Are terrestrial planets really that similar to Earth? What to know about their composition

• There are four terrestrial planets in our solar system, and billions of them are outside our solar system.
• Terrestrial planets are made of rock and silicate around a metallic core.
• Earth is the largest terrestrial planet in our solar system.

A terrestrial planet is a planet that is primarily made of rock and metals. Therefore, a terrestrial planet has a solid and firm planetary surface, like Earth.

That is why terrestrial planets are said to be Earth-like. But how true is that? And most importantly, could terrestrial planets other than Earth support life?

What are the terrestrial planets?

In our solar system, the four inner planets are all categorized as terrestrial. The inner planets are the four that are closest to the Sun: Mercury, Venus, Earth, and Mars.

That is where the terrestrial planets' list ends in the solar system, but there are also many terrestrial planets outside of it. NASA estimates that there could be more than 10 billion terrestrial planets just in the Milky Way.

However, just because a planet is classified as terrestrial, it doesn't mean that it's necessarily habitable. For example, in the solar system, Mercury and Venus are too hot for life to thrive, and Mars is likely too cold. Apart from this, Mercury and Mars do not have a protective, breathable atmosphere like Earth's, and Venus has an extremely dense and toxic one.

Outside of the Solar System, Kepler-10b was the first confirmed terrestrial planet discovered by the Kepler Space Telescope. However, it is too close to its Sun-like star, Kepler 10, and so too hot to support life, as far as we know.

In 2017, NASA discovered seven terrestrial planets orbiting TRAPPIST-1, an ultra-cool red dwarf star in the constellation Aquarius, about 40 light-years from Earth. These rocky, Earth-size worlds could contain surface water in different physical states depending on their distance from their host star.

However, only three of them are located in the habitable zone, where liquid water is most likely to exist. The planets closest to the star presumably contain water vapor, while the planets farthest away are most likely icy. This exoplanet system is called after the central star that keeps it together, TRAPPIST-1.

What are terrestrial planets made of?

Terrestrial planets are made of rock, silicate, metals, water, and carbon. All terrestrial planets in our solar system actually have a metallic core, mostly made of iron, and a silicate shell with a mantle and a crust.

Earth, Mars, and Mercury also have water. Mars likely has water in the form of ice in its polar regions, and Mercury may contain water ice at the bottom of craters located at its north pole. In contrast, planets with an icy surface of volatiles such as water, ammonia, and methane are classified as ice planets or icy planets.

Icy planets are mostly made up of volatiles such as (frozen) water, ammonia, nitrogen, carbon dioxide, and methane. Pluto was the only icy planet in our solar system before it was reclassified as a dwarf planet by the International Astronomical Union in 2006. Uranus and Neptune are classified as ice giants, but they are also classed as Jovian ("Jupiter-like") or gas giants. These are large planets composed mostly of gases, such as hydrogen and helium, and contain a relatively small rocky core. There are four gas giants in our solar system — Jupiter, Saturn, Uranus, and Neptune.

Terrestrial planets vs. Jovian planets

• Jovian planets are called "gas planets" because they are composed primarily of hydrogen, helium, methane, and other gases, including some in a liquid state. For example, Jupiter is likely made up partly of liquid metallic hydrogen, which creates a huge magnetic field. This means that these planets don't have solid, rocky surfaces. Anything dropped onto these planets would sink through the gases until it was crushed by atmospheric pressure.
• In contrast, terrestrial planets have metallic cores mostly surrounded by silicate and rock —so we have something to step on.
• Jovian planets are very massive. The Jovian planets in our solar system are Jupiter, Saturn, Neptune, and Uranus. According to NASA, Jupiter is about 11 times larger than Earth, Saturn is about nine times larger than Earth, and Uranus and Neptune are each about four times larger than Earth (measured in diameter). This is why they are also called "giant planets" or "gas giants."

Terrestrial planets tend to be smaller in comparison. Exoplanets called "super-Earths" can be bigger than the Jovian planets but are not necessarily terrestrial —they may be made up of gas, rock, or a combination of both.

• Jovian planets rotate faster on their axis than terrestrial planets. One of the most accepted hypotheses to explain this is that when they formed, these planets accumulated most of their mass from gas in the surrounding solar nebula. That gas carried a lot of angular momentum as it fell onto the planets' cores, causing the planets to spin faster and faster as they coalesced. For comparison: while Earth and Mars complete a full rotation in roughly 24 hours (what we call a day), Jupiter and Saturn complete a full rotation in roughly 10 hours.

• Objects with more mass have more gravity. Therefore, Jovian planets attract more objects to orbit around them as moons. Jupiter has approximately 79 moons, Saturn has 82 moons, Uranus has 27 known moons, and Neptune has 14 known moons. Earth has only one moon, Mars has two (Deimos and Phobos), and Mercury and Venus have none.

• Theoretically, the gravitational forces of the Jovian planets could also lead them to destroy their own moons, as well as comets and asteroids passing by. That is probably the reason why Jupiter and Saturn have rings of dust and debris around them. Terrestrial planets do not have rings.



Which is the largest terrestrial planet?

Earth is the largest terrestrial planet in the solar system. Outside the solar system, the largest terrestrial planet discovered so far is called TOI-849b, discovered in 2020 by NASA's space telescope TESS.

TOI-849b is 730 light-years away and 40 times more massive than Earth —pretty much as large as Neptune, but it is not icy. It is actually scorching, with an average temperature of 2780ºF (1500ºC), something that happens because it is very close to its star —so close that it takes about 18 hours for the planet to complete a full rotation around its host star.

Planets that size are expected to accumulate a lot of gas. Consequently, TOI-849b should be a gas giant. But it is not. In fact, it doesn't have an atmosphere, or it has a very thin one, so its case is pretty unique. It is also very dense. One hypothesis is that TOI-849b is actually the core of a gas giant, and it doesn't have a gassy atmosphere because its star's radiation has swept it away.

Other terrestrial planet facts and characteristics

• Scientists believe that there are many terrestrial exoplanets, but gas giants are easier to detect due to their size.
• Because they may have tectonic activity or erosive processes, terrestrial planets tend to have topological features such as valleys, volcanoes, canyons, mountains, and craters.
• Some asteroids and moons have a similar structure to terrestrial planets, such as rocky asteroid Vesta (one of the largest in the asteroid belt) and Pallas, or the Earth's moon.