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  • Writer's pictureolivershearman

Phenomenal planet profile - inner system rocky planets

Updated: Jul 31

This is a bit of a unique blog post set as I have included some of my ebook contexts to share all the incredible knowledge that I have dredged up on the planets.

This is the first of two parts in these blog posts. This post focuses on Mercury, Venus, Earth and Mars, while the other blog post focuses on the outer gas giants. Hopefully this might lead to some fantastic planet based lessons for you and your students in the future!


A natural starting point for any conversation regarding the planets should begin with the planets within our Solar system and even closer to home, our planet Earth. Earth is a beautiful blue marble in the cosmos that is like a tiny ship afloat in the ethereal ocean of the cosmos. Earth is one of eight – currently – known planets – sorry Pluto – and has been our home for millennia.

With oceans, landmass, reasonable gravity and temperatures for water, a strong magnetic field for protection and different minerals throughout our world, it makes for a seeming paradise for the development of life. However, since there are many great resources that explain in depth the details of our world and the interesting aspects of how it works, we will be quickly skipping over our own wonderful planet to focus more on the further away and harder to reach Solar system objects.

Yet, it is worth noting that the Earth orbits the Sun at a distance of 1.5 x 10^8 km or around 1 Astronomical Unit (1 AU), that’s right we have used the Earth’s distance from the Sun – on average – as a unit of measurement. This is very useful for considering how far other planets are from the Sun and similarly how far other astronomical bodies are from each other too.

Planet profile: Earth:

• Approximate diameter: is 12,742 km.

• Approximate mass: 5.972 x 10^24 kg.

• Distance from the Sun: 149.6 x 10^6 km (or 1 AU).

• Orbital period: 365.25 days per orbital year

• Spin period: approximately 24 hours

• Approximate composition: 32% Iron, 30% Oxygen, 15% Silicon, 14% Magnesium, 3% Sulfur and many other pieces including Nickel, Calcium, Aluminium etc.

• Approximate gravity: 1g or 9.8 (m/s^2)

• Axis tilt: 23.5 degrees

• Number of moons: 1 moon (called ‘the Moon’).

The Moon:

The moon holds a special place in the hearts of humanity, since it is our closest neighbor and behind the Sun, it is the brightest object in the sky. It is also the only other astronomical body that humanity has physically visited so far. The Moon is unique with regards to its ratio of size relative to its parent planet; no other planet has such a large moon relative to its own size as Earth and the Moon.

There are three main theories for how the Moon came to orbit the Earth, the most common and popular is the giant collision theory. In this method: the young Earth and a large body collided, which meant that the Earth and Moon re-formed in their current shapes. The second theory is co-formation, where both the Earth and Moon formed together at approximately the same time, however this theory is unlikely given how much lighter – less dense – than Earth the Moon is. If they had formed at the same time, then they would be made of approximately the same materials and have similar densities, which they do not. The final option for the Moon’s current position is capture, but this has even more issues than the co-formation theory as most captured objects are not roughly spherical in shape and they do not typically orbit in the right plane as their parent planet. This means that giant collision, which happens to also be the most exciting option, is likely to be the true method of how the Moon came to be our close friend.

The Greeks knew the Moon as the goddess Selene and she had various lovers and was playful in stories while riding her ghostly white chariot across the night sky. The Romans however, knew the moon as Luna, though in other parts she was known as Diana and associated with the goddess of the hunt.

Due to the way that the Moon orbits Earth, we only ever see slightly more than half of its surface. If you look at animations online about this phenomenon, the Moon will appear to ‘rock’ from East to West as it becomes brighter, then rotates and becomes dimmer once again. Yet it should be noted, that there is no true ‘dark side of the Moon’, since it does get sunlight, we simply don’t see it. So there is a ‘backside’ of the Moon – which we never see – but there is not a ‘dark side’ of the Moon.


Mercury is the first of our planets in the Solar system when moving from closest to the Sun to the edge of the Solar system. It is a rocky and dense little ball of massive heat on its sunward facing side with temperatures up to 427 C and chilly temperatures on the dark side down to around -173 C. The reason there is such wild variation is that Mercury has no atmosphere to regulate temperature. Mercury is actually the smallest planet in the Solar system, but is actually the seconded densest planet (slightly behind Earth). Remarkably for every 2 orbits that Mercury completes around the Sun it will have spun around 3 full times, meaning Mercury will have 3 full days every 2 orbits! Mercury is also beaten in terms of temperature by its nearer neighbor Venus which has a much thicker atmosphere

Mercury has been known to humanity for a long time, with the discoverer being unknown and the name showing up in Greek mythology as being associated with Hermes, the messenger of the gods. Later in Roman mythology, Mercury was the name of the god of commerce, wealth, trade and thievery and was still sometimes known as the messenger to the Gods. Of course, being the messenger of the gods makes sense given the rapid speed that the planet orbits around the Sun and soars across our sky, seemingly ‘flying across the heavens’.

Mercury’s core makes up about 42% of its volume, which is very high for a planet though more recent estimates might put it even higher. It is suspected that this may have been caused by a collision similar to how the Moon has formed and the associated origin story of a collision with Earth in the past. Another possibility is that in Mercury’s early life, during formation it might have been too close to the Sun, which vaporised much of its lighter surface material.

Planet profile: Mercury:

• Approximate size: Diameter: 4880 km

• Approximate mass: 3.3 x 10^23 kg

• Distance from the Sun: 5.79 x 10^7 km or 0.387 AU

• Orbital period: 88 days

• Spin period: 4222 hours or 176 days

• Approximate composition: Likely 70% metals and 30% silicates

• Approximate gravity: 3.7 (m/s^2) or 0.38 g’s

• Axis tilt: 0.034 degrees relative to its orbit

• Number of moons: 0

• Average temperature: 167 C


Venus is the second planet from the Sun and is covered in a thick atmosphere that exerts a pressure about 92 times that of Earth’s atmosphere. Venus has a runaway greenhouse effect, with temperatures high enough to melt lead on the surface (around 462 C). Some people point to Venus as an example of how a really strong runaway greenhouse effect could impact a planet on an extreme scale. Venus’s atmosphere has more acid per cubic centimeter than a common car battery and while it is similar in size and planetary composition to Earth, Venus is much more volcanically active than Earth.

Venus was previously associated with Aphrodite the Greek goddess of love, who corresponds with Venus the roman goddess of love and famously depicted in the painting, the birth of Venus painted probably around 1480.

A day on Venus lasts longer than a year (ie: to rotate takes longer than orbiting around the Sun). It is also a planet that has a retrograde spin, meaning it spins clockwise or opposite to most of the other planets. Venus also has the most circular orbit of all the planets in the solar system with very little eccentricity.

Planet profile: Venus:

• Approximate size: diameter: 12,104 km

• Approximate mass: 4.87 x 10^24 kg

• Distance from the Sun: 1.08 x 10^8 km or 0.72 AU

• Orbital period: 224.7 days

• Spin period: 5832.5 hours or 243 days

• Approximate gravity: 8.9 m/s^2 or .9 g’s

• Axis tilt: 177.4 degrees relative to its orbit

• Number of moons: 0

• Average temperature: 464C


Mars is the fourth planet from the Sun and is about half the size of Earth. The planet does not have a strong magnetic field, which means it likely has a relatively solid core. The surface is covered by a lot of Iron oxide, what we know as rust and that is what gives Mars its characteristic red coloring.

Mars has some of the most impressive geological features in the Solar system with the highest mountain – Olympus mons – that is about 25 km high and the largest (deepest) canyon in the Valles Marineris (10 times the length of the grand canyon and almost 5 times as deep). The Valles Marineris is about 4000 km long, 200 km wide and about 7 km deep, comparing it to the Grand Canyon, which is only 446 km long, 29 km wide (at the widest point) and about 1.8 km deep.

So why is Olympus Mons so high? A lot of this has to do with the lower gravity and also the fact that the plate tectonics don’t move away the volcanic hotspot allowing for higher and higher eruption to build up this currently dormant shield volcano and the highest mountain the Solar system. Mars does have an atmosphere of mainly carbon dioxide, but it is relatively thin and not much of an atmosphere with only about 0.6% the pressure of Earth’s atmosphere. Though this doesn’t mean that the atmosphere isn’t active, with some of the most extraordinary dust storms in the solar system.

The ancient Greeks called Mars ‘Ares’ as in the god of war. Then when the Romans came forth, they renamed Ares to Mars, who would ride his chariot across the sky in the color of blood.

Planet profile: Mars:

• Approximate size: Diameter: 6792 km

• Approximate mass: 6.43 x 10^23 kg

• Distance from the Sun: 2.28 x 10^8 km or 9.55 AU

• Orbital period: 687 days or 1.88 years

• Spin period: 24.7 hours

• Approximate gravity: 5.0 m/s^2 or 0.51 g’s

• Axis tilt: 25.2 degrees relative to orbit

• Number of moons: 2 moons (Deimos and Phobos)

• Average temperature: -65 C

The moons of Mars:

The moons of Mars are very small; their surface area is roughly that of some of the small states of Earth such as Luxembourg - slightly larger than both or Malta - which is slightly smaller than both. Phobos is on a very slow collision course for Mars and Deimos is on a very slow escape course from Mars; both these things will occur over many millions of years, but eventually it seems likely that Mars will be without moons in the future. Phobos and Deimos are both named for sons of the god Ares who follow their father into battle, Phobos was the god of fear and Deimos was the god of terror.

The below is an image of Phobos, being about 20km radius or so, it is a relatively tiny moon when compared against our own or some of the gas giants much larger moons.

Deimos looks relatively similar, though a little bit smaller and a bit less round as it is only about 14 km radius or so.

If you would like to learn more about the universe, then please do consider purchasing my good value ebook part 1, 2 and 3 - link here.

Thanks for reading.

Cheers and stay curious

Oliver - The Teaching Astrophysicist

(Note: This blog post was NOT generated by AI and is conceived, typed and uploaded by a real person.)

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