Terrestrial Planets: Definition & Facts About the Inner Planets

terrestrial planets
The terrestrial planets or our solar system: Mercury, Venus, Earth & Mars
Credit: Public domain

Terrestrial planets are Earth-like planets (in Latin, terra means Earth) made up of rocks or metals with a hard surface — making them different from other planets that lack a solid 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 creation of the solar system, there were likely more terrestrial planetoids, but they likely merged or were destroyed.


Mercury is the smallest terrestrial planet in the solar system, about a third of 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. 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 make it impossible to host life as we know it. 

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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. Much of its surface is marked with volcanoes and deep canyons — the biggest of which is 4,000 miles long. Few spacecraft have ever penetrated Venus’s 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.

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Of the four terrestrial planets, Earth is the largest, and the only one that has extensive regions of liquid water. Wateris 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.

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Mars has the largest mountain in the solar system, rising 78,000 feet 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 (in varying amounts) has also been found in some parts of the surface. Methane is produced from both living and non-living processes.

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Beyond the solar system

NASA's Kepler space observatory had discovered more than 1,000 confirmed alien planets — and thousands more possibilities — as of August 2015. Using the data from the telescope, scientists have calculated that there may be billions of Earthlike planets in the Milky Way galaxy. [Infographic: A Sky Full of Alien Planets]

Kepler scientists also found that super-Earths — planets slightly larger than our own — are abundant in the universe. If these super-Earths are rocky, it would make rocky planets a common phenomenon. However, not all of these are habitable; some are too close or far from their parent star, for example.

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Non-terrestrial planets

Not all planets are terrestrial. In our solar system, Jupiter, SaturnUranus 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. Landing on a gas giant is nearly impossible, since only the core is solid, and the atmosphere is storm-filled and very thick.  It's also inhospitable to life as we know it.

Additional reporting by Elizabeth Howell, contributor.

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