Terrestrial Planets: Rocky Worlds Close to the Sun

terrestrial planets
The terrestrial "rocky" worlds of our solar system, shown in scale to each other. NASA/JPL-JHU.

Today, we know what planets are: other worlds. But, that knowledge is pretty recent in terms of human history. Up until the 1600s, planets seemed like mysterious lights in the sky to early stargazers. They appeared to move through the sky, some more rapidly than others. The ancient Greeks used the term "planetes", which means "wanderer", to describe these mysterious objects and their apparent motions.

Many ancient cultures saw them as gods or heroes or goddesses.

It wasn't until the advent of the telescope that planets stopped being otherworldly beings and took their proper place in our minds as actual worlds in their own right. Planetary science began when Galileo Galilei and others began looking at planets and trying to describe their characteristics.

Sorting Planets

Planetary scientists have long since sorted planets into specific types. Mercury, Venus, Earth, and Mars are called "terrestrial planets". The name stems from the ancient term for Earth, which was "Terra". The outer planets Jupiter, Saturn, Uranus and Neptune are known as the "gas giants". That's because most of their mass lies in their huge atmospheres that smother the tiny rocky cores deep inside.

Exploring the Terrestrial Planets

Terrestrial worlds are also called "rocky worlds". That's because they're made mainly of rock.

We know a great deal about the terrestrial planets, based largely on exploration of our own planet and spacecraft flybys and mapping missions to the others. Earth is the main basis for comparison — the "typical" rocky world. However, there are major differences between Earth and the other terresrials.

Let's take a look at how they are alike and how they differ.

Earth: Our Home World and Third Rock from the Sun

Earth is a rocky world with an atmosphere, and so are two of its closest neighbors: Venus and Mars. Mercury is also rocky, but has little to no atmosphere. Earth has a molten metallic core region covered by a rocky mantle, and a rocky outer surface. About 75 percent of that surface is covered with water, mainly in the world's oceans. So, you could also say that Earth is a water world with seven continents breaking up the wide expanse of oceans. Earth also has volcanic and tectonic activity (which is responsible for earthquakes and mountain-building processes). Its atmosphere is thick, but not nearly so heavy or dense as those of the outer gas giants. The main gas is mostly nitrogen, with oxygen, and smaller amounts of other gases. There's also water vapor in the atmosphere, and the planet has a magnetic field generated from the core that extends out into space and helps to protect us from solar storms and other radiation.

Venus: Second Rock from the Sun

Venus is the next closest planetary neighbor to us. It is also a rocky world, wracked by volcanism, and covered with a stifling heavy atmosphere made up mostly of carbon dioxide.

There are clouds in that atmosphere that rain out sulfuric acide onto the dry, overheated surface. At one time in the very distant past, Venus may have had water oceans, but they are long gone — the victims of a runaway greenhouse effect. Venus does not have an internally generated magnetic field. It spins very slowly on its axis (243 Earth days equals one Venus day), and that may not be enough to stir up the action in its core needed to generate a magnetic field.

Mercury: Closest Rock to the Sun

The tiny, dark-colored planet Mercury orbits closest to the Sun and is a heavily iron-laden world. It has no atmosphere, no magnetic field, and no water. It may have some ice in the polar regions. Mercury was a volcanic world at one time, but today it is just a cratered ball of rock that alternately freezes and heats up as it orbits the Sun.

Mars: Fourth Rock from the Sun

Of all the terrestrials, Mars is the closest analog to Earth. It's made of rock, just as the other rocky planets are, and it has an atmosphere, although it's very thin. The magnetic field of Mars is very weak, and there's a thin, carbon-dioxide atmosphere. Of course, there are no oceans or flowing water on the planet, although there's a lot of evidence for a warmer, watery past.

The Rocky Worlds in Relation to the Sun

The terrestrial planets all share one very important characteristic: they orbit close to the Sun. They likely formed close to the Sun during the period when the Sun and planets were born. The close proximity to the Sun "baked away" much of the hydrogen gas and inventory of ices that existed close to the newly forming Sun at the beginning. Rocky elements could withstand the heat and so they survived the heat from the infant star. 

The gas giants may have formed somewhat close to the infant Sun, but they eventually migrated out to their present positions. The outer solar system is more hospitable to the hydrogen, helium, and other gases that make up the bulk of those gas giant planets. Up close to the Sun, however, the rocky worlds could withstand the heat of the Sun, and they remain close to its influence to this day.

As planetary scientists study the makeup of our fleet of rocky worlds, they are learning a lot that will help them understand the formation and existence of rocky planets circling other Suns. And, because science is serendipitous, what they learn at other stars will better help them learn more about the existence and formation history of the Sun's little collection of terrestrial planets.

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Petersen, Carolyn Collins. "Terrestrial Planets: Rocky Worlds Close to the Sun." ThoughtCo, Mar. 2, 2017, thoughtco.com/terrestrial-planets-rocky-worlds-close-to-the-sun-4125704. Petersen, Carolyn Collins. (2017, March 2). Terrestrial Planets: Rocky Worlds Close to the Sun. Retrieved from https://www.thoughtco.com/terrestrial-planets-rocky-worlds-close-to-the-sun-4125704 Petersen, Carolyn Collins. "Terrestrial Planets: Rocky Worlds Close to the Sun." ThoughtCo. https://www.thoughtco.com/terrestrial-planets-rocky-worlds-close-to-the-sun-4125704 (accessed January 16, 2018).