Terrestrial Planets

An assortment of strange and wonderful celestial bodies lie within the boundaries of our Solar System. Planets are of particular interest to amateur astronomers and professional scientists alike. Perhaps it has to do with how varied each is, or how they are the astronomical bodies we can relate to most. After all, our home is a planet. Our fascination also stems from our wanderlust; hoping to travel to planets both near and far, especially if anything should ever happen to the Earth. Lastly, there is the hope that by studying planets – and astronomy at large – we may discover intelligent life and prove we are not alone in this vast cosmos.

There are eight planets orbiting the Sun, also known as major planets to distinguish them for dwarf planets and other planetoids. These planets can be divided into three classes of planets: terrestrial/ rocky/ telluric, gas giants, and ice giants. The terrestrial planets are Mercury, Venus, Earth and Mars; the gas giants are Jupiter and Saturn; and the ice giants are Uranus and Neptune.

With Earth being a rocky planet, it is both interesting and important to study the other three terrestrial planets. Learning about the rocky planets reveals much about the creation of the Solar System, and the evolution of the Earth.

This guide to the terrestrial planets includes extensive facts about each of the rocky planets, including our own.

Basic Characteristics of Terrestrial Planets

The terrestrial planets in the Solar System are also the four innermost and smallest planets. They are MercuryVenusEarth and Mars. They share fundamental characteristics that define them as terrestrial planets. The internal structure of the terrestrial planets is the same. Each has a metallic core that is rich in iron and surrounded by a mantle of silicate. They all have solid surfaces which share many of the same features including canyons, craters, mountains, valleys and volcanoes.

Terrestrial planets also have a secondary atmosphere which sets them apart from the gas and ice giants. The atmosphere of giant planets formed as part of the planet, from the materials the Solar System was created from. The secondary atmosphere of rocky planets is a result of volcanic activity and comets impacting the planets billions of years ago.

Some of the Solar System’s natural satellites also share these features with rocky planets. The Moon is an obvious example; it has a similar internal structure to Earth but with a much smaller core. Two of the Galilean moons, Europa and Io, have internal structures resembling that of rocky planets. The Solar System’s dwarf planets have solid surfaces too, but their internal composition is much different from terrestrial planets.


Mercury has quite a few distinctions. Named after the messenger god of Roman mythology, it is the closest planet to the Sun, and the smallest planet in the Solar System. Although it is smaller than some of the larger natural satellites of other planets, it does have more mass than they do. Its proximity to the Sun means it has the shortest orbital period of all planets – both major and dwarf – at 87.97 earth days in one Mercurian year.

From Earth, we can only ever see Mercury early in the morning and early in the evening, as it appears to follow the Sun. It can reach apparent magnitudes that makes it brighter than Sirius (the brightest star). However, it can sometimes be hard to observe the planet because of the Sun’s glare. Mercury can be seen very clearly during a total solar eclipse. With the right protective filters, we can also watch Mercury transit the Sun (pass in front of the Sun’s disc). The Hubble Space Telescope cannot observe Mercury at all, for pointing that close to the Sun could harm the instrument.

Like Venus, it is an inferior planet or interior planet (relative to Earth); meaning that their orbits lie within the orbit of another planet.  The planet undergoes phases, like the Moon, which can be viewed telescopically.

The planet’s core is in three parts: a solid outer core of iron sulphide, a deeper liquid core, and a solid inner core. Its size compared to its density suggests a core very rich in iron – a higher iron content than any other planet. Studies show that the core alone may make up 35% of the planet’s volume.

The most accepted theory to why this is suggests an impact with another large rocky body stripped the planet of most of its crust and mantle. It is also possible that Mercury may have formed before the Sun’s energy output has stabilized. Being so close to the Sun, the extreme temperature of this environment would vaporise rock.

Surface geology of the planet closely resembles the Moon, especially with its plains and heavy cratering. Evidence shows that Mercury was bombarded by comets and asteroids shortly after its formation over four and a half billion years ago. Unlike the Moon, the planet’s stronger surface gravity lessens the areas affected by impacts. However, craters do range in size from small shaped bowls to basins hundreds of kilometres across. Though the planet once had volcanic activity, it has been geologically inactive for billions of years.

Surface features get their names from different sources. Craters are named for deceased artists, musicians and authors who have all had significant impacts in their respective fields. Ridges are named for scientists who contributed to our knowledge of Mercury, and depressions after works of architecture. Montes are named after the word ‘hot’ in several languages, whereas plains get their names for the word ‘Mercury’ in several languages. Escarpments are named for ships of scientific expeditions, and valleys get their names after radio telescope facilities.

Surface temperatures on the planet range from -173 to 427 degrees Celsius (-280 to 800 degrees Fahrenheit). Despite these soaring temperatures, craters at the planet’s north pole seem to contain water ice. This is likely because the deep crater floors never receive direct sunlight. Mercury is too small and too hot to have any significant atmosphere, but it does have an exosphere rich in various gases and elements. The planet also has a magnetosphere which is strong enough to protect it from solar winds.

Mercury’s orbit is the most eccentric of all the planets (its orbit is the most deviated from a perfect circle). It also has a very strange and rare rotation. It rotates about its own axis thrice for every two revolutions around the Sun!

Mercury has no natural satellites. The planet has been visited by the spacecraft MESSENGER and by Marina 10 in a flyby.


Venus is named after the Roman goddess of beauty and love. Also known as the Evening Star and the Morning Star, it has long been of fascination and inspiration for all of humanity’s cultures through the ages. It is the second brightest object in the night sky – outshined only by the Moon. Its apparent magnitude can reach a luminous -4.9. This is bright enough to see clearly in midday Sun! Considering its brilliance, it really is no wonder that it was the first planet to have its motion across the sky plotted. It also comes as no surprise that misinformed observers often report Venus as an UFO because of how bright it is!

Like Mercury, Venus is an inferior or interior planet; and like Mercury (and the Moon) Venus undergoes phases that we can observe telescopically from Earth. Venus also transits the Sun’s disc, an event that you can observe yourself with the correct protective filters. It is indeed beautiful to behold – as its name suggests. But don’t be fooled: underneath all that beauty is a planet ravaged by one of the most hostile environments in the Solar System.

Venus is often referred to as Earth’s ‘sister planet’ because the planets are most similar in size and mass, though that is definitely where the comparison ends (if anything, Mars and Earth are more like twins). For one, the atmospheric pressure is 92 times greater than on Earth. For another, Venus is scorching. It is the hottest planet in the Solar System, despite Mercury being closest to the Sun. Its dense atmosphere – the densest atmosphere of all four terrestrial planets – is made up of 96% carbon dioxide, which is a greenhouse gas. In this regard, understanding Venus is a very important study for what can happen to Earth if we continue to disrupt our climate with carbon emissions from burning fossil fuels. Temperatures at the planet’s surface reach 462 degrees Celsius (863 degrees Fahrenheit). It is believed that there was once liquid water of Venus until the runaway greenhouse effect caused it all to be evaporated.

Venus can experience strong winds of up to 300 kilometres per hour (185 miles per hour). About 80% of the Venusian surface is covered by smooth volcanic plains. There is strong evidence of volcanism, and what’s more is that such activity may be quite recent and/ or ongoing. One indication of this is the thick clouds of sulphuric acid in the planet’s atmosphere. Venus is also divided into two ‘continents’ which make up the rest of the planet’s surface; one on each hemisphere (north and south).

Oddly enough, The Venusian surface is littered with very few impact craters, suggesting that the surface is still relatively young – a few hundred million years at most. Another enigma is the lack of lava flows despite active or recent volcanism.

Scientists do not know much about Venus’ internal structure, but look to the similarities in size and density with Earth to draw logical conclusions about its build. Venus most likely has a core, mantle and crust; but lacks real evidence of plate tectonics, suggesting that the crust is particularly strong. The planet has a very weak magnetic field; lacking a magnetosphere and meaning that the planet is exposed to solar winds and cosmic radiation. One theory as to why the magnetic field is so weak suggests that Venus may lack a solid core.

The planet has a small axial tilt of approximately 3 degrees, meaning that the temperature across the planet is more or less constant and even. The only real temperature differences occur with changing altitude.

All planets orbit about the Sun in an anticlockwise direction when viewed from Earth’s North Pole. Most planets also rotate in a counter clockwise direction, but Venus rotates clockwise in retrograde rotation. It has the slowest rotation of the planets at 243 days. Venus orbits around the Sun every 224.7 earth days, so a day on Venus is longer than a Venusian year! Venus also has the most circular orbit of the planets.

Like Mercury, Venus has no natural satellites. Planetary scientists believe Venus may have had at least one moon, but that it was destroyed by a large impact event billions of years ago. It does however have several Trojan asteroids (asteroids lying in its orbital path, but that remain in fixed positions rather than orbiting the planet).

There have been several spacecraft sent to explore Venus in flybys, orbits, and landings including the Soviet Venera programs, the U.S Mariner programs, and the NASA Pioneer projects. The Venus Express of the European Space Agency entered the planet’s orbit from 2006 to 2014, and Japan’s Akatsuki has been in orbit around Venus since December 2015.


Mars is the fourth planet from the Sun, and the second smallest planet in the Solar System after Mercury. It was named for the Roman god of war, and with its blood red appearance it is easy to see why. For this same reason, it is commonly called the ‘Red Planet’. The planet’s red colouring clearly distinguishes it from others in the night sky, even to the naked eye. Iron oxide (rust) on Mars’ surface is what gives it this rouge hue. Mars is still quite a colourful planet. It shows gold, brown, tan, and even green surface features depending on the minerals present.

It is the fourth brightest object in the night sky after the Moon, Venus, and Jupiter. Its apparent magnitude can be as bright as -2.91. Ground based telescopic views can resolve Martian features around 300 kilometres (190 miles) across.

Though Venus is sometimes called Earth’s sister planet because of its similar size and mass, Earth has more in common with Mars than any other planet we know of so far. Its surface features resemble both the Moon with various impact craters, and the Earth’s with deserts, valleys and polar ice caps among other features. Mars is home to the largest volcano in the Solar System, Olympus Mons, which may be the tallest or second tallest mountain in the Solar System based on different methods of measuring. One of the largest canyons in the Solar System, Valles Marineris, is also found on the Red Planet.

Mars is only about half the diameter of Earth, far less dense (due to the planet being much less massive), and has around 38% less surface gravity than Earth. Around 40% of the Martian surface is covered by a smooth basin (the Borealis basin) on the planet’s northern hemisphere, which may be the result of a great impact with a large body.

The planet has a dense metallic core made primarily of iron and nickel, and containing sulphur too. A mantle of silicate surrounds the core. It formed many of the planet’s tectonic and volcanic features, but now appears to be dormant. The Martian crust is rich in elements: aluminium, calcium, iron, magnesium, oxygen, potassium and silicon. Observations shows that the planet’s crust may have once been magnetized, though presently there is no evidence of a magnetic field covering the planet. Despite lacking a magnetosphere, mars does experience aurora, due to multiple little magnetic pockets which may be the remains of a decayed magnetic field from billions of years ago.

While a Martian year is significantly longer than an Earth year (687 earth days), it is closest to Earth in rotation and tilt. A Martian day is 24 hours, 39 minutes, and 35.24 seconds. The planet is tilted on its axis by 25.19 degrees, and experiences seasons very much like Earth’s. The planet has an interesting weather system, including having the largest dust storms in the Solar System. These storms can be small or large enough to cover the entire planet.

Mars has the most natural satellites of the rocky planets: two moons named Phobos and Deimos. They are quite small and irregularly shaped, and are thought to be captured asteroids.

Features on Mars get their names from a wide selection of sources including deities from classical mythology, deceased scientists and writers, tiny towns and villages on Earth, and the words for ‘Mars’ and ‘star’ in different languages.

Mars lies within the habitable zone – a region around a star (not too close or too far) in which a planet could harbour life. There is plenty of evidence suggesting that Mars may have once hosted life, or that life may still exist on the planet. Data collected from the planet by Phoenix shows that Martian soil is slightly alkaline and contains nutrients also present in Earth soils, and which are essential for plant growth. Landforms visible on the planet’s surface strongly indicate the presence of liquid water at one stage of the planet’s history, though permanent liquid water is now impossible due to Mars’ low atmospheric pressure (only 1% of Earth’s).

However, we do know that the Martian polar ice caps are made up of 70% water ice. Studies suggest that if all the frozen water of the southern ice cap were melted, the entire planet would be covered in water 11 meters (36 feet) deep. Opportunity also discovered jarosite on the planet’s surface; a mineral which can only form in the presence of acidic water.

Dozens of spacecraft from a number of countries have flown by, orbited, and landed on Mars. It currently hosts eight spacecraft – six in orbit and two on the planet. In orbit are: 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN, Mars Orbiter Mission and ExoMars Trace Gas Orbiter. The Mars Exploration Rover Opportunity and the Mars Science Laboratory Curiosity roam the Martian surface.

Professionals of all fields have long thought to send humans to Mars – a challenging and expensive mission – and still hope to in the near future.


Planet Earth – our home. Without any doubt, our planet is the most fascinating, beautiful, and mysterious of them all. Of every astronomical object ever discovered, studied and explored, Earth is the only one we know of that has life.

The Earth formed roughly 4.5 billion years ago with the rest of the Solar System. The word earth has roots in all Germanic languages, and the planet has been personified as a deity in a number of cultures. The rocky planet is the third in the Solar System, and is 150 million kilometres (93 million miles) from the Sun. A day on our planet is approximately 23 hours, 56 minutes and 4.0989 seconds, and a year on Earth is 365.2564 days.

We have one natural satellite: the Moon. The Moon is a large, terrestrial natural satellite. It is suggested that the Moon was formed from materials ejected by our planet after a violent collision with a Mars –sized body (Theia). The relationship is a very special one. The gravitational attraction of the bodies is what causes the ocean’s tides. The Moon is tidally locked, meaning that its rotational period around its axis is the same as its orbital period around Earth. This causes the same side of the Moon to always face Earth. The Sun illuminates different parts of the Moon as it orbits Earth, causing the Moon’s phases. It’s good to note that despite how these phases appear to us, half the Moon is always lit by the Sun. Another unique feature of our Moon is that the apparent size of its disc matches the apparent size of the Sun’s disc. This means that the Moon can completely obscure the Sun when they align, causing a total solar eclipse.

Earth is made up of aluminium (1.4%), calcium (1.5%), iron (32.1%), magnesium (13.9%), nickel (1.8%), oxygen (30.1%), silicon (15.1%), sulphur (2.9%), and the remainder is made up of trace elements. The Earth’s core is comprised of 88% iron. Earth has a solid inner core and a liquid outer core, layered by a mantle which supports an outer layer of silicate crust.

The Earth’s magnetic field is generated at the planet’s core, and extends outwards up to the Earth’s surface, and defining the magnetosphere which protects our planet from solar wind and cosmic radiation. Solar winds interacting with the planet’s magnetosphere is what causes the beautiful dancing aurora (the northern and southern lights).

The crust and upper mantle form tectonic plates which move relative to each other. Tectonic plates can come together, pull apart, or slide passed one another. Earthquakes, mountain formations, oceanic trenches and volcanic activity can all occur as a result of these movements.

The Earth’s oceans were formed by a combination of volcanic activity and outgassing (when trapped, dissolved or frozen gas is released). The oceans cover 70% of the Earth’s surface, setting Earth apart as a ‘Blue Planet’ conducive to life. The remaining part of the surface is varied and busy. There are canyons, continental shelves, mountains, plateaus, ridge systems, trenches, volcanoes and an assortment of other geological features.

Also crucial to the Earth being habitable are our atmosphere and the 23.44 degree tilt of the planet’s axis. This tilt produces seasons on Earth. The Earth is tilted toward the Sun during summer months. The days are longer, the Sun rises higher in the sky, and the angle at which sunlight hits the Earth is more direct. The opposite is true in winter months. The effect of the tilt on seasons is most visible and most extreme the Polar Regions, where the Sun does not set for nearly half a year, and does not rise for the other half. It is because of the tilt that we also experience the summer and winter solstice (longest and shortest days of the year), and spring and autumnal equinoxes (all of earth experiences equally long days).

The composition of our atmosphere is mostly nitrogen, followed by oxygen, argon, carbon dioxide and water vapour. Oxygen is essential for us to breathe, and the amount of carbon is equally important. As a greenhouse gas, carbon dioxide helps the Earth retain the Sun’s heat. Without it, the planet would be too frigid for life. However, too much of it causes disruptions in our climate that are devastating to life. Venus is the perfect example of a runaway greenhouse effect. By burning fossil fuels, we are warming our planet to abnormal degrees. It is our responsibility as custodians of this planet to clean up our ways.

The variety of life on Earth is truly outstanding. We have found weird and wonderful creatures in every nook and cranny of the planet. As much as the Earth is a life bearing planet, it is also a dangerous killer. Tropical cyclones, hurricanes, typhoons, landslides, tsunamis, sinkholes, blizzards, droughts, wildfires and a range of other extreme occurrences claim the lives of many. There have been near extinction events from massive volcanic eruptions; and manmade disasters such as deforestation, hunting and exploiting life and resources to extinction, and global warming.

The fact that there are other terrestrial planets in the Solar System gives us hope that somewhere in our galaxy there may be other earths teeming with life. For now, this is our only home – a beautiful and perfect one – and we should do everything in our power to keep it that way.