Terrestrial Planets

This post is going to cover the rocky inner planets of our solar system, and what makes them different to other planets.

If you want, jump Straight to:

What is a Planet?

To this day, the definition of planet is up in the air. A planet is widely accepted as something that orbits a star, has a large gravitational force (but is still less than a star’s), and has a clear orbit path. Our knowledge of planets is still growing, and so our understanding of what a planet is will change as well.

What Kind of Planets Are There?

Just like stars, planets come in many types, our solar system being very rich in planet diversity. Planets are classified into four groups: terrestrial planets, gas giants, ice giants and dwarf planets. From that they can be classified even further. Think about which class you would place each planet in our solar system. Today, we’re going over Terrestrial planets.

Terrestrial Planets

Terrestrial planets are rocky planets, such as our own planet.  Terrestrial planets tend to all have a similar structure: a core (usually iron), mantle and crust. Our solar system has four terrestrial planets: Mercury, Venus, Earth and Mars.

What kinds of Terrestrial Planets are there?

As we find more and more exoplanets, the range of planet types broaden. Currently we have four main categories: silicate, iron, coreless, and carbon.

Silicate planets are like the ones in our solar system. These will have a metallic core of mostly iron, and a silicate mantle (silicate means silicon-based and oxygen-based).

Similar to this type is the iron planet, which has only been theorised. Iron planets are mostly made up of iron, so they’re very dense and would have to be small. Mercury follows the structure of the iron planet closely.

Coreless planets are also theorised but not proven to exist. They have no metallic core but are made of only silicate mantle. Many asteroids are good examples of this type.

Lastly, carbon planets are made up of a metallic core and a carbon-based mantle, unlike our own silicate mantle.

Terrestrial Planets in our Solar System




  • Radius: 2440km
  • Semi-major axis: 58 million km
  • Eccentricity: 0.21
  • Orbit: 88 Earth days
  • Sidereal day: 58 days
  • Surface temperature: 450-180°C
  • Gravitational constant: 3.7 m/s²
  • Tilt: 2.11°
  • Moons: 0

Mercury is the smallest of all principle planets, with a radius of 2440km, and is the closest to the sun. It’s semi-major axis (average distance between the planet and the sun) is 58 million kilometres, but these distances range notably; its orbit is very elliptical (0.2), so it’ll be orbiting much slower at its aphelion (furthest distance) than at its perihelion (closest point).

Mercury’s north polar region. The yellow highlighted craters mark locations that show evidence for water ice. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Since its distance to the sun is very small, a Mercurian year is only 88 days. The close proximity and lack of atmosphere also means its temperature can reach up to 450°C during the day, but -180°C at night. Although, a day on Mercury is surprisingly long compared to its year; mercury completes a full rotation in about 59 Earth days.

Mercury’s structure


Just like any other terrestrial planet, Mercury has a core and mantle. However, Mercury’s core is notably dense for a tiny planet, and since its core is so large and crust is so small, it has an extremely weak magnetic field. The core as we know it is made up of solid outer layer and liquid iron inner layer. The presence of a magnetic field suggests that Mercury has an internal dynamo within a liquid core, just like Earth.

Mercury’s surface is rich in craters, cliffs, impact basins, and even volcanoes. Caloris Basin, an enormous impact basin, contains 9 volcanic vents that all intersect each other. It’s fair to say that Mercury’s surface is quite similar to the Moon’s. Yet, it’s difficult to observe Mercury since the closeness to the Sun means dealing with harsh solar winds, and of course, the challenge of following an elliptical and rapidly orbiting planet.

Looking at Mercury Through a Telescope

I hate to be the bearer of bad news, but I wouldn’t recommend observing Mercury through a telescope without a filter. The planet is always close to the Sun, and unless you had a solar filter and telescope with good magnification, you’re not going to be very satisfied with the results (unless you want to run the risk of permanent blindness).




  • Radius: 6,052 km
  • Semi-major axis: 108 million km
  • Eccentricity: 0.0068
  • Orbit: 225 Earth days
  • Sidereal day: 243 days
  • Surface temperature: 462°C
  • Gravitational constant: 8.9 m/s²
  • Tilt: 177.3°
  • Moons: 0

Next up in our wonderful neighbourhood is Venus. Venus is similar in size to Earth, it’s radius being 6,052 km, and although its semi-major axis is 108 million kilometres (much larger than Mercury’s), it is the hottest planet in our solar system.

A Computer simulated picture of Venus’ surface by Magellan. Credit: NASA/JPL

Unlike Mercury, Venus’s gravitational force is big enough to let it have a large atmosphere. The greenhouse gases that make up the planet’s atmosphere trap incoming heat, which is why Venus is hotter that Mercury.

A Venusian year is about 225 days, but a day on Venus lasts longer than that. One full rotation takes 243 days, and rotates clockwise unlike any other planet in the solar system.

Venus’ Structure

An actual image of a piece of Venus’ surface by Magellan. Credit: NASA/JPL

Venus is a very volcanic planet, since the crust is so old. The surface never moves, unlike Earth’s plate tectonics. Instead the crust is weakened from the mantle heating up, and is moved into the mantle where new crust is formed. This takes about 100 million years.

Earth and Venus are both cooling by similar amounts, suggesting Venus also has a liquid core. From the crust reformation, we can tell that it also has a mantle and crust, but with such little data, it’s hard to tell the composition within the internal structure.

Looking at Venus Through a Telescope

Venus in UV. Credit: NASA.

Venus’ atmosphere is very dense and of high pressure, mostly composed of carbon dioxide, nitrogen, and sulphur dioxide.

Within this atmosphere are lots of thick Sulphuric clouds that cover the planet. If you were to observe Venus, I’m afraid to say that the surface would be nowhere in sight, although the clouds are a wonderful to observe.




  • Average radius: 6371 km
  • Semi-major axis: 149.6 million km
  • Eccentricity: 0.017
  • Orbit: 365 Earth days
  • Sidereal day: 23 hours 56 minutes
  • Surface temperature: 15°C on average
  • Gravitational constant: 9.81 m/s²
  • Tilt: 23.4°
  • Moons: 1 (called Luna)

The most well-known planet of all time: Earth. Rich in materials and biodiversity, this planet is unique among other terrestrials ( also because it is actually flat).

Earth orbit the sun in the Goldilocks zone, where the temperature is ideal for organisms to live in. Of course, other conditions are taken into account when determining habitability, such as atmospheric pressure, presence of a magnetosphere, and oxygen levels. Earth ticks all the boxes for life!

Structure of Earth

Earth is made up of four basic layers: a liquid/solid iron core, mantle, crust and an atmosphere.


The core is 3’500 km in radius and can be split into two. The inner core is solid and an iron-nickel mix, but the outer core is where the magic happens. Unlike the inner core, the outer core is liquid, giving the metal inside it the ability to flow. The liquid’s temperature is higher further in the core which causes a convection current where hot liquid rises away from the centre and colder liquid sinks closer to it. This repeats as the colder liquid heats up and the hotter liquid cools, creating a huge dynamo.

Why is that important? A core dynamo induces a magnetic field, which protects the atmosphere being stripped away by solar wind. Life on Earth as we know it wouldn’t exist without a magnetic field. Mars no longer has a magnetic field, but magnetised rocks on the surface suggest that there was a field a long time ago, and that Mars used to have a core dynamo until it cooled.

Magnetic and geographical pole of the Earth
Earth’s magnetic field, induced by the core dynamo.

The mantle and crust are where even more extraordinary events happen! The mantle is 2’900 km thick and made up of two main layers: the lower and upper mantle. The crust varies in depth, but is 70 km thick at most.

The lower mantle is composed of rock and high amounts of iron/ magnesium compounds. Temperature and pressure is high, so rock is tightly packed.

The upper mantle is split into two regions: the lithosphere and asthenosphere. The inner layer, the asthenosphere, experiences varying temperatures, sometimes hot enough to melt the rocks. Not all rock is liquid in the asthenosphere, but there’s enough to cause slight movement.

The lithosphere is attached to the crust and split into tectonic plates. Heat that travels up the mantle to the lithosphere elasticises the rocks within it, and causes tectonic movement. The borders of plates are weak, and volcanic activity can thrive there. High pressure in the mantle can cause the hot liquid rock(called magma) to travel upwards and break through to the surface. Magma that reaches the surface is called lava.

Augustine Volcano in Alaska. Credit: Alaska Volcano Observatory and photographer Cyrus Read.

Another interesting tectonic event are tsunamis, because they are caused by all kinds of tectonic movement such as underwater volcanoes, landslides, and earthquakes.

Each event moves water around to an unstable position. Water, being a liquid, will always try to find a stable position. This sometimes generates a large wave of water which grows as it approaches a shore.

2011 tsunami that hit Miyako, Japan.

Looking At Earth Through A Telescope

The ideal telescope for looking at Earth would be a refractor, because it uses lenses instead of mirrors. The functioning part of the lenses should be roughly 6-7 mm. Two lenses are preferable, as they add depth perception, however some would prefer the use of four, for clarity. If your lenses are cloudy, laser treatment may be able to fix them.

If you haven’t caught on, I’m talking about using your eyes. You’re standing on Earth!




  • Average radius: 3386 km
  • Semi-major axis: 230 million km
  • Eccentricity: 0.09
  • Orbit: 687 Earth days
  • Sidereal day: 24 hours 37 minutes
  • Surface temperature: -63°C
  • Gravitational constant: 3.7 m/s²
  • Tilt: 25.2°
  • Moons: 2 (Phobos and Deimos)

Just before the asteroid belt is Mars. This planet is slightly squashed at the poles, so its polar radius is 3,396 km, and at the equator it’s 3,376 km. Mars is further away from the sun than us (230 million km away), so it’s average temperature of -63 °C is much lower than Earth’s warm 14°C.

A day on Mars is similar to Earth’s, being 24 hours and 37 minutes, but its year is almost double Earth’s; It takes 687 days to complete a full orbit around the sun.

Mars’ Structure

Clay rock and minerals on Mars’ surface. Image taken by Opportunity.

The red planet and our own have a few common traits; both have polar caps, seasons, changes in climate, and roughly the same dry surface area. How amazing is that?

Just like all our terrestrial planets, Mars has a metallic core, mantle, and crust. Mars’ core is suggested to have sulphur compounds as well. The mantle has tectonic features similar to Earth, volcano formation being a result of that. However, Mars’ mantle seems to be very cool and inactive, and has a very thick crust on top.

Polar Caps on Mars

A picture capturing the northern polar cap on Mars. Image credit: NASA/ESA.

One of the most fascinating features on Mars are the ice caps. These are permanent and made mostly of water ice, like Earth’s, but also has a small amount of dry ice. Mars’ tilt is enough to put a pole in complete darkness during winter, which is when the carbon dioxide in the atmosphere chills on the surface as dry ice. As winter turns to spring the dry ice sublimes back into the atmosphere as carbon dioxide. Some of the dry ice at the south cap is permanent, whereas almost all of the dry ice in the North occurs only in winter.

Looking at Mars Through a Telescope

Similar to Venus, Mars’ atmosphere is mostly made of carbon dioxide, with little nitrogen and oxygen. The atmospheric pressure is very low (6 mbar), so water boils at 37°C. The atmosphere is very dusty but thin so you’ll be able to see some surface details through a big telescope. With a small telescope, you’ll probably see a big red dot, with a shadow where the sun’s light doesn’t reach.

You’d need a very high powered telescope with high magnification to see very intricate details, but don’t be put off! Give it a try!


What Makes Terrestrials Different to The Giants?

At the beginning of the solar system, the planets were merely debris in a large disk surrounding the sun. By gravitational force, the debris clumped together into larger chunks. This repeated more and more as the planets grew bigger, because as the total mass increased, so did its gravitational pull.

This is how we expect a planet’s core is formed.

When the sun started its main sequence, it started to produce solar wind. Hydrogen and helium were blown further into the solar system by the wind.

Terrestrial planets had small cores and very little gas nearby them. Their lack of mass gave them a small gravitational force, meaning that they couldn’t pull in as much dust and gas as the giants. Since the light gases like Hydrogen and helium were blown away, terrestrial planets couldn’t pull much of it in. The gases that make up the majority of the atmosphere of a terrestrial planet are heavy, like oxygen, carbon dioxide and nitrogen, but there’s much less of those in the solar system.

As the planets finished forming, what we end up with are small terrestrial planets with very little atmosphere.

Other Terrestrial Planets

Beyond our solar system, we call planets exoplanets. Determining the composition and type of exoplanet becomes much harder, since they are very far away and our method of viewing them isn’t optimal.


Have a look at the Kepler data above, showing new potential exoplanets with their size and orbital period.

There’s an enormous amount of exoplanets that are Earth-sized, and looking back at what makes a terrestrial planet, it would be a fair assumption that most of the exoplanets near Earth’s size are terrestrial planets.

We’ve stated that terrestrials are smaller than gas giants (hence, they’re called giants). Any planet that’s much bigger than Earth (i.e nearing Neptune’s size) has a large enough gravitational pull to accumulate more material, basically becoming a gas giant. So, almost all of the planets between 0.5 and 1 Earth sizes should be terrestrial. Below 0.5 and the planet will probably class as a dwarf.

To summarise the above into one sentence: We have discovered an abundance of terrestrial exoplanets using Kepler, along with many gas and ice giants.

That will be it for terrestrials today. Next up, gas giants!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s