Our Cosmic Backyard: The Inner Planets

Our Solar System was formed 4.5 billion years ago when gravity pulled interstellar gas and dust together. The intense gravitational force on these particles caused an explosion of nuclear fusion, resulting in the formation of a star, our Sun.

Our Sun was the first resident of our Solar System. The newly formed star began to blow stellar winds to help clean up the leftover debris of gas, dust and rock. This stellar wind also helped the star from collapsing under its own gravity. The Sun was now fully functional and was ready for company – Planets.

As the Sun heated up its surroundings, helium and hydrogen molecules were pushed away to the outer parts of the would-be planetary system, while the heavier elements and particles remained closer to the Sun. As a result, the planets that formed closer to the Sun were rockier (Mercury, Venus, Earth, Mars) compared to their very gassy siblings further away (Jupiter, Saturn, Uranus, Neptune).

The leftover debris that surrounded the Sun were now, once again, under the control of gravity, bumping and smashing into each other; which eventually combined to form the planets of the Solar System.

The Planets

It took approximately 10 – 100 million years before planets started to form around the Sun. With the right amount of force, during the collision of debris, planets formed. If the particles collided too hard they would shatter each other. If they collided too softly there would not be enough energy to fuse. So, the collisions of these particles needed to be just right. Now, let’s explore the inner planets that formed.


Mercury was first observed through a telescope in the seventeenth century by famous astronomers Galileo Galilei and Thomas Harriot.
It is the smallest (Radius: 2,440km) and closest planet to the Sun (Distance: 58 million km) with temperatures ranging from -180°C to 430°C. Due to its fast revolution around the Sun, one year on Mercury is equivalent to only 88 Earth days! But here is something even more fascinating about this small planet. One day on Mercury is equivalent to 59 days on Earth!

Here is why.

Although the planet revolves around the Sun very quickly, its rotation is very slow, making the days on Mercury very long. This says that a Mercurian year is only about a day and a half – which is amazing!

Aside from its short years, what other mysteries does Mercury hold? What would a day on Mercury be like? The sunrise and sunset, for sure, would leave us baffled. The orbit of the planet is slightly elongated; this coupled with its slow rotation causes the planet to appear to have two sunrises and two sunsets in a single day. At sunrise, the Sun would rise briefly and then appear to sink beyond the horizon only to rise again. The same goes for the sunset where the Sun would appear to set twice before its final goodbye for the day. Just imagine, you get to see two blazing but yet beautiful sunsets everyday.

But alas, the planet is way too hot for us, so let’s pass on that. Instead, we’ll observe it right here from Earth.

Mercury is one of the most difficult planets to observe in the night sky because of its closeness to the Sun. For this reason, Mercury is best observed during twilight hours – just before the Sun rises or just before it sets. At these times, the planet is always close to the horizon. When it is on the opposite side of the Sun it is at its brightest as the sunlight is reflected off the planet’s surface right back at us.


The second planet from the Sun (Distance: 108 million km) and the hottest. Yes, the hottest! It is logical to think that Mercury should be the hottest planet given that it is the closest to the Sun, but Venus has decided to take up the title. Temperatures on the surface of the planet can reach a mean of 462°C. This is owing to the planet’s dense atmosphere. The dense clouds formed by multiple eruptions of volcanoes on the surface of the planet prevents the heat from the Sun from returning into space. The temperature remains the same whether it is day or night. This phenomenon is deemed the Greenhouse Effect. The planet is similar in size to our own planet, Earth, with a radius of 6,052km.

It takes Venus 225 Earth days to orbit the sun. The planet’s rotation is even slower than that of Mercury, taking it 243 Earth days to fully rotate on its axis (A sidereal day). However, a solar day takes only 117 Earth days to be completed; a solar day being the time it takes for the sun to return to the same point in the sky. Interestingly, a day on Venus is longer than a year on the planet, if we’re counting sidereal days.

Venus, unlike Mercury, is much easier to find in the night sky. It could usually be found three hours before dawn or after Sunset – giving the planet its name ‘Morning and Evening Star’.

Earth: The Paradise Planet

Our very own planet is the third from the Sun at a distance of 147.18 million kilometers. It is the fifth largest planet in the Solar System with a diameter of 12, 742 kilometers (Radius: 6,375km).

Earth is the only planet in the Solar System that is capable of sustaining life, or as far as we know. At least 70% of the planet’s surface is covered in water. Its atmosphere consists of free, unbounded oxygen (21%) that is essential for life as we know it. The other atmospheric components include nitrogen (78%), argon (0.9%) and other gases such as carbon dioxide, nitrous oxide and methane, that make up about only a tenth of the atmosphere. The Earth’s magnetic field, created by the planet’s rotation on its axis, also plays an important part in our protection. The magnetic field surrounding the Earth keeps out most of the Sun’s radiation, preventing our oceans from drying up and our crops and animals from dying.

However, this paradise planet was not always this sustaining. About four and a half billion years ago the Earth was nothing more than a floating mass of molten rock. Aside from the constant heated attacks from the Sun, it was always constantly pelted with incredibly large meteorites and asteroids. There was no atmosphere and no magnetic field. All of this packed together practically caused the planet itself to start melting. Nevertheless, this was all a necessary process for the achievement of the wonderful planet we know today!

Although we would describe Earth as a globe, the planet is not actually completely round; rather, it is more of an oblate spheroid. As the Earth spins, the force causes the planet to be squashed at its poles and bulged at its sides (equator), giving it more of an oval shape than a completely round one.


Mars, the Red planet, is the fourth planet from the Sun (Distance: 250 million km). It is one of the most studied planets in the Solar System. Astronomers believe that billions of years ago – Mars was much like Earth in that there was a dense atmosphere and the planet was much warmer than it is today. There was water – lots of water! Scientists believe that the now dry gullies littering the surface of the planet is, in fact, evidence that maybe, sometime long ago, water once flowed there. Could you imagine that possibility? Mars with rivers and lakes, and even oceans! With the launch of the Mars rover, Curiosity, in 2011 scientists have confirmed the presence on water on Mars by studying the soil on the surface of the planet. Nevertheless, Mars is now nothing more than an icy rock filled with lots of red sand. Any water that might have been there has long since disappeared.

More evidence that Mars was once like Earth could be seen in the planet’s atmospheric composition. Although the atmosphere around Mars is very thin, there are still some elements to be found.

Carbon dioxide makes up most of the planets atmosphere (95%) along with other molecular elements such as Nitrogen (2.7%), Oxygen (0.13%), carbon monoxide (0.07%) etc.

The composition of Mars’ atmosphere in addition to the fact that there is no water on the surface eliminates any chance of life, as we know it, ever being able to survive on the planet. But even if these were not an issue to us, what would life have been like on Mars? Would it be too strange? For one, there would be much less space to go around seeing as Mars is about half a size smaller than Earth. We would also have two moons instead of one, Phobos and Deimos, and they would be much smaller than what we are used to. What about during the day?

Sunset on Mars. Image credit: NASA

From Mars the Sun would seem to have shrunk because we have now moved further away from it. The weather would definitely be much cooler, even on the sunniest of days with average temperatures of -63°C. That’s freezing!

Mars takes 687 days to complete its journey around the Sun, that’s almost twice as long as Earth. On the bright side, or maybe not, Mars rotates at just about the same speed as Earth, so a day takes a little over 24 hours to end.

As astronomers continue to ‘probe’ into the mysteries of Mars, we are still left to wonder much about the Red Planet. Still, come February 18, they might actually find something new to report as NASA is slated to land its newest Mars rover, Perseverance, on the surface of the planet. Excited? Keep an eye out for that!

And speaking of keeping an eye out…

From Earth, Mars will appear in the sky as a bright orange-red star. Depending on the time of the year, Mars will appear low in the western sky after the sun has set. The closer Mars is to Earth, the brighter it appears in the sky. The planet made its closest approach to Earth back in 2003 when it was 35 million miles away. That’s pretty close, when you’re used to it being hundreds of millions of miles away.

Learning about our cosmic neighbours can be fascinating! In my next blog, I venture further in the Solar System to the outer planets – The Gas Giants. I hope you join me for that adventure.

Clear Skies!

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