Caoimhín's Content

After taking a c loser look at all the other planets, we are finally moving on to Neptune. Previously, we took a look at the dwarf planets as a group and some other minor bodies. Once we’ve dealt with Neptune, I will move on to taking a closer look at those distant objects, as well as going back and review the other planets, there’s only so much I can say in a few minutes and there’s plenty to discuss about every planet in our solar system. If you missed my videos about any of the other planets, they are all in the arc hive here and on my YouTube channel.

Unlike all the other planets, I can’t start this one with how Neptune appears in the sky. Neptune definitely isn’t visible to the naked eye, and no one’s saying otherwise. Technically Uranus is just about visible under perfect conditions, although I’ve never managed to see it myself, and all the other planets are easy to spot, even with some light pollution. You can certainly see Uranus as a sort of tealy-blue dot in reasonably low powered magnification, even with a big pair of binoculars, but Neptune requires a good bit of magnification to see at all. Uranus is up at the moment as well, over by the Pleiades, near Jupiter and Mars. From the city Uranus certainly isn’t visible, but with the light pollution removed Uranus is at least highlighted by Stellarium. This won’t be the case with Neptune. With four other planets up in the sky, it’s easy to trace out the path of the ecliptic, arcing up to Jupiter and Mars, through Uranus and across to Saturn. Despite being quite distant, Neptune is a planet and follows the ecliptic pretty closely, closer than the more distant dwarf planets. At the moment Neptune is reasonably close to Saturn in the sky, and they will both be quite high reasonably early in the morning, before the glow of the Sun interferes. Right now in early August the Moon is still close to new and will leave the sky by the time Neptune is risen. This get’s the sky nice and dark which is important for observing such a faint object. Neptune isn’t at opposition yet, so our view is still set to get a little better, but right now does provide a great opportunity to take a closer look. Thanks to Saturn being so bright and easy to spot, it almost acts as a landmark or way point, giving you an idea of roughly where to look for Neptune.

Neptune isn’t visible to the naked eye, just like Saturn it was discovered using telescopes. Or, at least telescopes were used to confirm its existence. Initially, astronomers began to suspect the existence of a massive planet beyond Uranus thanks to Uranus’s orbit around the Sun. By observing the motion of Uranus over the course of years and years, astronomers noticed that it wasn’t behaving as predicted. Charts predicting the positions of planets were a common tool for astronomers, but it was noticed that the charts predicting Uranus’s position became less and less accurate over time. Uranus’s orbit around the Sun didn’t match up with what the maths predicted, leading astronomers to hypothesize that there was some influence on Uranus’s orbit. This is what prompted people to go looking for Neptune, and it was maths that told them where to look. Several astronomers and mathematicians, usually scientist who were both at the same time, began using the deviations in Uranus orbit to predict where the object causing the deviations should be. In the end, the work of a French scientist, Urbain le Verrier, predicted the location of Neptune well enough that an astronomer, Johann Gottfried Galle, was able to use the calculations as a guide and observe the planet through a telescope. By looking at how the planet moved differently to the stars, it was confirmed to be a planet. Neptune was officially discovered on the 23rd of September 1846, by an astronomer working off the mathematical predictions of another scientist. There was a lot of direct collaboration in astronomy at this time in the past. Indeed, there is still plenty of collaboration in science today, but in what one might call the golden age of telescopic astronomy back in the 1800’s, a lot of these astronomers were writing letters to each other directly, sharing their research directly with each other, not just publishing it and leaving it out there for other people to discover but actively sharing it with their community. This of course, it led to some of these kinds of discoveries. With Neptune only first observed directly on the 23rd of September, it may be surprising that Neptune’s largest moon Triton was discovered as early as the 10th of October 1846. Just 17 days apart, a matter of a few weeks. This was accomplished because Triton is so large and so bright. Triton is an icy moon, and I put up a piece recently about icy moons if you’d like to take a look at it, but what matters here is that Triton is one., It is a moon that has a coating or shell of ice, which makes these moons very pale, very reflective, and therefore very bright. The high albedo, or reflectivity, also prevents the absorption of much heat from the Sun, keeping it very very cold, the temperature is only a few tens of degrees above absolute zero on the surface of Triton.

Triton is a reasonably big moon, the seventh biggest in our solar system, and by far the biggest moon of Neptune. In the video, I say that is is 95% the mass of the whole system of Neptune’s moons, but it is actually 99.5%. All of the other moons, and Neptune has 15 others that we know of, only contribute half of one percent f the total mass of the moon system, the rest is all Neptune. Triton is so large, that it is considered one of the big moons in the solar system, in the same group as the Galilean Moons and Titan orbiting Saturn. These larger moons are all big enough to be round, and in cases of moons like Ganymede, big enough to be counted as planet if they were orbiting the Sun on their own. Triton is one of those very big moons, larger than any known dwarf planet at least, and it is unique among the big moons for orbiting Neptune the wrong way. It’s tough to spot, as day to day the moons move fast enough for Triton to be in a significantly different position each night, but it doesn’t move enough in one night for it’s direction of motion to be easily spotted. By looking at Triton orbiting Neptune from space, with no ground or atmosphere to get in the way, it becomes a little easier to see. This hypothetical view also let’s us see The other moons of Neptune, which makes it easier to tell that Triton is moving differently to the rest. Triton is orbiting Neptune in retrograde, and retrograde orbits are a little rarer even in smaller moons, let alone bigger ones like Triton. Thanks to its strange retrograde path, some scientists have concluded that Triton might actually be a dwarf planet, similar to Pluto, similar to the other dwarf planets we looked at recently, that was captured by Neptune from the edge of the Kuiper Belt. Although Triton is bigger than any of the dwarf planets discovered so far, it does have a similar icy composition and Neptune is close enough to the Kuiper Belt to influence objects there. Many Kuiper Belt objects resonate with Neptune, and Pluto even crosses Neptune’s orbit. Most other moons are though to have formed around their planet, or form in the inner solar system and then being captured by the planets, as is likely with Mars’s moons. It looks like Neptune captured a dwarf planet rather than an asteroid or planetoid and made it one of its moons, providing a nice link between the dwarf planets and the full planets. A lot of the dwarf planets we know today were discovered because of quirks in Neptune’s orbit, the same way that astronomers looked for Neptune because of strange things about Uranus’s orbit. Percival Lowell was one astronomer who calculated possible locations for this hypothetical planet, and at Lowell observatory, Clyde Tombaugh discovered Pluto in the course of that search. Pluto however is too small to explain the quirks of Neptune’s orbit and the orbits of other distant objects. We have not yet found it this distant plant, called the 10th planet or planet X since the discovery of Pluto, it is still often referred to as planet X, even though it would now be the ninth planet, wit the Roman Numerals IX.

Unlike Uranus, for the moons of Neptune the naming scheme returned to using creatures from classical mythology. These moons are named after spirits and creatures of the sea that were associated with Neptune, the god of the sea, the Roman equivalent to the Greek Poseidon. Some of these moons include Naiad and Thalassa, which gives its name to thalassaphobia, a fear of the open ocean. Proteus was the third moon of Neptune’s discovered, as recently as 1989. Another moon of Neptune’s is called Hippocamp, after the half-horse half-fish creatures that are associated with waves in mythology, and of course Capricornus is a sort of hippocamp, being half- goat half-fish, another nice link between different things in the sky. Nereid was the second moon of Neptune discover, in 1949, a good bit before Proteus but still long after Triton. It is the second largest moon, but that still makes it a small fraction of Triton’s size. Nereid is an interesting moon because it has a highly eccentric orbit, like the orbit of a comet though on a much smaller scale. It changes its distance from Neptune pretty drastically between its closest and it’s furthest point, it’s about 7 times further away at its furthest than it is at its closest point, making this a very interesting moon to take a look at.

Moving through time will bring the Great Blue Spot or the Dark Blue Spot into view, a dark patch in the upper atmosphere of Neptune. It is an ice giant so again we’re talking about upper atmosphere here rather than a surface feature. There is no real surface, just a liquid metallic hydrogen ocean deep under the atmosphere. Neptune’s storm is similar to the Great Red Spot on Jupiter, it is a massive storm in the upper atmosphere of the planet, with very high wind speeds, way bigger than any storm on Earth of course. However, the Great Dark Spot on Neptune is really a sequence of several storm that have been observed in the atmosphere of Neptune. While there is often, or even usually, a dark storm visible on Neptune, it isn’t as permanent as the Great Red Spot on Jupiter seems to be. The magnetic fields of Neptune and Uranus are a little bit titled, they don’t line up perfectly with the actual motion of the planet, which has led scientists to conclude that there is some separation between the rotation seen in the upper atmosphere and the metallic hydrogen beneath it. Even here on Earth, our magnetic poles and our physical poles are not perfectly in line. There is a chance that some liquid hydrogen is moving quite free of the atmosphere and more solid hydrogen. There is probably not much friction when everything’s being lubricated by liquid hydrogen and liquid helium. Although those gases make up the bulk of Neptune, there is a lot of methane in Neptune’s upper atmosphere as well. The methane is what gives Neptune this lovely blue colour along with Uranus. However, the colour of Neptune is not the lovely, rich almost royal blue, or a kind of a bright sky blue, that we normally see in images. In reality Neptune is a paler more washed out bluey-teal colour, as we now understand it, Neptune and Uranus are actually much much more similar in colour then we originally believed, This is mostly due to early images from Voyager, had to be processed in certain ways and it was difficult to make sure that the image represented what a human eye would actually see. Neptune is almost unobservable, even with quite a powerful telescope, it only appears as a blue dot. Jupiter’s moons, certainly the Galilean Moons, are brighter and easier to see through a telescope then this planet. This planet is just so incredibly distant that even the nearest dwarf planet Ceres is easier to see through a telescope or binoculars than this blue dot. You should see a distinct blue colour at least, which helps this little dot of light a stand out from the stars. At the moment, it doesn’t seem possible to see the moons of Saturn and Neptune together. They aren’t quite close enough together in the sky, you would need a very big field of view. However, this is just for the current moment, of course those planets can end up closer to being in line with each other at other times and hopefully that’s something we’ll get to investigate in the near future.

That is at least a brief overview of Neptune. If you’d like to learn more about ice giants, you can go back to my previous piece describing the difference between Ice Giants like Uranus and Neptune and Gas Giants like Saturn and Jupiter, that talks a lot more about the composition of these planets. Neptune is out there, though it is so tough to spot that it is a planet that we discovered thanks to maths. Thanks to astronomers paying careful attention to the orbit of Uranus and calculating where the body perturbing Uranus’s orbit should have been, that’s what led us to Neptune. Hopefully I have now led you to Neptune, quite close to Saturn for you to take a look at over the course of the month of August, so I hope you get a chance to see it. I hope that you enjoyed this little piece about Neptune. If you did and if you’d like to see more from me in the future, you can help by subscribing to this website or my YouTube channel. Whether you get to see Neptune or not, hopefully I’ll see you back here next time.