Terrestrial planet

Masses of terrestrial planets

Telluric planets size comparison

Size of Kepler Planet Candidates

Carbon Planet

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Fountains of Enceladus PIA07758

Terrestrial planet refers to a category of planets that are primarily composed of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, namely Mercury, Venus, Earth, and Mars. These planets are characterized by their dense, rocky compositions, thin atmospheres, and, with the exception of Mercury, the presence of various geological features such as valleys, volcanoes, and mountains.

Characteristics

Terrestrial planets have a number of defining characteristics that distinguish them from gas giants and ice giants. These include:

• Solid Surfaces: Unlike gas giants, terrestrial planets have solid, rocky surfaces, making them capable of supporting landforms and, potentially, life.
• Small Size and Mass: Terrestrial planets are significantly smaller and less massive than gas giants. This smaller size and mass contribute to their higher densities.
• Dense Composition: They are composed mainly of silicate rocks and metals, with a core of metal (iron, nickel) and a mantle of silicate material.
• Thin Atmospheres: While the atmospheres of terrestrial planets vary, they are generally thinner than those of gas giants, with Earth being an exception due to its robust biosphere and magnetic field which helps retain a thicker atmosphere.
• Lack of Rings and Few Moons: Terrestrial planets typically have few, if any, moons and lack the ring systems that are characteristic of the larger gas giants.

Formation

The formation of terrestrial planets is thought to occur through the process of accretion, where dust and rock in the protoplanetary disk surrounding a young star collide and stick together, gradually forming larger bodies. Over time, these bodies become large enough to attract more material through gravity, eventually forming planets. The terrestrial planets in the Solar System formed closer to the Sun, where it was too hot for volatile molecules like water and methane to condense, leading to their rocky compositions.

Habitability

The potential for habitability on terrestrial planets is a subject of significant interest in astrobiology. Factors that influence habitability include the planet's distance from its star (which affects surface temperature), the presence of water, and the thickness and composition of the atmosphere. Earth is currently the only known terrestrial planet to support life, though Mars and Venus are subjects of research into past habitability and the potential for microbial life.

Exploration

Terrestrial planets have been the focus of numerous space exploration missions. Mercury has been explored by missions such as Mariner 10 and MESSENGER, Venus by Venera and Magellan, Mars by a fleet of orbiters, landers, and rovers such as Mars Pathfinder, Mars Exploration Rovers, Mars Science Laboratory, and the Mars Reconnaissance Orbiter, and Earth through both ground-based observations and satellites.

Exoplanets

Beyond the Solar System, the search for exoplanets has revealed many terrestrial-like planets orbiting other stars. These exoplanets offer the exciting possibility of studying terrestrial planet formation and habitability in a variety of environments and conditions vastly different from those found in the Solar System.