Planets in our solar system fall into one of two major categories: Jovian planets and terrestrial planets.
Jovian planets encompass the gas giants and ice giants of the outer solar system, whereas terrestrial planets include the small rocky planets within the inner solar system.
These rocky terrestrial planets include the four closest to our sun: Mercury, Venus, Earth and Mars. What else makes these celestial bodies terrestrial planets, and how do they compare to some of the other wondrous planets in the solar system and beyond? We'll explain.
The short answer is yes. All terrestrial planets in the universe share the same characteristics as the four terrestrial planets in the inner region of our own solar system. Some include a rocky core or metal core, but all terrestrial planets are surrounded by a silicon-based rocky mantle or a solid surface comprised of primarily carbon-based minerals.
A terrestrial planet's surface is sloped and carved with topographical features like mountains, valleys and canyons due to volcanic activity, shifting tectonic plates and in Earth's case, fast-moving liquid water.
A terrestrial planet is large enough to form a secondary atmosphere but is typically too small, and orbiting within high-temperature regions close to the sun, to form planetary ring systems like those found on larger, more distant Jovian planets.
Terrestrial Planets in Our Solar System
Let's discuss the terrestrial planets in our local solar system: Mercury, Venus, Earth and Mars.
This terrestrial planet is a place of extremes. It is the smallest of all the planets in the solar system, barely outsizing Earth's moon and other natural satellites. Mercury is also the closest planet to the sun, providing a glaring visible light exposure nearly seven times as bright as a sunny summer day on our planet.
Due to its small mass and close proximity to the sun, Mercury has the fastest orbital period around the sun of any planet. It takes just 87.97 Earth days for Mercury to make a complete orbit around the sun (one Mercurian year). That's nearly 29 miles (46.6 kilometers) per second!
Although you may think that Mercury's position as the nearest planet to the sun would logically make it the hottest planet, its thin atmosphere makes it difficult to retain heat.
During the day, temperatures can soar to a high as of 800 degrees Fahrenheit (430 degrees Celsius), but then plummet at night as low as minus 290 degrees Fahrenheit (minus 180 degrees Celsius).
One interesting fact about Mercury: Its closeness to the sun has caused the planet to develop an unusual rotation known as orbital resonance. That means it rotates three times on its axis for every two orbits around the sun.
"A weird consequence of this phenomena means that if you could stand on Mercury and withstand the 400-degree Celsius temperatures of high noon, you would have to count 176 Earth days (two Mercurian years) until you experienced the next one," explains Dr. Vahé Peroomian, a physics and astronomy professor at the University of Southern California.
Thanks to its thick atmosphere, Venus is the hottest of the four terrestrial planets. Its atmosphere is toxic and comprised of carbon dioxide and yellowish sulfuric acid cloud cover. This greenhouse effect, mixed with Venus' close proximity to the sun, produces scorching surface temperatures as high as 900 degrees Fahrenheit (475 degrees Celsius). Venus also has many geological surface features similar to other terrestrial planets, including mountains and (potentially 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.
Humans really won the cosmic lottery by being lucky enough to call Earth our home planet. Earth's orbit lands within the "Goldilocks" habitable zone — not too hot to boil Earth's mostly liquid surface and not too cold to freeze-dry the flora and fauna on our planet's rocky surface. Earth is the terrestrial planet that is just right.
Much of this hospitable environment is also thanks to Earth's atmosphere, which is composed primarily of nitrogen, oxygen and trace gases. This multilayered forcefield provides our planet's inhabitants with breathable air, drinkable water in the form of natural springs and precipitation, and protection from solar radiation.
You may have even had the pleasure of witnessing this life-saving solar shielding when you see the swirling "Northern lights" (aurora borealis) dancing across the night sky.
Mars is a dense planet. Although it has a much smaller iron core than Earth, its comparable mass still makes up roughly half the planet's size.
Mars' most notable characteristic is its vast, desolate landscape, spotted with giant peaks and deserts of red iron oxide dust. There is evidence that the Red Planet's surface was once covered in tributaries that carved out canyons and other geographical features. But the thin atmosphere keeps water from lasting long on the surface. The only signs of water or water vapor is as ice in 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 also theorize that these terrestrial exoplanets vary wildly in size and mineral composition. Other solar systems likely produce planets with different chemical and mineral makeups, and the spinning protoplanetary disk of an early solar system surrounding a young star could produce a wide range of carbon planets, dwarf planets and super-Earths like Proxima b.
A dwarf planet is not much larger than a moon or "minor planet" from an asteroid belt, so these planets are more challenging to identify and track in space than an exoplanet from the super-Earth size category.
The planetary structure of many extrasolar planets may also diverge from our "iron planet" terrestrial definition if they have a solid surface without a dense metallic core. It is also impossible without further exploration to conclude if any of these hypothetical coreless planets in the "Goldilocks zones" of their respective star's inner solar system would have the atmosphere and liquid water reserves to sustain human life.
So, for the time being, scientists continue to study nearby inner planets to gain more insight into which terrestrial exoplanets have the best chance of becoming Earth 2.0. However, other members of our species can do their fair share to protect the planet we already have and avoid the need to seek out other terrestrial planets as a plan B.
Now That's Massive
One of the key distinctions between terrestrial "Earth-size" planets and Jovian planets (sometimes called "gas giants") is an extreme difference in planetary mass and volume. For example, Jupiter, the largest of the gas-giant planets, is roughly 318 times the mass of Earth. This size disparity becomes even greater when we account for volume. If we created an imaginary empty globe the size of Jupiter, you would need a whopping 1,323 Earths to fill it. In fact, Jupiter is so large that if you were to combine all the other planets in our solar system into a massive, transformer-like celestial body, Jupiter would still be more than double its size.
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