What is a terrestrial planet?
By studying our solar system over several centuries, astronomers have learned a lot about the types of planets that exist in our universe. This knowledge has since expanded with the discovery of extrasolar planets, many of which are similar to what we have observed here at home.
For example, while hundreds of gas giants of varying sizes have been detected (which are easier to detect due to their size), many Earth-like planets have also been spotted. “Like the Earth”. These are known as terrestrial planets, a name that says a lot about how a planet was born.
Also known as a terrestrial or rocky planet, a terrestrial planet is a celestial body composed mostly of rocks or silicate metals and having a solid surface. This sets them apart from gas giants, which are mostly made up of gases like hydrogen and helium, water, and some heavier elements in various states.
The term terrestrial planet is derived from the Latin “Terra” (ie Earth). The terrestrial planets are therefore those which are “similar to the Earth”, which means that they have a structure and composition similar to the planet Earth.
Composition and characteristics:
All terrestrial planets have roughly the same type of structure: a central metallic core composed mostly of iron, surrounded by a silicate mantle. These planets have common surface features, including canyons, craters, mountains, volcanoes, and other similar structures, depending on the presence of water and tectonic activity.
Terrestrial planets also have secondary atmospheres, which are generated by volcanism or comet impacts. This also differentiates them from gas giants, where planetary atmospheres are primary and were captured directly from the original solar nebula.
Terrestrial planets are also known to have few or no moons. Venus and Mercury do not have moons, while Earth only has one (the Moon). Mars has two satellites, Phobos and Deimos, but these are more like large asteroids than real moons. Unlike gas giants, terrestrial planets also do not have planetary ring systems.
Terrestrial Solar Planets:
All of these planets found in the inner solar system – Mercury, Venus, Earth, and Mars – are examples of terrestrial planets. Each is composed primarily of silicate rock and metal, which are differentiated between a dense metal core and a silicate mantle. The Moon is similar, but has a much smaller iron core.
Io and Europa are also satellites which have internal structures similar to those of terrestrial planets. In the case of the former, models of the composition of the moon suggest that the mantle is composed primarily of silicate rock and iron, which surrounds a core of iron and iron sulphide. Europa, on the other hand, is said to have an iron core surrounded by an outer layer of water.
Dwarf planets, like Ceres and Pluto, and other large asteroids are similar to terrestrial planets in that they have a solid surface. However, they differ in that they are, on average, made of materials more icy than rock.
Terrestrial extrasolar planets:
Most of the planets detected outside the solar system are gas giants because they are easier to spot. However, since 2005, hundreds of potentially terrestrial extrasolar planets have been discovered, mainly by the Kepler space mission. Most of them were what are called “super-Earths” (ie planets whose mass is between that of the Earth and that of Neptune).
Examples of extrasolar terrestrial planets include Gliese 876 d, a planet that has a mass 7 to 9 times that of Earth. This planet orbits the red dwarf Gliese 876, located about 15 light years from Earth. The existence of three (or perhaps four) terrestrial exoplanets was also confirmed between 2007 and 2010 in the Gliese 581 system, another red dwarf located about 20 light years from Earth.
The smallest of them, Gliese 581st, has only about 1.9 earth mass, but orbits very close to the star. Two more, Gliese 581 c and Gliese 581 d, along with a proposed fourth planet (Gliese 581 g) are more massive super-Earths orbiting in or near the star’s habitable zone. If this is true, it could mean that these worlds are potentially habitable Earth-like planets.
The first confirmed terrestrial exoplanet, Kepler-10b – a planet with between 3 and 4 land masses and located some 460 light years from Earth – was discovered in 2011 by the Kepler space mission. That same year, the Kepler Space Observatory team published a list of 1,235 candidate extrasolar planets, including six that were “the size of Earth” or “super-Earth” (i.e. say less than 2 terrestrial rays) and which were located in their habitable zones of the stars.
Since then, Kepler has discovered hundreds of planets ranging from the size of the Moon to super-Earths, with many more candidates in that size range. In January 2013, 2,740 candidate planets were discovered.
Scientists have proposed several categories to classify terrestrial planets. Silicate planets are the standard type of terrestrial planet seen in the solar system, which are primarily composed of a silicon-based rock mantle and a metallic (iron) core.
Iron planets are a theoretical type of terrestrial planet that is composed almost entirely of iron and therefore has a higher density and a smaller radius than other terrestrial planets of comparable mass. Planets of this type are believed to form in high temperature regions near a star and where the protoplanetary disk is rich in iron. Mercury is a possible example, which formed near our Sun and has a metallic core equal to 60-70% of its planetary mass.
Coreless planets are another theoretical type of terrestrial planet, made up of silicate rocks but without a metallic core. In other words, coreless planets are the opposite of an iron planet. It is believed that coreless planets form further away from the star, where volatile oxidizing materials are more common. Although the solar system does not have coreless planets, asteroids and chondrite meteorites are common.
And then there are the carbon planets (aka “diamond planets”), a theoretical class of planets made up of a metallic core surrounded by mostly carbon-based minerals. Again, the solar system does not have any planets that fit this description, but has an abundance of carbonaceous asteroids.
Until recently, everything scientists knew about planets – including their formation and the different types that exist – came from studying our own solar system. But with the explosion in the discovery of exoplanets over the past decade, what we know about planets has increased dramatically.
On the one hand, we understood that the size and scale of the planets are larger than previously thought. Additionally, we have seen for the first time that many Earth-like planets (which could also include being habitable) actually exist in other solar systems.
Who knows what we’ll find once we have the ability to send probes and manned missions to other terrestrial planets?
Measuring the mass of an exoplanet the size of Mars
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