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Terrestrial planet

The first word of this story is Terrestrial, a term derived from the Latin words Terra and Tellus, meaning Earth, which immediately signals that these worlds share a fundamental kinship with our own home. A terrestrial planet is not a floating gas cloud or a distant ice ball, but a solid body composed primarily of silicate rocks and metals, possessing a surface you could theoretically stand upon. Within the Solar System, the International Astronomical Union recognizes four such planets orbiting closest to the Sun: Mercury, Venus, Earth, and Mars. These worlds share a specific internal architecture that distinguishes them from their larger, gaseous counterparts. Each possesses a central metallic core, predominantly iron, surrounded by a silicate mantle. This structure is so defining that even some of our solar system's moons, such as Earth's Moon and Jupiter's moon Io, are sometimes classified as terrestrial planets due to their geophysical similarities. The distinction is crucial because it separates these rocky worlds from the outer giants, which are composed mostly of hydrogen, helium, and water existing in various physical states, lacking the solid ground that defines the terrestrial experience.

The Silent Cores

Beneath the surface of these worlds lies a complex history of formation and differentiation that often remains hidden from casual observation. While Mercury, Venus, Earth, and Mars all follow the standard pattern of a metallic core and silicate mantle, the story of their formation reveals a chaotic early solar system. During the birth of the Solar System, countless terrestrial planetesimals and proto-planets merged or were ejected, leaving only the four major planets and a few survivors like the large rocky asteroids Pallas and Vesta. Pallas, for instance, is about the same size as Vesta but is significantly less dense, appearing to have never differentiated into a core and mantle. This suggests that some protoplanets began to accrete and differentiate but suffered catastrophic collisions that left only a metallic or rocky core, such as 16 Psyche or 8 Flora. The Earth's Moon, while geophysically similar to terrestrial planets, has a much smaller iron core than expected, hinting at a violent origin story involving a massive impact. Even Jupiter's moon Europa, with its significant ice layer, is sometimes considered an icy planet rather than terrestrial, yet it shares a similar density and may possess a metallic core like the Moon and Io, blurring the lines between these classifications.

Atmospheres of Fire and Ice

The air that surrounds these worlds is not a primordial gift from the solar nebula but a secondary creation born of volcanic fury and cosmic impact. Unlike the giant planets whose atmospheres were captured directly from the original solar nebula, terrestrial planets possess secondary atmospheres generated by volcanic out-gassing or from comet impact debris. This process has shaped the surface structures of these worlds, creating canyons, craters, mountains, and volcanoes depending on the presence of erosive liquids or tectonic activity. Earth stands alone in the inner solar system with an active surface hydrosphere, a feature that has allowed for the erosion and reshaping of its landscape over billions of years. Yet, the story extends beyond Earth. Europa is believed to have an active hydrosphere under its ice layer, and Titan, an icy moon, even has surface bodies of liquid, albeit liquid methane rather than water. The density of these worlds provides further clues to their history, with uncompressed density indicating metal content. The Galilean satellites show a trend of decreasing density moving outward from Jupiter, similar to the trend observed in the terrestrial planets moving outward from the Sun, consistent with the temperature gradient that existed within the primordial solar nebula.

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Common questions

What is a terrestrial planet?

A terrestrial planet is a solid body composed primarily of silicate rocks and metals, possessing a surface you could theoretically stand upon. This definition distinguishes these worlds from floating gas clouds or distant ice balls. The International Astronomical Union recognizes four such planets orbiting closest to the Sun: Mercury, Venus, Earth, and Mars.

When was the first confirmed terrestrial exoplanet discovered?

The first confirmed terrestrial exoplanet, Kepler-10b, was found in 2011 by the Kepler space telescope. This discovery utilized the transit method to identify Earth-size planets around other stars. The find opened the floodgates for the identification of hundreds of planets ranging from Moon-sized to super-Earths.

How do terrestrial planets form their atmospheres?

Terrestrial planets possess secondary atmospheres generated by volcanic out-gassing or from comet impact debris. Unlike giant planets whose atmospheres were captured directly from the original solar nebula, these worlds create their air through volcanic fury and cosmic impact. This process has shaped surface structures by creating canyons, craters, mountains, and volcanoes.

What is a carbon planet?

A carbon planet, also known as a diamond planet, is a theoretical class composed of a metal core surrounded by primarily carbon-based minerals. While the Solar System contains no carbon planets, it does have carbonaceous asteroids such as Ceres and Hygiea. These theoretical worlds challenge our understanding of planetary chemistry.

When was the first Earth-mass rogue planet detected?

In September 2020, astronomers using microlensing techniques reported the detection of an Earth-mass rogue planet named OGLE-2016-BLG-1928. This planet is unbounded by any star and free-floating in the Milky Way galaxy. The discovery highlights the diversity of terrestrial bodies that can exist without the warmth of a parent star.

What is uncompressed density?

The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure. A greater uncompressed density indicates a greater metal content within the planet. This differs from the true average density, also often called bulk density, because compression within planet cores increases their density.

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The Super-Earth Revolution

For decades, astronomers believed that the planets discovered outside the Solar System would be giant gas worlds, simply because they were easier to detect. However, the discovery of hundreds of potentially terrestrial extrasolar planets since 2005 has rewritten the narrative of planetary formation. The first confirmed terrestrial exoplanet, Kepler-10b, was found in 2011 by the Kepler space telescope, which was specifically designed to discover Earth-size planets around other stars using the transit method. This discovery opened the floodgates, leading to the identification of hundreds of planets ranging from Moon-sized to super-Earths. A super-Earth is defined as a planet with a mass between Earth's and Neptune's, but whether it is a gas planet or a terrestrial world depends on its mass and other parameters. In 2016, statistical modeling suggested that the transition point between rocky, terrestrial worlds and mini-Neptunes without a defined surface was very close to Earth and Venus's, implying that rocky worlds much larger than our own are actually quite rare. As of 2024, the expected transition point between rocky and intermediate-mass planets sits at roughly 4.4 earth masses and roughly 1.6 earth radii, challenging previous assumptions about the prevalence of massive rocky worlds.

The Carbon and Iron Mysteries

Beyond the standard silicate classification, theoretical models propose exotic types of terrestrial planets that challenge our understanding of planetary chemistry. A carbon planet, also known as a diamond planet, is a theoretical class composed of a metal core surrounded by primarily carbon-based minerals. While the Solar System contains no carbon planets, it does have carbonaceous asteroids such as Ceres and Hygiea, and it remains unknown whether Ceres has a rocky or metallic core. Another theoretical type is the iron planet, which consists almost entirely of iron and therefore has a greater density and a smaller radius than other solid planets of comparable mass. Mercury in the Solar System has a metallic core equal to 60 to 70 percent of its planetary mass and is sometimes called an iron planet, though its surface is made of silicates and is iron-poor. These iron planets are thought to form in high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron. Conversely, a coreless planet is a theoretical type that consists of silicate rock but has no metallic core, the opposite of an iron planet. Although the Solar System contains no coreless planets, chondrite asteroids and meteorites are common, and Ceres and Pallas have mineral compositions similar to carbonaceous chondrites, suggesting that such worlds might exist in the outer regions of other star systems.

The Rogue Wanderers

The search for terrestrial worlds has expanded beyond the boundaries of star systems to include planets that drift freely through the galaxy. In September 2020, astronomers using microlensing techniques reported the detection, for the first time, of an Earth-mass rogue planet named OGLE-2016-BLG-1928, which is unbounded by any star and free-floating in the Milky Way galaxy. This discovery highlights the diversity of terrestrial bodies, which can exist without the warmth of a parent star. The frequency of these worlds is staggering; in 2013, astronomers reported 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. Eleven billion of these estimated planets may be orbiting Sun-like stars, with the nearest such planet potentially just 12 light-years away. However, these statistics do not guarantee habitability. Estimates show that about 80 percent of potentially habitable worlds are covered by land, and about 20 percent are ocean planets. Planets with ratios more like those of Earth, which was 30 percent land and 70 percent ocean, only make up 1 percent of these worlds, suggesting that 'pale blue dots' like Earth may be rare among habitable worlds.

The Density Divide

The physical properties of these worlds provide the most direct evidence of their composition and history, yet measuring them requires sophisticated models and data from orbiting spacecraft. The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure, and a greater uncompressed density indicates a greater metal content. This 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. Calculations to estimate uncompressed density inherently require a model of the planet's structure, and 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's orbits. Where such data is not available, uncertainties are inevitably higher. The data reveals that the icy worlds typically have densities less than 2 grams per cubic centimeter, while Eris is significantly denser and may be mostly rocky with some surface ice, like Europa. The density trends of the rounded terrestrial bodies directly orbiting the Sun trend towards lower values as the distance from the Sun increases, consistent with the temperature gradient that would have existed within the primordial solar nebula.