The Far Future of Uranus

Uranus is often one of the most ignored planets in the solar system. When first looking at it, it doesn't seem like there's anything interesting going on. It's a featureless pale blue cue ball. This, however, is untrue. Voyager 2 just so happened to fly by Uranus when its atmosphere was very calm, and Hubble observations show the Uranian atmosphere has storms and bands just like Neptune pretty often. But there's something weird going on with Uranus upon closer inspection. The most obvious thing is that the entire planet is tipped over on its side. The poles face the sun and get decades of never-ending sunlight for every orbit, while the other half of the planet is in total darkness, and it switches as Uranus completes its orbit around the sun. Obviously, something must have caused this. The best explanation is that a collision with another planet in the early solar system was the culprit. However, this is not confirmed. The only reason it's the most popular theory is because, so far, nobody has seriously proposed an alternative. Unfortunately, Uranus has only ever been visited once, and so we know very little about it. But there are some interesting things we can say about its future. When the sun becomes a red giant in several billion years, a similar trend will unfold for every planet. It will get hot. How hot depends on, obviously, its distance from the sun, and it ranges from being vaporized like Mercury to maintaining temperate Earth-like temperatures until the sun becomes a white dwarf like Neptune. But when you look closer, every planet has a weird and different future. Mercury will likely survive inside the sun for millions of years. Mars' axial tilt is all over the place, and sometimes it may actually flip it on its side like Uranus is today. Titan is migrating away from Saturn, which could one day destabilize the entire Saturnian moon system. Jupiter's Galilean moons could become the largest comets in the history of the solar system. Neptune will become a temperate ice giant and will destroy its moon Triton. And Uranus is no different. Its unique environment and moon system will make for a pretty interesting future. Welcome to the seventh episode in this series on the far future of the solar system. This episode will be about Uranus, and so far, I've done every planet except Venus. Make sure to watch the far futures of Mercury, Mars, Jupiter, Saturn, and Neptune, and Earth one billion years in the future when you're done watching this. The far future of Venus, and probably the far future of the Kuiper belt, will come out sometime later. Anyways, Uranus is unique among the giant planets because it doesn't have a super large moon. Jupiter has four, Saturn and Neptune have one, and even Earth has one. Don't get me wrong, there are five major Uranian moons, but the biggest one, Titania, is only 5% the mass of Earth's moon. But that doesn't mean they aren't interesting. Miranda, Ariel, Umbriel, Titania, and Oberon are all expected to have subsurface oceans, though with differing degrees of confidence. In the case of Miranda, it may have had one in the past, but no longer does. This is typical of the outer solar system, where ice is the dominant material for the building of large objects instead of rock. Before we get into how an expanding sun will affect Uranus and its major moons, we should talk about the minor moons as well, because several of them may not last for much longer. While the five major Uranian moons have stable orbits, the Uranian minor moon system seems to be unstable on long timescales. Meaning within the next few million years, the small moons of Uranus may get ejected or collide with one another. Specifically, the moons Cupid and Belinda are expected to collide sometime within the next 10 million years, which will create a new thin ring system around Uranus that may reform into new moons in the future. And they're not the only ones. Cressida and Desdemona, two more small asteroid moons, are also expected to collide with one another in less than 1 million years. However, Desdemona also has a chance of colliding with Juliet instead. The more likely scenario though is Desdemona and Cressida collide first, producing a moon the authors have a paper about that's called Cressdemona, before then colliding with Juliet. Or, assuming that doesn't happen, Juliet may collide with Perdita instead, or even the objects that result from the collision of Cupid and Belinda. Because apparently Juliet is hated by the entire Uranian moon system, and everyone wants it annihilated as quickly as possible. And these timescales can become shorter or longer depending on the density of the minor moons, which is not currently known with a high amount of confidence. In most simulations run in the paper, Cupid and Belinda collide first, followed by Cressida and Desdemona, and most interestingly, they mention that the has the chance of happening in as little as 1,000 years from now. Once these collisions happen, the resulting material may form new moons like Cressdemona, which I've already mentioned, and some combination of the names Cupid and Belinda. These collisions will also likely produce debris around Uranus that could form additional moons or rings aside from Cressdemona and Cupid-Belinda. And this cycle, like the one present on Mars, is likely to continue, with new moons forming on short timescales, colliding with other moons, forming new moons again and again. This is similar to the potential origins of Phobos and Deimos around Mars. I made another video about this already, but there's good evidence that Mars used to have a larger moon in the past that was torn apart to form new moons, which were then torn apart, over and over again until Phobos and Deimos were produced. And Phobos will be torn apart by Mars again in roughly 50 million years, continuing the cycle. This could be similar to what's going on with Uranus today, and I think that's pretty interesting. Something like that may have happened on a larger scale with Saturn as well, forming its rings. If we have 3 planets in the solar system that have had, at one point or another, unstable systems of moons, that may be something pretty common across the universe. Especially when you consider that Triton, a captured dwarf planet, is going to be destroyed by Neptune in 4 billion years too. But that's just my own speculation, so take it with a grain of salt. Anyways, the next topic is the major Uranian moons, Miranda, Ariel, Umbriel, Titania, and Oberon. Because when you hear about the sun becoming a red giant, one of the first things brought up after despair about Earth being destroyed is that the gas giants, and more importantly, their moons, will enter the habitable zone, where temperatures are right for liquid water to exist. And as already mentioned, in the outer solar system, water ice isn't just common, it's a major or even dominant ingredient in the composition of nearly every major solid object past Ceres except for Io. That means only one thing, when the sun becomes a red giant, everything past the asteroid belt is going to start melting fast. This is true for Uranus and its moons as well, but due to it being further from the sun, it'll take longer to happen compared to Jupiter and Saturn. Unfortunately, because we don't know exactly how big the sun will get or how fast, we can't say exactly when Uranus will enter the habitable zone or for how long. However, somewhere around 7 billion years from now is a decent estimate, and when it does, it won't stay in the habitable zone for longer than a few hundred million years, and that's at the absolute max. And unlike Neptune, which may end its life in the habitable zone or at least close to it, Uranus will just keep warming up. It'll at some point get too hot for liquid water, and things will go from bad to worse as the Uranian moons go from melting to evaporating. But things get worse, because it's unlikely the Uranian moons will ever have habitable environments to begin with. I elaborate on this much more in The Far Future of Jupiter, which I highly recommend you watch. But for a brief summary, it seems very unlikely that any icy moon, when it enters the habitable zone, will become habitable, and if it does, it will be an extremely short window. This is because water is an extremely powerful greenhouse gas, and these moons are literally made of it. When it starts evaporating, it will increase the temperatures, evaporating more ice, in a runaway greenhouse effect until you get ultra-hot steam planets with no liquid water and no oceans. In order to avoid this, you need to get rid of the atmosphere faster than it can form. And we unfortunately don't know any of the specifics enough to predict this, but it seems unlikely that any solar system moon will reach the necessary balance between losing and building an atmosphere to create habitable conditions. And if a moon or two do get lucky, the sun is still warming up, and any equilibrium it reaches will quickly be broken by the increasing temperatures. This is true for the Uranian moons as well. And as I say in every Far Future video that deals with icy objects, instead of becoming habitable ocean worlds, all the moons of Uranus, large and small, will become comets. Some of the largest comets in the solar system, in the case of Titania and Oberon. Water will evaporate and be blasted into space, creating massive comet tails that surround Uranus and all of its moons. Unfortunately this will almost certainly destroy all of the interesting surface features the Uranian moons have. For example, Verona Rupes on Miranda, the tallest cliff in the solar system, will probably be vaporized. Miranda in general has some of the most dramatic terrain of any large solar system object, and that'll all probably be smoothed out as the ice evaporates. And as Uranus leaves the habitable zone, this will only become more extreme. Depending on how long the sun lasts before it becomes a white dwarf, the major Uranian moons could have most or all of their icing material stripped away, leaving just their much smaller rocky cores behind. But because Uranus is so far from the sun, it also may not spend enough time in the hot regions before the sun becomes a white dwarf and things cool right back down. But enough about the moons, let's talk about Uranus itself. Like Neptune, Uranus will spend at least a brief period of time as a temperate ice giant, which is one of my personal favorite types of planet. Due to its low density, Uranus's surface gravity is similar to Earth's, and when the sun heats up, there could definitely be a region in its atmosphere where the pressure and temperature are both habitable, and with a nice surface gravity, that could be a pretty comfortable environment. Especially because Uranus contains a ton of water. The exact amount varies based on who you ask, and there are some estimates that say the ice giants have more rock than water, but some models show that Uranus and Neptune have the largest water oceans in the entire solar system. Some of that could easily become water clouds when it heats up, which honestly is a pretty habitable environment. Good temperatures, good pressures, good amount of sunlight, good surface gravity, and even good access to water. Ironically, the best place to look for a habitable environment around Uranus might be Uranus itself rather than any of its moons, if it weren't for the fact that there isn't a solid surface. But again, this environment will not last long, because the sun is still warming up. What exactly Uranus will look like during this time is unknown and depends on the time period you're looking at, but it's likely that methane haze creating the blue color will decompose, and it'll become white due to water clouds, or brown and yellow with other types of hazes, the same ones that give Saturn its color. Though I will say, due to Uranus's high axial tilt, it may have some interesting climates and wind patterns not seen anywhere else in the solar system, but it's impossible to know exactly what those would look like. Unfortunately, there's not much else to say. Uranus could become a relatively temperate planet, surrounded by enormous comet moons, before things get too hot. After that, the sun becomes a white dwarf, and things cool down fast. Any ice still left on the moons refreezes, and the whole Uranian system is trapped in pitch darkness. The sun loses about half its mass, and every planet moves outwards. This might be good for both Uranus and Neptune, as the further orbit increases the time they spend in the habitable zone. But after all is said and done, Uranus becomes a cold, dark world where nothing interesting happens again. After trillions of years, random stellar encounters slowly eject the surviving solar system planets away. We don't know when this will happen, but know it's inevitable, because things constantly move around the Milky Way, and it's only a matter of time before something passes close enough to mess everything up. Uranus, being far from the sun, will likely be one of the first planets to go, after Neptune. Then it becomes a rogue planet, wandering the dying Milky Way for all eternity. But at least no matter what happens, Juliet is long gone. Thank you for watching. If you enjoyed, check out my other videos about planets in space exploration.