The Rocky Planets

There are two types of planets in the universe, solid and gaseous. The gaseous planets are enormous balls of gas and are called “giants” for a reason. The other planets are rocky, or terrestrial planets and have a thing or two in common with the planet we call home.

A Look at the Rocky Planets

The term “terrestrial” comes from the Latin word for Earth, Terra. Thus, terrestrial planets are “Earth-like” in some fashion, starting with the fact that they are mostly comprised of a silicate rock base.

Terrestrial planets can be further broken down into categories. Solar terrestrial planets are what we call the four such planets in our own solar system, Mercury, Venus, Earth and Mars, of which Earth is the largest, and is the only such planet with an active hydrosphere. Extra solar terrestrial planets are, as one might imagine, rocky planets found outside of our solar system.

As Earth is the largest terrestrial planet that we are familiar with we use it as a measuring tool in order to categorise terrestrial planets even further. A super-Earth is a rocky planet that is over ten Earth-masses. A planet this size is not uncommon—we have a few in our solar system—but these are gas giants, planets that are enormous but not solid. A super-Earth is both enormous and solid, and there have been a few discovered in recent years.

The Structure of a Rocky Planet

There are a number of features that are unique to rocky planets, all thanks to their solid make up. A central metallic core, often made primarily of iron, is common, covered with a rocky, silicate mantle. The core can be molten or solid or some combination of the two. The surface of the rocky planet will be covered with features like volcanoes, mountains, craters and canyons. While both gas planets and rocky planets have atmospheres more often than not, a gas giant has what is known as a primary atmosphere, which is captured from the surrounding space during its formation, while a terrestrial planet has a secondary atmosphere, one that is created by internal processes such as volcanism. A terrestrial planet’s atmosphere is therefore often replenishing itself.

Discovering Rocky Planets

Extrasolar terrestrial planets are out there but have been difficult to spot in relation to the gas giants, which are typically so much larger. In fact, the first discovery of Extrasolar terrestrial planets was made quite by accident. Alexander Wolszczan discovered the first three such planets in 1992 while studying a pulsar (a star that emits radio waves in a predictable pattern.)

As these three planets, one much smaller than Earth (0.02 Earth Mass) and the others much larger (4.3 and 3.9) passed between the pulsar and the observer they blocked the radio emissions. Had they been orbiting a non-pulsar they would have remained undiscovered.

Final Thoughts

Rocky planets hold the best hope for discovering life in the universe outside that which we know on our own rocky planet. While there remains the possibility of discovering life on a gas giant in some as-yet unknown form, the conditions for life to exist are more readily supported on those terrestrial planets that are most like our own.

Related Articles in the 'Our Solar System' Category...

• A Guide To The Outer Solar System
• How Planetary Surfaces Are Formed
• Observing Dark Matter And The Lifespan Of The Universe
• Orbits and How They Work
• Origins of the Solar System
• Planetary Atmospheres
• The Asteroid Belt
• The Discovery of our Dented Solar System
• The Dwarf Planets
• The Gas Giant Planets
• The Kuiper