Terrestrial planets are Earth-like planets made up of rocks or metals with a hard surface. Terrestrial planets also have a molten heavy-metal core, few moons and topological features such as valleys, volcanoes and craters.
In our solar system, there are four terrestrial planets, which also happen to be the four closest to the sun: Mercury, Venus, Earth and Mars.
During the formation of the solar system, there were likely more terrestrial planetoids, but they either merged with each other or were destroyed.
The definition of "planet" from the International Astronomical Union is controversial. The IAU defines a planet as a celestial body that is in orbit around the sun, has a nearly round shape, and has mostly cleared its orbital neighborhood of debris. Scientists are divided in particular on the third point, with some saying that it's hard to define how much clearing a planet does, while others saying a world like Pluto would clear less than a world like Earth. This means that some astronomers argue that the dwarf planet Pluto should be classified as a planet, along with various other dwarf planets scattered throughout the solar system.
Mercury
Mercury is the smallest terrestrial planet in the solar system, about a third the size of Earth. It has a thin atmosphere, which causes it to swing between burning and freezing temperatures. Mercury is also a dense planet, composed mostly of iron and nickel with an iron core. Its magnetic field is only about 1 percent that of Earth's, and the planet has no known moons. The surface of Mercury has many deep craters and is covered by a thin layer of tiny particle silicates.
In 2012, scientists found extensive evidence of organics — the building blocks of life — as well as water ice in craters shaded from the sun. Mercury's thin atmosphere and close proximity to the sun mean it's impossible for the planet to host life as we know it.
Venus
Venus, which is about the same size as Earth, has a thick, toxic carbon-monoxide-dominated atmosphere that traps heat, making it the hottest planet in the solar system. Venus has no known moons. Much of the planet's surface is marked with volcanoes and deep canyons.
The biggest canyon on Venus stretches across the surface for 4,000 miles (nearly 6,500 kilometers). And it's possible that at least some of the planet's volcanoes are still active. Few spacecraft have ever penetrated Venus' thick atmosphere and survived. And it's not just spacecraft that have trouble getting through the atmosphere — there are fewer crater impacts on Venus than other planets because only the largest meteors can make it. The planet is hostile to life as we know it.
Earth
Of the four terrestrial planets, Earth is the largest, and the only one with extensive regions of liquid water. Water is necessary for life as we know it, and life is abundant on Earth — from the deepest oceans to the highest mountains. Like the other terrestrial planets, Earth has a rocky surface with mountains and canyons, and a heavy-metal core. Earth's atmosphere contains water vapor, which helps to moderate daily temperatures.
The planet has regular seasons for much of its surface; regions closer to the equator tend to stay warm, while spots closer to the poles are cooler and in the winter, icy. The Earth's climate, however, is warming up due to climate change associated with human-generated greenhouse gases, which act as a trap for escaping heat. Earth has a northern magnetic pole that is wandering considerably, by dozens of miles a year; some scientists suggest it might be an early sign of the north and south magnetic poles flipping. The last major flip was 780,000 years ago. Earth has one large moon that astronauts visited in the 1960s and 1970s.
Mars
Mars has the largest mountain in the solar system, rising 78,000 feet (nearly 24 km) above the surface. Much of the surface is very old and filled with craters, but there are geologically newer areas of the planet as well. At the Martian poles are polar ice caps that shrink in size during the Martian spring and summer. Mars is less dense than Earth and has a smaller magnetic field, which is indicative of a solid core, rather than a liquid one.
While scientists have found no evidence of life yet, Mars is known to have water ice and organics — some of the ingredients for living things. Evidence of methane has also been found in some parts of the surface. Methane is produced from both living and non-living processes. Mars has two small moons, Phobos and Deimos. The Red Planet is also a popular destination for spacecraft, given that the planet may have been habitable in the ancient past.
Beyond the solar system
During its lifetime, NASA's Kepler space observatory discovered more than 2,300 confirmed alien planets — and thousands more possibilities — as of January 2019. Kepler ran out of fuel in 2018, but many of its possible planet discoveries still need to be confirmed with follow-up observations from other telescopes. Using the data from the telescope, scientists calculated that there may be billions of Earth-like planets in the Milky Way galaxy.
In 2017 another telescope – The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) – discovered a system of Earth-like planets orbiting a star more than 39 light-years away. Within the system are 7 planets, thought to be terrestrial – 4 of which are super-Earth sized. This exoplanet system is known as the TRAPPIST-1.
A 2021 study published in the Planetary Science Journal suggests that all the planets in this system share a similar density. This could mean they share the same ratio of materials – such as iron, silicon and oxygen – commonly found on rocky planets.
There are 3 planets in the TRAPPIST-1 system that sit in the habitable zone of its central star – TRAPPIST-1 e, f and g. The habitable zone is the distance from the star whereby liquid water could exist on the planet's surface. Planets in this region of a solar system are potentially capable of hosting life.
A successor mission to Kepler, called TESS (Transiting Exoplanet Survey Satellite), began operations in 2018. The spacecraft is designed to look for Earth-size planets that are only a few light-years away from our planet, allowing for quick observations by other telescopes on Earth. As of early 2019, TESS has already discovered a handful of planets; its first confirmed find was in September 2018.
Non-terrestrial planets
Not all planets are terrestrial. In our solar system, Jupiter, Saturn, Uranus and Neptune are gas giants, also known as Jovian planets. It's unclear what the dividing line is between a rocky planet and a terrestrial planet; some super-Earths may have a liquid surface, for example. In our solar system, gas giants are much bigger than terrestrial planets, and they have thick atmospheres full of hydrogen and helium. On Jupiter and Saturn, hydrogen and helium make up most of the planet, while on Uranus and Neptune, the elements make up just the outer envelope. These planets are also inhospitable to life as we know it, although this region of the solar system has icy moons that could have habitable oceans.
Additional resources
For an in-depth look into our solar system, check out NASA's interactive Solar System Exploration webpage. The Planets: The Definitive Visual Guide to Our Solar System by DK, is also an excellent illustrative guide to all things planets.
Bibliography
Courtney Dressing et al, "The mass of Kepler-93b and the composition of terrestrial planets," The Astrophysical Journal, Volume 800, February 2015.
Simon Grimm et al, "The nature of the TRAPPIST-1 exoplanets", Astronomy & Astrophysics, Volume 813, May 2019, https://doi.org/10.1051/0004-6361/201732233.
George Ricker et al, "Transiting Exoplanet Survey Satellite (TESS)," American Astronomical Society, Volume 42, January 2010.
NASA, "Mercury: Solar System Exploration," January 2022.
NASA, "Venus: Solar System Exploration," January 2022.
NASA, "Mars: Solar System Exploration," January 2022.
NASA, "Earth: Solar System Exploration," January 2022.
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