terrestrial planets can have canyons, craters, mountains, and volcanoes. Terrestrial planets possess secondary atmospheres, generated through internal volcanism or comet impacts, in contrast to the gas giants, whose atmospheres are primary, captured directly from the original solar nebula.

Solar terrestrial planets

Relative masses of the terrestrial planets of the Solar System, including the Moon

Earth's Solar System has four terrestrial planets: Mercury, Venus, Earth, and Mars. Only one terrestrial planet, Earth, is known to have an active hydrosphere.

During the formation of the Solar System, there were probably many more "terrestrial" planetesimals, but they have all merged with, or been ejected by, the four remaining worlds in the solar nebula.

Dwarf planets, like Ceres and Pluto, and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface, but are, on average, composed of more icy materials (Ceres and Pluto have a density of 2.1 g cm⁻³, and Haumea's density is similar to Pallas's 2.8 g cm⁻³).

Density trends

The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure. A greater uncompressed density indicates greater metal content. Uncompressed density differs from the true average density because compression within planet cores increases their density; the average density depends on planet size as well as composition.

The densities of the solar terrestrial planets, the Moon, and the three largest asteroids are shown below. Densities generally trend towards lower values as the distance from the Sun increases.

Object	mean density	uncompressed density	semi-major axis
Mercury	5.4 g cm−3	5.3 g cm−3	0.39 AU
Venus	5.2 g cm−3	4.4 g cm−3	0.72 AU
Earth	5.5 g cm−3	4.4 g cm−3	1.0 AU
Moon	3.3 g cm−3	3.3 g cm−3	1.0 AU
Mars	3.9 g cm−3	3.8 g cm−3	1.5 AU
Vesta	3.4 g cm−3	3.4 g cm−3	2.3 AU
Ceres	2.1 g cm−3	2.1 g cm−3	2.8 AU
Pallas	2.8 g cm−3	2.8 g cm−3	2.8 AU

The main exception to this rule is the density of the Moon, which probably owes its lesser density to its unusual origin.

It is unknown whether extrasolar terrestrial planets in general will also follow this trend. E.g. Kepler-10b does: it has a density of 8.8+2.1
−2.9 g cm⁻³, and orbits its star much closer than Mercury orbits the Sun.

Extrasolar terrestrial planets

See also: Super-Earth, Pulsar planet, and List of nearest terrestrial exoplanet candidates

Artist's rendering of an Earthlike planet

The majority of planets found outside the Solar System have been gas giants, since they produce more pronounced wobbles in the host stars and are thus more easily detectable. However, a number of extrasolar planets are suspected to be terrestrial.

During the early 1990s, the first extrasolar planets were discovered orbiting the pulsar PSR B1257+12, with masses of 0.02, 4.3, and 3.9 times that of Earth's, by pulsar timing (a method only applicable to pulsars).

When 51 Pegasi b, the first planet found around a star still undergoing fusion, 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 may be terrestrial was found orbiting around the red dwarf star Gliese 876, 15 light years away. That planet, Gliese 876 d, has a mass 7 to 9 times that of Earth and an orbital period of just two Earth days. But the radius and composition of the planet is unknown.

On 10 August 2005, Probing Lensing Anomalies NETwork/Robotic Telescope Network (PLANET/RoboNet) and Optical Gravitational Lensing Experiment (OGLE) observed the signs 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 the Solar System's asteroid belt. The planet revealed its existence through a technique known as gravitational microlensing, currently unique in its capability to detect planets with masses down to that of Earth.

In April 2007, a team of 11 European scientists announced the discovery of a planet outside the 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 wavelengths. Those wobbles can reveal the existence of other worlds. What they revealed is planets circling the red dwarf star, Gliese 581. Gliese 581 c was considered to be habitable at first, but more recent study (April 2009) suggests Gliese 581 d is a better candidate. Regardless, it has increased interest in examining planets circling dimmer stars. About 80 percent of the stars near Earth are red dwarfs. The Gliese 581 (c and d) planets are about five to seven times heavier than Earth, classifying them as super-Earths.

Gliese 581 e is only about 1.9 Earth mass, but could have 2 orders of magnitude more tidal heating than Jupiter’s volcanic satellite Io. An ideal terrestrial planet would be 2 Earth masses with a 25-day orbital period around an M dwarf star.

The discovery of Gliese 581 g was announced in September 2010, and is believed to be the first so-called "Goldilocks planet" ever found, the most Earth-like planet, and the best exoplanet candidate with the potential for sheltering life found to date. However, its discovery has been called into doubt - recent analyses of the data by different researchers have yielded different conclusions as to the existence of the planet.

Using the HARPS facility, scientists discovered a Goldilocks planet named HD85512b with 3.6 times the mass of Earth and the right conditions for liquid water. The acceleration of gravity is 1.4 times that on Earth. The orange dwarf star orbited by HD85512b is 5.6 billion years old and resides 36 light years from the solar system in the constellation Vela.

The Kepler Mission endeavours to discover Earth-like planets orbiting around other stars by observing their transits across the star. The Kepler spacecraft was launched on 6 March 2009. The duration of the mission will need to be about three and a half years long to detect and confirm an Earth-like planet orbiting at an Earth-like distance from the host star. Since it will take intervals of one year for a truly Earth-like planet to transit (cross in front of its star), it will take about four transits for a reliable reading.

Dimitar Sasselov, the Kepler mission co-investigator, recently mentioned at the 2010 TED Conference that there have been hundreds more candidate terrestrial planets discovered since Kepler went online. If these planets are confirmed via further investigation, then it will represent the largest find of extrasolar planets to date. The Kepler science teams are, for now, keeping the initial results of any candidate planets a secret so they can confirm their results. The first public announcement of any such results is expected during the early part of 2011.

On 2 February 2011, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the "habitable zone." Some of these candidates were "Earth-size" and "super-Earth-size" (defined as "less than or equal to 2 Earth radii [Re]" [or, Rp ≤ 2.0 Re] - Table 5). Six of these candidates (namely: KOI 326.01 [Rp=0.85], KOI 701.03 [Rp=1.73], KOI 268.01 [Rp=1.75], KOI 1026.01 [Rp=1.77], KOI 854.01 [Rp=1.91], KOI 70.03 [Rp=1.96] - Table 6) are in the "habitable zone." A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.

A number of other telescopes capable of directly imaging extrasolar terrestrial planets are also being designed. These include the Terrestrial Planet Finder, Space Interferometry Mission, Darwin, New Worlds Mission, and Overwhelmingly Large Telescope.

Types

Several possible classifications for terrestrial planets have been proposed:

Silicate planet

The standard type of terrestrial planet seen in the Solar System, made primarily of silicon-based rocky mantle with a metallic (iron) core.

Iron planet

A theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass. Iron planets are believed to form in the high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron.

Coreless planet

A theoretical type of terrestrial planet that consists of silicate rock but has no metallic core, i.e. the opposite of an iron planet. The Solar System contains no coreless planets, but chondrite asteroids and meteorites are common in it. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common.

Artist's impression of a carbon planet

Carbon planet (also called "diamond planet")

A theoretical class of planets, composed of a metal core surrounded by primarily carbon-based minerals. They may be considered a type of terrestrial planet if the metal content dominates. The Solar System contains no carbon planets, but does have carbonaceous asteroids.

Super-Earth

Super-Earths are planets with masses between Earth's and Neptune's. They may be gas planets or terrestrial, depending on their mass and other parameters. The latter represent the upper-end of the terrestrial-planet mass range.

See also

• Dwarf planet
• Earth analog
• Gas giant — also known as Jovian or Giant planets.
  • Chthonian planet
• List of nearest terrestrial exoplanet candidates
• Planetary habitability

External links

• SPACE.com: Q&A: The IAU's Proposed Planet Definition 16 August 2006
• BBC News: Q&A New planets proposal Wednesday, 16 August 2006