_{An illustration of the Solar system. (Not up to scale) Image by Phillippe Put, Flickr, CCO.}

Stardusts and Galaxies, Asteroids, spirals, Moons and Stars and all. Isn't it pretty amazing how the environment outside of earth sounds? With the recent surges in the field of astronomy over the past 3 to 4 centuries, Man has gotten to explore the vacuum to a reasonable extent, enough to realize that there's a lot more to take in that we have always perceived. With the constantly evolving technologies and methods available to science today (such as the space probes and powerful telescopes), we have amassed quite a large body of data, We have been able to perform spacewalks, documented 9 planets in the solar system and seen the shimmering rings of Saturn. We have executed manned and unmanned space missions and dug and explored the Martian soil. We have gone on to discover planets like our earth, found craters and canyons, icefields and volcanoes. Never before has so much information about the cosmos (and every other thing in general) been revealed in such a short time.

Today, in the spirit of science, how about we a brisk stroll around our solar system just to get an overview and general characteristics of the kinds of stuff you'd find out there? No promises on this end, but it might just be worth your time.

The Solar system! -- Pierre Laplace's hypothesis

Research data suggests that our solar system was created from a gigantic cloud of interstellar dust and gases that contracted under its own gravity some 4600 million years ago. As all of that matter contracted, they spun and collided into each other, converting gravitational energy into thermal energy to make the the process increasingly hotter, until a larger chunk (about 95%)^source fell toward the center of the dust cloud to form a nebula, which, ultimately, gave birth to our star; The Sun (called the Protosun at its earlier stage).

The rest of the outlying matter that wasn't part of the sun was blown away and clustered at different places to form protoplanets. Over time, the protosun, - still being pulled by its gravity inwards- continued to shrink and increase in density until about 5 billion years ago, when it was warm enough to instigate a nuclear reaction by fusion, and give off an enormous amount of radiant energy!

The radiant energy was powerful enough to push away protoplanets even farther out into space, spinning them as they go. These protoplanets being acted upon by their own gravity, attracted even more space dust till they were big enough to become Planets on their own, and then smaller chunks of matter circled around them as natural satellites or moons.

This theory, (also known as the Nebula hypothesis) propounded by Pierre Simon Laplace has lasted long enough to take its place as the most accepted definition amongst others. By no means are these claims sufficiently elucidated at the moment. They're just our best possible answers yet as we continue to glean the solar system in search of every data we could get our hands on. However, this knowledge has helped humans form an understanding and perspective of our place in the universe!

The Planets of the Solar system.

Our solar system is made up of eight planets yet discovered, with each planet having distinctive and unique features from the others, as well as striking similarities. In order of their closeness to the sun, I present to you:

• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune
• Pluto, the oddball, the outcast, the declassified.

_{The relative distances of the earth on a reduced scale. Diameters are not up to scale. Image: No machine-readable author provided. Aeddub~commonswiki assumed (based on copyright claims). CC BY-SA 2.5, 3.0, via Wikimedia Commons}

Through studies based on a couple of factors like their size, location, density and other physical attributes, The planets are classified into 2 major groups, namely:

• The Inner Planets (Terrestrial Planets)
• The Outer Planets (Jovian Planets.) excluding Pluto.

The first four planets (the inner planets) - Mercury, Venus, Earth, Mars - are relatively small, denser and crowded-in closer to the sun in comparison to the other planets.
Apart from location, they share very similar properties that made astronomers group them that way. For one, due to their proximity from the Sun, the radiant heat boiled off the lighter elements that surround them, leaving each terrestrial planet with a liquid core made up of Iron and Nickel deep beneath kilometers of landmass!

Unlike the terrestrial inner planets, the Jovian planets -Jupiter, Saturn, Uranus, and Neptune- are relatively bigger and have lower densities in comparison to the inner planets. The drop in density is because they are mostly made up of gases. The term 'Jovian' comes from the fact that they share a lot of similarities to Jupiter. Their distances apart are widely spaced and as well very far away from the sun.

Astronomy measures distance in space using a unit of measurement termed as 'Astronomical units', with the earth taken as reference. 1AU (which is approximately equal to 150x10⁶Km) basically is the distance between the Earth the Sun. This would imply that the other inner planets before earth (Mercury and Venus) would have their distances from the sun less than 1AU, and that planets beyond Earth would have distance values greater than 1AU. You'll find the table below showing their mean distances from the sun in Astronomical units and Kilometres

Planet	Distance (Astronomical Units)	Distance (Kilometres)
Mercury	0.39	57.9 x 106
Venus	0.72	108.2 x 106
Earth	1.0	149.6 x 106
Mars	1.52	227.9 x 106
Jupiter	5.19	778.3 x 106
Saturn	9.5	1,427 x 106
Uranus	19.1	2,871 x 106
Neptune	30	4,497 x 106
Pluto	39.53	5,913 x 106

_{Noticed how the distance from the between the sun and the first outer planet (Jupiter) is over three times the distance from the sun to the last of the inner planets (Mars)? And also how the distance between a previous Jovian planet and the next one almost always doubles as we descend down the table? This shows how far away they are from each other and the sun, which rightly earns them the name Outer planets.}

Surface-Temperature difference.

By intuition, we understand that the distances of the planets from the Sun definitely would affect the time at which the radiant energy would take to travel to the surfaces of these planets. this would indicate that temperatures at each planet would differ significantly as well. That said, Mercury would obviously be the hottest of the planets since it is barely 0.39AU from the sun (this isn't entirely true though, as we shall see). With a peak temperature as high as about 329^oC on afternoons in Mercury, most of Earth's day to day materials like plastic and metals with lower melting points would melt under the heat. On earth, we could experience as high as 32^oC along the equator on afternoons, and about 27^oC on Mars' equator.

The Jovian planets which are well-spaced from each other, and beyond the inner planets, receive lesser solar radiation in comparison to the inner planets. Hence, they are relatively cooler. The typical temperature on Jupiter for example, gets as low as about -122^oC and almost doubles on Neptune at -210^oC. Hence, these planets are significantly cold. All of which proves that temperature plays a major role in determining which planet exists as an oven or a deep freezer!

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Fact: I know I mentioned that Mercury is closer to the Sun and by intuition, it should be the hottest planet among the bunch. But really, would that be correct? Is Mercury the hottest planet in our solar system?.

Well, that isn't true. Despite the fact that mercury is closer to the sun in that the other planets, it isn't the hottest. Venus which stands at a distance of 0.72AU from the sun is comparatively hotter than Mercury, with a peak temperature of about 744K during the daytime. This is due to the same phenomenon we have on earth that's responsible for heating and the prevailing global warming:

Yeah, you guessed right, it's the Greenhouse effect.

You see, amongst the terrestrial planets, Mercury has no atmosphere at all. Venus, on the other hand, has an incredibly thick atmosphere which contains about 96% CO₂ gas by volume. CO₂ is quite notorious in Earth's atmosphere for its capacity to retain heat and increase global warming. With that, even though Venus has lands and mountains that could probably support life, such temperature makes it impossible for 99.9999% of the life we know on earth yet, and It's hard to tell how much longer life's toughest organisms (I.e; The Tardigrades) can survive such a hellish atmosphere.

Physical properties.

Comparing the physical properties this time, we still see similarities within each group member planets, and differences between them as well. Terrestrial planets -as the name implies- are terrestrial. This means they have physical properties made up of lands, rocks, sands and an atmosphere and they have mass as well as unique densities. Mercury is the only planet terrestrial planet without an atmosphere while Mars has the solar system's tallest mountains and it's deepest canyons.

So it is, that Venus has CO₂ as its main atmospheric gas while Earth has its atmosphere filled with 70% nitrogen, and is the only planet with atmospheric oxygen mostly due to living organisms such as plants that help keep the cycle going.

The Jovian planets have substantially greater masses than the terrestrial planets, with Jupiter taking first place with its mass value over 318 greater than our earth's. However, the Jovian planets look bigger because they occupy more volume because they are made up of gases.

Using the formula M/V to divide mass by volume, it's easy to see that the Jovian planets aren't compact at all. This would imply that they are less dense than the terrestrial planets. Saturn, for example, has its average density less than that of water (1000Kg/m³) and all of that information suggests that the two possible gases that fit the puzzle are Hydrogen and Helium.

I should mention at this point that Pluto is the oddball out because it doesn't scale at all. it is no more classified as a planet but rather as a new class of planets called Plutoids or Dwrf planets for a couple of reasons.

First off, Pluto meets just two of the four basic requirements of what makes a planet a planet, and since it's an official rule by the International Astronomical Union (IAU), I'll be glad to lift it off a site and list it below:

1. A planet must orbit around a star.

2. A planet must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

3. A planet must neither be a star or a satellite.

4. A planet must have cleared the neighbourhood around its orbit.^source

Of the above requirements, Pluto fails in numbers 2 and 4, though not every member of the science community agrees. It has been delisted from among the planets.

Pluto isn't as big as the Jovian planets. Rather, it is smaller than all of the inner planets. For that reason, we would expect it to be more compact than the Jovian planets and just as dense as the terrestrial planets. However, with all that compactness, it is not! Pluto's density is much similar to the Jovian planets and its physical properties do not seem to fit in any of the categories of planets, leading astronomers to suggest that Pluto might have been an escape-moon from Neptune. To explain its weird compactness, scientists have discovered that it is so cold out there on Pluto that even gases exist as ice, as it should be, considering that it is 39.53 times as far away from the sun as earth is

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Well, there's a lot more about Pluto we could discuss, but I suppose this is enough brain-dump for the day. I'd rather have my readers understand and enjoy my articles than lose concentration along the way.

Hence, I shall continue next time with the movement ( rotation, and revolution) exhibited by the planets in the solar system, as well as their natural satellites (moons), and space debris like asteroids. I will be putting up a more complete table for quick facts we could use as referential purposes as well. Until then, stay safe.

Thank you for reading.

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References

_{Answers in Gen. -- Pierre Laplace Nebular hypothesis.
Big history -- Our Solar system.
The talking Democrat -- Why Pluto isn't a planet anymore.}