All terrestrial planets in the Solar System have the same basic structure, such as a central metallic core (mostly iron) with a surrounding silicate mantle.
The large rocky asteroid 4 Vesta has a similar structure; possibly so does the smaller one 21 Lutetia. Another rocky asteroid 2 Pallas is about the same size as Vesta, but is significantly less dense; it appears to have never differentiated a core and a mantle. The Earth's Moon and Jupiter's moon Io have similar structures to the terrestrial planets, but Earth's Moon has a much smaller iron core. Another Jovian moon Europa has a similar density but has a significant ice layer on the surface.
During the formation of the Solar System, there were many terrestrial planetesimals and proto-planets, but most merged with or were ejected by the four terrestrial planets, leaving only Pallas and Vesta to survive. Some began to accrete and differentiate, but suffered catastrophic collisions that left only a metallic or rocky core, like 16 Psyche or 8 Flora respectively. Many S-type and M-type asteroids may be such fragments.
Dwarf planets, such as Ceres, Pluto and Eris, are found today only from the asteroid belt outwards (although Vesta was likely a dwarf planet in the past), in the regions beyond the formation snow line (where water ice was stable under direct sunlight in the early Solar System). They are similar to terrestrial planets in that they have a solid surface, but are composed of ice and rock rather than of rock and metal.
Some small Solar System bodies in the inner solar system such as Vesta are quite rocky, or in the case of Psyche even metallic like Mercury, while others such as Pallas are icier.
Most planetary-mass moons are ice-rock (e.g. Ganymede, Callisto, Titan, and Triton) or even primarily ice (e.g. Mimas, Tethys, and Iapetus). The three exceptions are Earth's moon, which has a composition much like Earth's mantle, Jupiter's Io, which is silicate and volcanic, and Jupiter's Europa, which is believed to have an active hydrosphere. Jupiter's Ganymede, though icy, does have a metallic core like the Moon, Io, Europa, and the terrestrial planets.
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 (also often called "bulk" density) because compression within planet cores increases their density; the average density depends on planet size, temperature distribution, and material stiffness as well as composition.
Densities of the terrestrial planets and largest rocky asteroids
The uncompressed density of terrestrial planets (and the large rocky remaining protoplanets Vesta and Pallas) trends towards lower values as the distance from the Sun increases.
Earth's Moon has a density of 3.3 g·cm−3 and Jupiter's satellites Io and Europa are 3.5 and 3.0 g·cm−3; other large satellites are icier and typically have densities less than 2 g·cm−3 (e.g. Ganymede 1.94 g·cm−3, Callisto 1.83 g·cm−3, Titan 1.88 g·cm−3).
The dwarf planets Ceres, Pluto and Eris have densities of 2.2, 1.9 and 2.5 g·cm−3, respectively. (At one point Ceres was sometimes distinguished as a 'terrestrial dwarf', vs Pluto as an 'ice dwarf', but the distinction is no longer tenable. It now appears that Ceres formed in the outer Solar System and is itself quite icy.)
Calculations to estimate uncompressed density inherently require a model of the planet's structure. Where there have been landers or multiple orbiting spacecraft, these models are constrained by seismological data and also moment of inertia data derived from the spacecraft orbits. Where such data is not available, uncertainties are inevitably higher. It is unknown whether extrasolar terrestrial planets in general will show to follow this trend.
Extrasolar terrestrial planets
Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable. But since 2005, hundreds of potentially terrestrial extrasolar planets have also been found, with several being confirmed as terrestrial. Most of these are super-Earths, i.e. planets with masses between Earth's and Neptune's; super-Earths may be gas planets or terrestrial, depending on their mass and other parameters.
It is likely that most known super-Earths are in fact gas planets similar to Neptune, as examination of the relationship between mass and radius of exoplanets (and thus density trends) shows a transition point at about two Earth masses. This suggests that this is the point at which significant gas envelopes accumulate, so that the maximum size of a rocky planet may not be much larger than that of Earth. Exceptions to this are very close to their stars (and thus would have had their volatile atmospheres boiled away).
During the early 1990s, the first extrasolar planets were discovered orbiting the pulsarPSR B1257+12, with masses of 0.02, 4.3, and 3.9 times that of Earth's, by pulsar timing.
When 51 Pegasi b, the first planet found around a star still undergoing fusion, was discovered, many astronomers assumed it to be a gigantic terrestrial, because it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. It was later found to be a gas giant.
In 2005, the first planets orbiting a main-sequence star and which show signs of being terrestrial planets were found: Gliese 876 d and OGLE-2005-BLG-390Lb. Gliese 876 d orbits the red dwarf Gliese 876, 15 light years from Earth, and has a mass seven to nine times that of Earth and an orbital period of just two Earth days. OGLE-2005-BLG-390Lb has about 5.5 times the mass of Earth, orbits a star about 21,000 light years away in the constellation Scorpius.
From 2007 to 2010, three (possibly four) potential terrestrial planets were found orbiting within the Gliese 581 planetary system. The smallest, Gliese 581e, is only about 1.9 Earth masses, but orbits very close to the star. Two others, Gliese 581c and Gliese 581d, as well as a disputed planet, Gliese 581g, are more-massive super-Earths orbiting in or close to the habitable zone of the star, so they could potentially be habitable, with Earth-like temperatures.
Another possibly terrestrial planet, HD 85512 b, was discovered in 2011; it has at least 3.6 times the mass of Earth.
The radius and composition of all these planets are unknown.
In 2013, astronomers reported, 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 Sun-like stars and red dwarfs within the Milky Way. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. However, this does not give estimates for the number of extrasolar terrestrial planets, because there are planets as small as Earth that have been shown to be gas planets (see Kepler-138d).
Several possible classifications for terrestrial planets have been proposed:
The standard type of terrestrial planet seen in the Solar System, made primarily of silicon-based rocky mantle with a metallic (iron) core.
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, such as Ceres and 10 Hygiea. It is unknown if Ceres has a rocky or a metallic core.
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, and is sometimes called an iron planet, though its surface is made of silicates and is iron-poor. Iron planets are thought to form in the high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron.
A type of terrestrial planet with an icy surface of volatiles. The Solar System contains no known icy planets under the dynamical definition, but does under the geophysical definition: most planetary-mass moons (such as Titan and Enceladus) and many dwarf planets (such as Pluto and Eris) have such a composition. Europa is sometimes considered an icy planet due to its surface ice, but its higher density indicates that its interior is mostly rocky. Such planets can have internal saltwater oceans and cryovolcanoes erupting liquid water (i.e. an internal hydrosphere, like Europa or Enceladus); they can have an atmosphere and hydrosphere made from methane or nitrogen (like Titan). A metallic core is possible, as exists on Ganymede.
A theoretical type of terrestrial planet that consists of silicate rock but has no metallic core, i.e. the opposite of an iron planet. Although the Solar System contains no coreless planets, chondrite asteroids and meteorites are common in the Solar System. Ceres and Pallas have mineral compositions similar to carbonaceous chondrites, though Pallas is significantly less hydrated. Coreless planets are thought to form farther from the star where volatile oxidizing material is more common.
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