Terrestrial planet

• 1. Introduction

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  Mercury, Venus, Earth, and Mars share something no other planets in our Solar System can claim: they are made of rock and metal rather than gas. These four worlds, which the International Astronomical Union recognizes as the Solar System's terrestrial planets, are the closest to the Sun among the planets. They are named after the Latin words for Earth, Terra and Tellus, because in terms of structure, they are Earth-like. But how different can a rocky planet actually be? The answer stretches from planets with iron cores making up 60-70% of their mass, to theoretical worlds made almost entirely of carbon, to icy bodies that blur the boundary between rock and frozen ocean. What makes something a terrestrial planet, and how far does that category extend across the galaxy?

• 2. The Rocky Core

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  Every terrestrial planet in our Solar System shares the same basic architecture: a central metallic core, mostly iron, wrapped in a surrounding silicate mantle. Mercury takes this to an extreme. Its metallic core makes up 60-70% of its total planetary mass, earning it the informal label of an iron planet, even though its surface is made of silicates and is actually iron-poor. Iron planets like Mercury are thought to form in high-temperature regions close to a star, where the protoplanetary disk is rich in iron.

  Not all rocky bodies went through the same separation process. The 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 at all, making it a geologically distinct case.

  Some protoplanets began to differentiate but suffered catastrophic collisions that stripped away everything except a metallic or rocky core. The asteroid 16 Psyche is thought to be one such bare metallic remnant, and 8 Flora a rocky one. Many of the S-type and M-type asteroids scattered through the Solar System may be fragments of these ancient collisions.

  The surfaces of terrestrial planets can carry canyons, craters, mountains, and volcanoes, depending on whether an erosive liquid or tectonic activity was ever present. Their atmospheres are secondary, generated by volcanic outgassing or from comet impact debris, rather than the primary atmospheres of the giant planets, which were captured directly from the original solar nebula.

• 3. Where Rocky Meets Icy

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  Jupiter's moon Io has a structure similar to a terrestrial planet, but Earth's Moon has a much smaller iron core by comparison. Europa, another Jovian moon, has a similar density to some terrestrial bodies but carries a significant ice layer on its surface, which is why it is sometimes classified as an icy planet instead.

  The boundary between rocky and icy worlds runs through some surprising places. Ganymede, though predominantly icy, does have a metallic core like the Moon, Io, Europa, and the terrestrial planets. Titan has surface bodies of liquid, but that liquid is methane rather than water. Bodies such as Ganymede, Callisto, Enceladus, and Titan are known to have subsurface hydrospheres, much like Europa is believed to.

  Among the terrestrial planets of the Solar System, only Earth has an active surface hydrosphere. Europa is believed to maintain one beneath its ice layer.

  Some researchers have proposed the name Terran world to cover all solid bodies that have assumed a rounded shape, regardless of composition. This broader category would bring both rocky and icy planets under one term, sidestepping the question of where exactly the silicate family ends.

• 4. Density and Distance

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  The uncompressed density of a planet, meaning the average density its materials would have at zero pressure, tells scientists how much metal is present. A greater uncompressed density points to greater metal content. This figure differs from bulk density because compression inside a planet's core raises density further; the true average depends on planet size, temperature distribution, and the stiffness of the materials involved.

  A clear pattern emerges when looking at the Solar System's terrestrial bodies: uncompressed density drops as distance from the Sun increases. Mercury sits at 0.39 AU from the Sun and has an uncompressed density of 5.3 g per cubic centimeter. Venus and Earth, farther out at 0.72 and 1.0 AU respectively, both measure 4.4. Mars, at 1.52 AU, comes in at 3.8. Vesta, at 2.36 AU, measures 3.5, and Pallas, at 2.77 AU, falls to 2.9. This gradient is consistent with the temperature pattern that would have existed in the primordial solar nebula.

  The Galilean satellites of Jupiter show a similar trend moving outward from that planet. No such trend is observable, however, for the icy satellites of Saturn or Uranus. Icy worlds typically have densities below 2 g per cubic centimeter. Eris is a notable exception, measuring 2.43, and may be mostly rocky with only some surface ice, resembling Europa in composition.

  Where landers or multiple orbiting spacecraft have visited, scientists can constrain structural models using seismological data and moment of inertia measurements derived from orbital tracking. Where such data is absent, uncertainties remain substantially higher.

• 5. Rocky Worlds Beyond the Sun

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  Most planets discovered outside our Solar System are giant planets, simply because they are easier to detect. Since 2005, however, hundreds of potentially terrestrial extrasolar planets have been found, with several confirmed as terrestrial.

  The first extrasolar planets ever discovered, in the early 1990s, orbited not a sunlike star but the pulsar PSR B1257+12, detected by pulsar timing. They had masses of 0.02, 4.3, and 3.9 times that of Earth. When 51 Pegasi b was discovered as the first planet found around a star still undergoing fusion, many astronomers assumed it was a gigantic terrestrial because no gas giant was thought capable of existing as close as 0.052 AU from its star. It turned out to be a gas giant.

  In 2005, Gliese 876 d and OGLE-2005-BLG-390Lb became the first planets orbiting a main-sequence star to show signs of being terrestrial. Gliese 876 d orbits the red dwarf Gliese 876, just 15 light-years from Earth, and has a mass seven to nine times that of Earth with an orbital period of only two Earth days. OGLE-2005-BLG-390Lb carries about 5.5 times Earth's mass and orbits a star roughly 21,000 light-years away in the constellation Scorpius.

  The Kepler space telescope, specifically designed to find Earth-size planets using the transit method, confirmed the first verified terrestrial exoplanet, Kepler-10b, in 2011. That same year, Kepler's mission team released a list of 1,235 extrasolar planet candidates, including six that were Earth-size or super-Earth-size and located within the habitable zone of their stars.

  In September 2020, astronomers using microlensing reported the first detection of an Earth-mass rogue planet, named OGLE-2016-BLG-1928L, not bound to any star and floating freely through the Milky Way.

• 6. Rethinking Super-Earths

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  In 2016, statistical modeling using a broken power law to relate a planet's mass and radius suggested that the transition point between rocky terrestrial worlds and mini-Neptunes without a defined surface fell very close to the sizes of Earth and Venus. The implication was that rocky worlds much larger than those two are quite rare, and some researchers called for retiring the term super-Earth as scientifically misleading.

  As of 2024, refinements to the mass-radius model place the expected transition between rocky and intermediate-mass planets at roughly 4.4 Earth masses and roughly 1.6 Earth radii. Most super-Earths discovered may in fact be gas planets depending on their mass and other parameters, not guaranteed to be rocky at all.

  Frequency estimates paint a striking picture of how common rocky worlds might be across the galaxy. Based on Kepler data, astronomers reported in 2013 that as many as 40 billion Earth- and super-Earth-sized planets could orbit within the habitable zones of sunlike stars and red dwarfs inside the Milky Way alone. Around 11 billion of these may orbit stars like the Sun. The nearest such planet, scientists estimated, could be as close as 12 light-years away. Estimates also suggest that roughly 80% of potentially habitable worlds are land-covered, with about 20% being ocean planets. Worlds with ratios resembling Earth, at 30% land and 70% ocean, make up only about 1% of these potentially habitable planets.