Our Cosmic Backyard: The Outer Planets

In my previous blog, we took a look at our Sun and Inner Planets. Now let’s travel beyond the asteroid belt, to our Outer Planets – the Gas Giants

But wait! What is the asteroid belt?

Let’s consider the asteroid belt as our Solar System’s junkyard. It is made up of the leftover debris from the creation of the planets. It is filled with millions of icy rock and dust particles existing between the planets Mars and Jupiter.  Most of the asteroids trapped in this circle are relatively small. They can range from the size of cars and trucks to a couple thousand feet in diameter.

Some scientists believe that the debris are the remains of a planet that was never formed. According to NASA, however, even if all the rocks and dust were to be combined, the total mass would not be enough to actually form a planet. In fact, the total mass of the asteroid belt put together would actually weigh less than our own moon.

Still, these asteroids had the potential to merge with other planet size masses to make it even bigger; for instance, Mars might have actually been bigger. Unfortunately, they are now stuck in this ring because the gravitational pull of Jupiter is so strong. This made it quite impossible for them to escape the force acting upon them both from the Sun and the big planet. Even so, some asteroids still manage to escape.

Now, onwards, to the Gas Giants.

Jupiter

Jupiter (Greek: King of Gods) is the fifth planet from the Sun at an average distance of 484 million miles and is by far the largest planet of our Solar System with a radius of 69,911 kilometres. Although it is millions of miles away from Earth, this planet is visible to the naked eye and with the help of a small telescope. It is one of the brightest objects in the night sky after the moon, Venus and sometimes Mars.  It orbits the Sun once every 12 years and because it spins very fast on its axis, a Jovian day lasts only 10 hours.

The mere size of this planet alone is mind-blowing. To think that if we were to combine all the other planets in the Solar System, Jupiter would still be about 2.5 times bigger. Even more, theoretically, Earth could fit into it as much as 1,300 times before all the space is taken up. If this planet had gained about 80 times more it’s current mass, we might have had a second sun in the Solar System. In Guyana, we already cry out about the heat; imagine the heat of two suns.  

Of course, for a planet carrying this size it is no surprise that it has acquired quite a few followers. To this day, Jupiter has 79 known moons and many moonlets (tiny broken fragments of the moons). The four largest moons, Ganymede, Io, Callisto and Europa, are known as Galilean Satellites as they were discovered in 1610 by Galileo Galilei himself. Ganymede is the largest moon ever discovered in the Solar System; it is even larger than the planet Mercury with a radius of 2,634km. It is also the only known moon in our solar system that has a magnetic field of its own.

Io (eye-oh) is one of the most violent moons of the Solar System as its surface is covered with many active volcanoes.

Aside from its size, another eye-catching feature of this planet is its colours. Jupiter is very vibrant with its bands of clouds of red, brown and white twisting and swirling across the planet. But, as calm as these beautiful stripes and swirls may seem to us, they are actually thick layers of chaotic clouds made of ammonia and water vapours floating in a translucent atmosphere of hydrogen and helium. These bands are formed by persisting storms that carry winds of up to 335 mph – that’s faster than any recorded wind speeds here on Earth. This, though, is nothing compared to the wind speeds recorded in the massive hurricane-like storm that rages on the planet. Yes, we are talking about the famous Great Red Spot. Winds in this anticyclone can reach up to 400 mph – this is, by far, much stronger than any category 5 hurricane ever reported here on Earth. 

The Great Red Spot is about twice the size of Earth and has been raging for over 300 years since it’s discovery. Over the years, it was noted that the Red Spot has been shrinking although the rate at which it grows smaller has now decreased in recent years.

The Great Red Spot

The thickness of Jupiter’s atmosphere is calculated at an estimated 44 miles thick and is made up of mostly hydrogen and helium. Here on Earth, hydrogen commonly exists as a gas, however, beneath the clouds Jupiter it is much denser – even more so than water – and therefore exists in a liquid, metallic state. This phenomenon is possible because of the immense pressure and temperature that exists below the layers of clouds. The deeper into the planet you go the more the pressure and temperature increases. The pressures and temperatures are so extreme below the troposphere that hydrogen atoms are compressed into a liquid-metallic state. The hydrogen molecules lose their electrons leaving behind a gooey substance that is able to hold electrical charge, much like a metal. In theory, this electrical is responsible for Jupiter’s powerful magnetic field.

When compared to Earth Jupiter’s magnetic field is 15-54 times more powerful, making it the strongest magnetic field of any planet; the gravitational pull is just as powerful. Anything that passes too close to the planet is trapped in its gravity and is usually unable to escape. For this reason, Jupiter has oftentimes been referred to as Protector.

The planet is able to act as a sort of shield for Earth and the other inner planets by deflecting any asteroids or comets that could be headed our way. A perfect example of this is the destruction of the Comet P/Shoemaker-Levy 9 by the planet in 1992. This asteroid was captured by Jupiter at least 20 years before and had kept it in its orbit until it finally swallowed it completely in 1992.

On the other hand, Jupiter’s gravity can also be a double-edged sword. Just as it can deflect an asteroid or Comet, it could also send them hurtling towards the inner planets.

But enough of the doom and gloom of the planet. Let’s look at another interesting fact about Jupiter.

Although Saturn is known as the Ringed Planet, did you know that Jupiter also has rings? In 1979, the Voyager I spacecraft, just as it was about to leave Jupiter’s orbit and head further out to space, it looked back one last time towards Jupiter and the Sun; it was then that the discovery was made. The spacecraft sent back images and scientists were stunned to see that this massive planet actually had rings.

Rings of Jupiter. Taken by the Voyager I Spacecraft in 1979. NASA

Yes, in fact, there are three rings – The Gossamer rings (which are two furthest rings from the planet) and the Main Ring. The rings are very faint and can only be seen if lit by the Sun and viewed from behind the planet or if viewed in infrared where they appear to glow. These fragile rings are made up only of dust particles and are therefore not as vibrant and colourful as Saturn’s.

SATURN

Saturn is the sixth planet from the Sun (Distance: 890 million miles) and the second largest of the Solar System with a radius of 58, 232 kilometres. It takes Saturn twenty-nine Earth years to complete one revolution around the Sun and just ten hours to complete a day.

This planet with its alluring rings has earned the nickname ‘Jewel of the Solar System’.  It may not be the only ringed planet of the Solar System but it sure is the most beautiful.

The rings are made up of water ice and rock that are illuminated by the Sun and are extended hundreds of thousands of kilometres from the planet. These are so named; Ring A, B, C, D, E, F, G. Simple, right?  Although, generally, there are only seven rings, they are actually made up of thousands tinier ringlets.

Between the rings are gaps that can measure thousands of miles across. The gaps are created by the gravitational pulls of the planet and the planet’s moons some of which exists between the rings itself. The Cassini Division for instance, is about 2,920 miles wide. This gap separates two of Saturn’s brightest rings and is quite easy to spot.

Now, despite Saturn being the second largest planet in the Solar System, did you know that it is also the lightest of them all?

What, with Saturn having only about one-third of Jupiter’s mass and a density lesser than that of water and all; theoretically, it would actually be able to float if placed in a pool large enough to hold it.   

Again, it is no surprise that this ‘Jewel’ is also famous when it comes to the number of followers it has collected. Saturn has 53 confirmed and named moons with 29 others patiently waiting in line to be confirmed and named as well.

While a few moons orbit within the rings, others orbit closer to and further from the planet. Titan, the planet’s largest moon and second largest in the Solar System, is situated a good distance away from the planet itself (Distance: 1, 221, 870 km) and is known to have a dense atmosphere of its own. It is the only known body in the Solar System, aside from Earth of course, to have liquids in the form of rivers and lakes and even seas on its surface. Not liquid water, sadly, but liquid methane and ethane. The atmosphere around Titan is made up of nitrogen and there are actually clouds there that occasionally would allow it to rain on the moon.

NASA’s Dragonfly mission set for 2034, plans to explore and find out everything it can about this moon that holds the possibility of sustaining life as we know it.

As for Saturn’s atmosphere, it is made up of hydrogen and helium. The hydrogen, like on Jupiter, exists in a liquid metallic state due to high pressures and temperatures. This is also believed to influence the planet’s magnetic field.

From Earth, Saturn will appear as a yellowish-brown star in the night sky. When viewed through a relatively powerful telescope the planet will appear to look like a yellow or gold marble because of the darkened edges of the planet.

The yellow-gold bands seen on the surface are actually due to high-speed winds in the planet’s upper atmosphere as well at the immense heat that is emanated from the planet’s interior. These winds can reach up to 1,100 mph (1,800 km/h) and temperatures can get as high as 11,700 °C at its core.

Uranus

We have now arrived at our first ice giant and the third largest planet (Radius: 25, 362 km) in the Solar System – Uranus. Being the seventh planet from the Sun (Distance: 1.8 billion miles) with barely any sunlight to keep warm, this planet is nothing more than a barren and frozen land. It takes light from the Sun two hours and forty minutes to reach Uranus. The average temperatures may range from -218°C to -158°C. Still, this icy giant has its own unique features for us to marvel at.  

In ancient times, Uranus was thought to be a bright star among the many others in the night sky. It was only until 1781 when astronomer, William Herschel discovered that this star was actually a planet. This then lead the scientists of that age to debate what they should call the planet. ‘Georgium Sidus’, ‘Herschel’, ‘Neptunk’ were some of the suggested names until they finally settled for ‘Uranus’ after much contemplation.

This planet is nicknamed “The Sideways Planet” due to the fact that it orbits the Sun on its side. It seems Uranus does not agree with following the trend of rotating on its axis, instead it opted to rotate on its equator with its poles pointing towards and away from the Sun and not up and down – unique. Astronomers believe that Uranus might have been in a slight accident with another celestial body soon after it was born which caused the planet to tilt so far off its axis (97.7°) that it was never able to recover making it appear to be rotating on its side.


This ice blue world is surrounded by thirteen faintly lit rings. These rings were first discovered in 1977 when a few astronomers were trying to observe the atmosphere of Uranus. Afterwards, many telescopes had provided visual evidence that there were indeed rings around the planet – even Voyager II spacecraft sent back some photos of its fly-by of Uranus and its rings.  

Photo of Uranus and its rings taken by the Voyager II spacecraft in 1986. Credit: NASA

When it comes to planets with the most natural satellites, Uranus comes in third with its 27 moons after Jupiter and Saturn. If you’ve ever read Shakespeare’s stories and plays you would recognize most of the names of Uranus’ moons as they were mostly named after Shakespearean characters.

Oberon and Titania were the first of Uranus’ moons to be discovered and they are also the largest, followed by their siblings Ariel and Umbriel.

The moons orbiting Uranus may vary in size from 980 miles in diameter to just 11 miles in diameter (Cupid). Most of the inner moons of the Uranian System are made up of half water-ice and half rock but not much is known of the composition of the outer moons – they may just be asteroids captured by the planet many years ago.  

A day on Uranus last just bout 17 Earth hours and it orbits the Sun once every 84 Earth years. The extreme tilt of the planet results in the most extreme seasons on the planet. A quarter year on Uranus is equivalent to 21 Earth years; therefore, every season lasts as long as 21 years each. In theory, if humans were to live on Uranus, they would only be able to witness each season once before they die.

The atmosphere is made up of mostly hydrogen and helium with some small amounts of methane which is responsible for the planet’s blue appearance, as well as ammonia.

Astronomers have found that Uranus is capable of producing very aggressive clouds in its atmosphere which causes the rapid change of the planet’s colour; wind speeds can reach up to 560 mph.

This ice world is visible from Earth albeit a little tricky to catch. The best time to view Uranus in the night sky is when it is in the Opposition position. This means that the planet is directly opposite the Sun with Earth sitting between them.  But this is not all – it also depends on what time of the year Uranus is in opposition and whether you are in the Southern or Northern Hemisphere. If Uranus meets opposition during the summer months of the Northern Hemisphere, then this is not the ideal time to find the planet as the Sun is high in the northern sky at this time. On the other hand, if Uranus is in opposition when the Sun us low in the northern sky, December, then this is the time to get your telescopes out and track that planet down.

Neptune

Finally, we have arrived at the eight and last planet of our Solar System. Named after the Roman god of the sea, Neptune is an ice world of the bluest hues of any planet in the Solar System and is the fourth largest planet with a radius of 24, 622 km. This distant planet stands 2.8 billion miles away from the Sun, that’s 30 times the distance of the Earth from the Sun – and has average temperatures of about -200°C. Barely any sunlight is able to reach this planet, even at noon when the Sun is at its highest point, the sky would seem dull, dim maybe a little bit gloomy.

Neptune is the only planet that is not visible to the naked eye in the night sky and can only be seen with the aid of a telescope.            

Here is an interesting fact: in 2011, Neptune made its first full revolution around the Sun since its discovery in 1846! That’s right, it takes this blue planet 165 Earth years to journey around the Sun. On the other hand, there are only sixteen hours in a day on Neptune.

The way in which this planet was ‘discovered’ is quite unique. After the discovery of Uranus, a French astronomer, Alexis Bouvard, continued to study the planet. He noticed that the way in which Uranus orbits the Sun seem irregular at some point and suggested that perhaps there was another gravitational influence on the planet aside from that of the Sun. A German astronomer, Johann Galli, using this theory, did subsequent calculations helping to navigate the telescopes of astronomers to find the ‘external influencer’; and this is how Neptune was discovered – the first planet to be mathematically predicted.       

The Voyager II spacecraft was the only human-made device to ever make it to Neptune. With its fly-by mission, Voyager II sent back photos that helped astronomers to understand little more about the planet.

Neptune has fourteen known moons with Triton being its largest and the only spherical one. Triton is known as the only moon in the Solar System that orbits its planet in the opposite direction to which the planet itself is spinning – this is a retrograde orbit. This phenomenon may be due to Triton having been a small planet which Neptune captured seeing as all the other moons of the planet have no definite shape, NASA explains.

Rings of Neptune. Photographed by Voyager II. CrediT: NASA

There are five bright but unstable rings that surround the planet. This is the fourth ringed planet of the Solar System but this may soon change. The rings around the planet are believed to be recent, in astronomical terms, and also have a short life span as observations have shown that some of the rings are actually disappearing and at quite a rapid pace.

Like all the other Gas Giants, Neptune’s atmosphere is made up mainly of hydrogen and helium as well as small amounts of methane. Because Neptune is a more vividly blue planet than Uranus, scientists believe that there may be an unknown component to the planet’s atmosphere that results in this colour and not just the trace amount of methane present. Even the clouds themselves have a rather vivid blue tint to them.

Neptune boasts some of the highest recorded wind speeds in the entire Solar System; some winds reach up to 1,500 mph. The winds on Neptune are linked to the Great Dark Spots that disappear and reform at different times on the surface of the planet. These storms can be as large as Earth itself, the latest storm discovered in 2018 is wider than the Atlantic Ocean. These storms are no joke; but as big as they are, they are relatively short lived with an average life span of 3-5 years.

According to NASA, it is believed that the core of Neptune is just about the same mass as Earth. The core is thought to be made up of iron and magnesium-silicate. About 80% of the planets mass is a hot, dense mixture of icy materials including water, methane and ammonia. Of the four Gas Giants, Neptune is the densest of them all. The planet has a magnetic field that is 27 times more powerful than our own.

There are still many mysteries to the outer planets left to be discovered. As our technology advances, perhaps sometime in the near future we will be able to venture deeper into the secrets of the Gas Giants. I am excited to learn what else these foreign worlds hold deep beneath their clouds.

Thank you for joining me on this journey of our Solar System. The universe is vast and there is much more to be explored.

Keep your eyes and ears out for my next blog.

Clear Skies!

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