The rocky planets in the Solar System, known as terrestrial planets, include Mercury, Venus, Earth, and Mars.

Planets in our Solar System are divided into two primary categories: Jovian planets and terrestrial planets.

Jovian planets consist of the gas and ice giants found in the outer regions of the Solar System, while terrestrial planets are the rocky, smaller planets within the inner Solar System.

The rocky terrestrial planets, which include the four planets closest to the Sun—Mercury, Venus, Earth, and Mars—are the focus here. What makes these planets 'terrestrial' and how do they compare with other remarkable planets in our Solar System and beyond? Let's explore.

Are All Rocky Planets Considered Terrestrial?

The answer is yes. All terrestrial planets share key characteristics, much like the four inner planets of our Solar System. They may have a rocky or metallic core, but all terrestrial planets are enveloped by a silicon-based rocky mantle or a solid surface predominantly made up of carbon-based minerals.

A terrestrial planet’s surface is often marked by slopes and diverse topographical features, including mountains, valleys, and canyons. These are shaped by volcanic processes, tectonic plate movements, and, in Earth’s case, the flow of liquid water.

A terrestrial planet is large enough to develop a secondary atmosphere. However, due to its relatively small size and proximity to the Sun in a high-temperature region, it is typically unable to form planetary rings like the larger, more distant Jovian planets.

Exploring Terrestrial Planets in Our Solar System

Let's take a closer look at the terrestrial planets in our own solar system: Mercury, Venus, Earth, and Mars.

Mercury

Mercury, the smallest planet in the solar system, is a place of extremes. It’s even smaller than Earth's moon and some other moons, and is the planet closest to the sun. This proximity results in it being bombarded by an intense light that is about seven times brighter than a summer day on Earth.

Mercury orbits the sun faster than any other planet due to its small size and proximity. In just 87.97 Earth days, Mercury completes one full orbit, traveling at a speed of nearly 29 miles (46.6 kilometers) per second.

While it might seem that Mercury's position as the closest planet to the sun would make it the hottest, its thin atmosphere prevents it from holding on to heat.

During the day, temperatures on Mercury can soar as high as 800 degrees Fahrenheit (430 degrees Celsius), but at night, they can plummet to as low as minus 290 degrees Fahrenheit (minus 180 degrees Celsius).

A unique feature of Mercury is its unusual rotation pattern, known as orbital resonance. The planet rotates three times on its axis for every two orbits it completes around the sun.

"An odd consequence of this phenomenon is that if you could stand on Mercury and survive the extreme heat of 400 degrees Celsius at high noon, you would have to wait 176 Earth days (equivalent to two Mercurian years) to see another one," explains Dr. Vahé Peroomian, a professor of physics and astronomy at the University of Southern California.

Venus

Due to its dense atmosphere, Venus holds the title of the hottest among the four terrestrial planets. The atmosphere, made up of carbon dioxide and thick sulfuric acid clouds, creates an extreme greenhouse effect. Combined with its close orbit to the sun, Venus experiences surface temperatures reaching as high as 900 degrees Fahrenheit (475 degrees Celsius). The planet also features geological formations similar to other terrestrial planets, including mountains and (possibly active) volcanoes.

Although Venus is commonly nicknamed "Earth's twin" since it is about the same size and mass, these two iron planets couldn't be more different, especially when you compare how well their environments support life. Venus, for instance, has crushing air pressure at its surface.

"If you stood on the surface of Venus, the air pressure would be the same as if you were 3,000 feet [914 meters] beneath the ocean's surface," Peroomian says. You'd have to go 164,042 feet (50 kilometers) high into Venus' atmosphere to reach a layer where the atmospheric pressure and average was similar to Earth at sea level, he says.

Earth

Humanity truly hit the cosmic jackpot by being fortunate enough to call Earth our home. Earth's orbit lies within the "Goldilocks" zone — it's not too hot to vaporize the planet's mostly liquid surface, nor too cold to freeze-dry all life on its rocky terrain. Earth is the terrestrial planet that has just the right conditions for life.

A large part of Earth's welcoming environment can be attributed to its atmosphere, which is mostly made up of nitrogen, oxygen, and trace gases. This multi-layered shield provides breathable air, drinkable water from natural springs and precipitation, and a protective barrier against harmful solar radiation.

You might have even experienced this protective shield when witnessing the stunning "Northern lights" (aurora borealis) light up the night sky in a dazzling display.

Mars, the fourth planet from the sun, is known for its red hue and barren landscape.

Mars is a relatively dense planet. Despite having a smaller iron core than Earth, its mass is still significant, accounting for roughly half the size of the planet.

Mars' most distinctive feature is its expansive, barren landscape, dotted with towering peaks and deserts filled with red iron oxide dust. There is evidence suggesting that the planet's surface was once traversed by rivers, carving canyons and other geological formations. However, the thin atmosphere prevents water from remaining on the surface for long, and the only visible traces of water or vapor exist as ice at the polar regions.

Extrasolar Terrestrial Planets Outside the Solar System

Scientists hypothesize that there are many extrasolar terrestrial planets — or exoplanets — in nearby galaxies and solar systems. These are planets, like Earth, that orbit their own stars. The main challenge to prove which extrasolar planets share similarities with Earth's surface is the distance they are from our solar system.

For instance, the closest of the potentially terrestrial extrasolar planets to our solar system lies in the Proxima Centauri system, over 4 light-years and 25 trillion miles from Earth. It's called Proxima b and it's one of the super Earths.

Astronomers can only make educated guesses about whether Proxima b is part of the terrestrial or Jovian planets using information gathered from the data when the planet passes between Earth and its local star.

Dwarf Planets and Super-Earths

Astronomers and physicists hypothesize that terrestrial exoplanets exhibit significant variation in both size and mineral composition. Other solar systems likely generate planets with distinct chemical and mineral makeups, and the rotating protoplanetary disk around a young star in an early solar system can give rise to a diverse range of carbon planets, dwarf planets, and super-Earths like Proxima b.

A dwarf planet is typically not much larger than a moon or a "minor planet" from an asteroid belt, which makes these objects more difficult to detect and track in space compared to exoplanets that fall into the super-Earth category.

Coreless Planets

The structure of many extrasolar planets may differ from our own terrestrial definition of an "iron planet" if they have a solid surface without a dense metallic core. Without further exploration, it is impossible to determine whether these hypothetical coreless planets, located within the "Goldilocks zones" of their star's inner solar system, could sustain human life with an atmosphere and liquid water reserves.

For now, scientists continue to focus on studying nearby inner planets to gain insights into which terrestrial exoplanets may offer the best chance of becoming Earth 2.0. However, it's also important for us to do our part in protecting the planet we already call home and avoid relying on the search for a new home as a backup plan.