Caoimhín's Content

Every month or so, I usually make a video talking about a particular planet. beginning with several months ago, up Jupiter and Saturn in the past couple of months. Having looked at the gas giants, the ice giants are next. Before giving them a specific video, I though it would be nice to compare the gas giants and ice giants as a group first, to highlight why they are commonly separated out in the modern day. Gas giant is still the term used to describe all four of the largest planets in our solar system, and it is an older term, from the time before the differences between the planets was truly understood. Ice giants, being a much newer term specifically for Neptune and Uranus is still less common. This may change as we discover more planets of each type around other stars, and as our understanding of Neptune and Uranus grows. This ice giants are a lot further from the Earth and less studied than gas giants, but as we learn more about them, the term may become more prevalent.

Looking towards the end of June provides a good reason and setting to talk about these planets, as all of them will be above the horizon as the Sun rises. Just before the Sun actually comes up, we can see both of the gas giants, the two biggest planets in our solar system, Jupiter the biggest and Saturday second biggest. They will both be in the sky together, with Mars between them on the ecliptic. Right at the end of the month, they will be joined by the waning crescent Moon, providing a nice range of objects to look at, all visible with the naked eye from the city. Jupiter is harder to spot than the rest at this time, being much lower to the horizon, sitting in the dawn glow of the Sun. If you are closer to the equator than Ireland, you will be able to see all of these planets in the morning much earlier in the month, but you will still have to wait for the Moon to move past full and join them. On the other hand, if you are further North than Ireland, you may need to wait a little longer for Jupiter to become visible. Either way, by the end of the month it should be far enough from the Sun to be visible almost anywhere with a clear horizon.

This means that we can certainly see the gas giants, but unfortunately the ice giants are not quite visible to the naked eye. Looking at the sky from a city location, with light pollution, definitely makes the ice giants impossible to see, even if there wasn’t a sunrise glow in the sky. From a dark sky location in the countryside, all of the previously visible planets will be easier to see, as will other objects in the sky. For example, the Pleiades will be visible just above Jupiter, which is a nice thing to spot and practically impossible from the city. However, there is still no sign of the ice giants, and even the faint early morning glow of the Sun will obscure them. To deal with this, I’ll need to remove the atmosphere, which ensures Uranus at least gets highlighted. Under these perfect conditions, with no atmosphere at all, even stars that would be invisible to the naked eye in the countryside become visible. This is because the atmosphere will scatter the light coming through it a little, even if the air is clean and dry, much more so if it is dusty or humid. Even under these conditions, Uranus is a pretty faint dot. It is technically of a high enough magnitude to be seen with the naked eye, just over 5th magnitude, at least under these conditions. Uranus would be a little brighter when it is at opposition, rather than practically on the far side of the Sun, but it would also appear a little less bright when there’s atmosphere in the way. The unaided human eye can supposedly see close to 6th magnitude objects under perfect conditions, but this will vary from person to person. Uranus is right on the edge of unaided visibility, which means that even a little assistance from a binoculars or telescope should bring it into view as a dot at least. However, due to the distances involved, it takes significantly more magnification to see Uranus as a coloured disc, like the other planets. Despite the fact that Uranus is quite big, it is much further away than the bigger gas giants, which keeps it looking small through low powered magnification. Even at a level of magnification that reveals the Pleiades or Seven Sister to be thousands of individual stars, Uranus still isn’t much more than a faint dot, possibly starting to look a little blue in colour.

With a more powerful telescope of course we do begin to see that it’s a planet rather than a bluish star. Just like the other big planets, Uranus has a large number of moons, some of which are quite large, and should start to become visible as little dots beside the planet. Even without such powerful magnification, by observing Uranus over longer periods of time, it can be seen that Uranus is moving around the sky like the other planets, not staying in a relatively fixed position like the stars. This is the technique that was used by ancient people to determine the motions of the planets that are unequivocally visible to the naked eye. Along with being only barely visible, Uranus has a much longer year than the other planets, close to the Sun, about 84 Earth years long. This makes it much harder to tell that it is moving, and requires precise measurements over a much longer period of time. Taking a look at the moons, you might recognize the names of a few of them, such as Juliet, Puck, as characters from Shakespeare’s plays. Its not just Shakespeare, three of the moons are named after characters from a poem by another British writer and poet, Alexander Pope. Being a large planet, Uranus has quite a few moons, 28, and all but those three are named after characters from various Shakespeare plays. This little different form the more mythological origin of the names of other moons in the solar system, leading some to cal; the moons of Uranus the “literary moons”. Uranus should be visible as a little pale, tealy-blue coloured dot through a telescope, but even if you can see it with your naked eye sometimes, right now the glow of the sunrise will almost certainly obscure it from view.

Neptune is never visible to the naked eye, even under the atmosphere free conditions simulated in the video. It is quite close to Saturn in the sky, on the same line as Mars but closer to Saturn. By taking a closer look towards Saturn, the Stellarium software will highlight Neptune, but without some assistance like this it can be pretty tough to pinpoint. The Moon will be moving past this region of the sky in late June as well, and the extra light can make Neptune even more difficult to spot. Being further away than Uranus again, it takes even longer to orbit the Sun, around 165 Earth years for 1 Neptunian year. With a closer look, we can see what appears to be a much deeper blue colour than Uranus. Neptune has relatively few confirmed moons, just 16, but there is likely more waiting to be discovered. Triton, Neptune’s largest moon, is quite bright, and my be more prominent when looking at Neptune than Titania, Uranus’s largest moon, would be when looking at it. Despite being big and bright, and having a similar name, like Titan, Triton has a very different composition. Titan, the second largest moon in the solar system, has a dense atmosphere of gasses like methane and ethane, reflecting light much like the atmosphere of Venus. Triton, the seventh largest moon, is also reflective, but most likely due to an icy surface. Titania, the eight biggest, is less reflective and thus a bit harder to spot. Triton also orbits very close to Neptune, making it easier to see even by accident if you are looking straight at the planet itself.

By taking a close look at these two distant planets, we can see their respective colours, the deep blue of Neptune and the pale, tealy colour of Uranus. In many representations, based on images taken by one of the Voyager probes, they look quite different. However, more recent studies indicate that they are certainly both blue, with Uranus only being a little plaer really. Whenever we look at the planets through our atmosphere, the atmosphere scatters some light and this can effect what colours we see. The only things that have seen the planets outside our atmosphere are cameras, which work a little different to the human eye. This makes it difficult to know for sure what they might look like if you were floating next to them in space. No actual biological eyes have ever gotten close to the ice giants, that we know of at least. Space probes have flown past these planets and taken close up images, but they are only images. The colour of images often needs to be adjusted, or even added back in, through software. This can lead to some uncertainty. However, even with less than perfect images, or our less than perfect view through our own atmosphere, if we were to look at Jupiter and Saturn, we’ll see that they are very different colours, and this is one of the differences that we see between gas giants and ice giants. Jupiter of course is a stripy planet, with alternating shades of brown and beige, along wit the famous Great Red Spot and less famous Little White Ovals. Saturn of course is far less stripy, but shows a similar light brown or beige colour. These planets, Jupiter and Saturn, the gas giants, have a very different composition to the ice giants of Neptune and Uranus, and this difference in composition is visible as a difference in colour. Similar to Jupiter, Neptune has a large storm visible in its upper atmosphere. Despite some similarities to the Great Red Spot, it still has a different colour, being the Great Blue Spot or Dark Blue Spot instead. It is visible as a dark blue region, close to the middle of the planet, when it is facing the Earth. Just like Jupiter and its storm, sometimes the storm is on the worn side of thee planet to observe. However, also like Jupiter, Neptune has a much shorter day than the Earth, just 16 hours, rather than 24, ensuring that the Dark Blue Spot is regularly facing the Earth. Even though these planet take a long time to orbit the Sun, because they’re so far away, most of them don’t take too long to rotate.

The different colour is due to the different composition is why we say ice giants for Uranus and Neptune. In this context “ices” aren’t limited to frozen water. Just like an astronomical day being measured from noon to noon, in astronomy ices are elements that freeze at temperatures warmer than about 100 degrees Kelvin, around -100 degrees Celsius. There are multiple ways to measure temperature, with Fahrenheit, Celsius and Kelvin being the most famous and popular, the Rankine and many other scales exist as well. Fahrenheit and Celsius are quite different. 0 degrees Celsius is -32 degrees Fahrenheit, and the gaps between each number on a Celsius scale are bigger than those on a Fahrenheit scale, 1 degree Celsius is about 1.8 degrees Fahrenheit. Kelvin and Celsius are less different, as their degrees are the same size or distance apart, but 0 degrees Celsius or 0°C is 273 degrees Kelvin. Although Kelvin is most commonly used in astronomy and other sciences, I will use Celsius here. To convert to Kelvin, just deduct 273, to convert to Fahrenheit, multiply by nine fifths and then deduct 32. Whatever scale suits you best, hydrogen and helium need to be very cold to be liquid, they boil at temperatures below -200°C, where as comparatively volatile chemicals, such as water, methane and ammonia won’t boil until closer to -100°C, as high as 0°C for water. These elements can be solid in the colder regions of space, where as hydrogen and helium are not known to do so. It is these ices that make up the majority of Uranus and Neptune. They both do have a fair chunk of hydrogen and helium, but Saturn and Jupiter are almost entirely hydrogen and helium, around 90%. The gas giants are so big and so rich in these lighter elements, it is believed that they turn metallic under all of the pressure inside these planets. Uranus and Neptune are only about 20% hydrogen and helium, and they are too small to generate the necessary pressure to create the metallic form. Instead, they are believed have a supercritical liquid under their atmospheres. These planets do receive some heat from the Sun, as well as retaining heat from their formation and containing radioactive elements that decay producing more heat. They are certainly cold compared to the Earth, but not cold enough for liquid methane under normal pressure. Of course, deep under the atmosphere the gasses end up under much higher pressure, compressing them into a liquid state that blends seamlessly into the gaseous atmosphere, a supercritical liquid.

We can see that on the outside, gas giants and ice giants have a different colour. In their upper atmospheres they have a different composition of gases, and deep down they most likely have a different composition as well. It is hard to be exact regarding the core or mantle of the gas giants and ice giants, as they are obscured by hundreds of kilometers of clouds. However, we can learn about the interiors of these planets indirectly. This is done by studying their gravitational field, their magnetic field and any radiation they emit. The magnetic field and gravity of a planet can vary in different areas around it due to the composition of the planet, even here on Earth. Earth’s magnetic field has been mapped out, and in certain areas the magnetic field is stronger or weaker. The Earth’s gravity has been mapped out, and the pull of the Earth varies a little depending on where you are, if you have a lot of very dense stuff underneath you the local gravity can be a little bit higher. This isn’t really anything that would make a difference to us but sensitive instruments can pick it up. This has been used to find deposits deep underground, by measuring their effect on gravity and magnetism. In the same vein, satellites orbiting around these distant giant planets have figured out a little about their interiors, and by learning what their atmospheres are made of more directly, through spectrographic measurements, we can estimate what they will act like further down. Even only relying on measurements of the gasses of the upper atmosphere, there is definitely differences in the makeup of the atmosphere between these different giant planets. Of course Jupiter and Saturn are also way bigger than Uranus and Neptune, even though the ice giants are way bigger than the Earth and Venus, the gas giants are significantly bigger again. Uranus and Neptune are quite similar in size, with Uranus being a little wider and lighter. The difference between Saturn and Jupiter is greater, with Jupiter being about 1.7 times bigger than Saturn, not quite double. Jupiter is a little over 20 times the size of Neptune or Uranus, with Saturn being a over 12 times the size of the ice giants. Of course, this still leaves the Earth at about 57 times smaller than Neptune, the smallest planet by volume discussed here.

There may be 4 massive planets in our solar system, all giants compared to the Earth, but I hope this write up and attached video give some idea as to why they fit into 2 distinct groups. I will go into more depth specifically about Uranus and specifically about Neptune, separately, in the coming months, so if you enjoyed this and think you would enjoy those, you can subscribe to this website, you can also subscribe to my YouTube, and you can find me on Instagram under the same name. However you do so, I hope you come back next time.