Feb. 28, 2022

Episode 8 - Planets II

Welcome to Untangling Science, a podcast about science that is for everyone, with me Darragh Ennis. You probably know me as The Menace from ITVs quiz show The Chase, but my day job is as a scientist at the University of Oxford. In this podcast I want to bring the world of science to people who think it’s too complicated to understand in a way that is fun and straightforward. We have a website, www.untanglingscience.com and you can follow us on Twitter @untanglings . I have a blog on the website that I leave useful information, links and diagrams from each episode in, so check that out. First of all, apologies about the big gap since the last episode. I took some time off as I was moving house and that chaos was not a huge help to having time to write podcast episodes. We’re all settled now in our new place so I hope to get episodes out more regularly. Anyway, on to today’s episode. I left you all hanging last time, half way through our trip through the planets of the solar system. This time we will be heading back into space to talk about the colder parts of the solar system with the giants of Jupiter and Saturn and the frozen worlds of Uranus and Neptune. Hope that sounds good, and let’s start with the greatest of all the planets, mighty Jupiter.


Jupiter is named after the chief of the gods and it’s a good name for such a truly vast planet. And it really is massive. After the sun it is easily the biggest object in the solar system, with a mass about two and a half times more than all of the other planets added together. It is so big that this is even obvious from Earth and it was this obvious size that led to it being named after the king of the gods. Jupiter is quite easy to see from Earth and after the Moon and Venus it is the next brightest thing in the night sky. Jupiter is about 5 times further from the Sun than Earth is, about 750 million kilometres, so the fact it is so bright and easy to see should give you an idea of it’s scale. I know I’m banging on about how big it is, but if you take nothing else from this episode than the fact that Jupiter is absolutely immense then that’s not a bad start to understanding this planet. But what’s it like on Jupiter? Is it rocky and hot or freezing like Mercury? Or is it roasting in a choking acidic atmosphere like Venus? Or is it cold and dusty like Mars? Well, it’s like none of those, as Jupiter is a totally different sort of planet to the ones we’ve talked about before as it’s a gas giant. Gas giants, as the name suggests, are huge balls of gas. So the surface of Jupiter is not solid, it is instead mostly made up of Hydrogen and Helium swirling around a small rocky core. This ball of gas rotates at a fair speed, with a day on Jupiter lasting just under 10 hours. When you consider the sheer size of the planet this means the surface is moving pretty quickly, about two and a half times faster than earth does. This causes the planet to bulge slightly at the equator and flatten at the poles so it is what is called an oblate spheroid, which is basically the shape of a ball that’s been squeezed a bit. So now we know what it looks like let’s get closer and find out what it’s like on the surface. Well unlike Mercury, Venus and Mars, we can’t actually land on Jupiter as it’s not got a solid surface. But if we were to fly close to the surface one word would come to mind, stormy. Jupiter has the most spectacular storms in the solar system, they make earth’s hurricanes and tornadoes look pretty puny in comparison. The whole surface of the planet is whipped by raging winds up to 400 miles per hour. The most famous of these storms is called the great red spot and it is so big that we can see it from Earth. This might sound impossible but the sheer size of this storm is truly staggering. It is bigger across than our planet is, with a diameter of over 16,000km and an estimated height about the same as mount Everest. These measurements change a bit over time, because the great red spot isn’t just absolutely vast, it’s also really old. It’s believed that the first sighting of the spot was in 1665 and it shows no sign of disappearing any time soon. Added to these terrifying storms, Jupiter’s gravity is two and a half times stronger than Earth’s, and without a solid surface to stop you, if you get too close you would sink into the planet and be crushed by the intense pressure. So best not to do that then. Zooming back out into orbit we can see another effect of Jupiter’s size and gravity, it’s moons. The current count is about 80 moons as the huge planet sweeps up anything in its path as it orbits the Sun. The moons tend to be named after people Jupiter/Zeus seduced in ancient myths and luckily for the scientists naming the moons there’s no shortage of those as Zeus got about a bit. Going through all 80 would take all day, so a quick word about the four biggest of these, known as the Galilean moons.


First of all, they are called the Galilean moons as they were discovered by that great scientist and feature of Bohemian Rhapsody, Galileo Galilei. I think we should do an episode on him soon, maybe next time. Until then let’s just talk about how he found these moons. In early 1610, Galileo was observing Jupiter using a newly re-designed telescope. This telescope that he invented was able to see 3 bright objects near Jupiter that were previously unknown. Galileo assumed they were faint stars but after some months of observations he found a fourth bright spot and realised they were orbiting around the planet. This was the first time anything other than our own moon was seen to orbit a planet and Galileo wrote about these observations in letters that are the first record of the discovery. The moons are called Ganymede, Europa, Io and Calisto. Ganymede is the biggest moon in the solar system, and at over 5,000km across its actually larger than Mercury. It doesn’t have much of an atmosphere, but it is thought to have huge underground oceans that are believed to hold more water than all of the earth’s oceans combined. This would make it a potential site for future human landings, but a more likely moon for that is the smallest of these four satellites, Europa. Europa is extremely smooth, the smoothest surface of any of the solid large objects in the solar system. This is thought to be caused by a huge liquid salt water ocean under the surface of the moon, and what is believed to be plumes of water spraying up like geysers were spotted by the Hubble Space telescope a few years ago. All of this has led to much speculation that there could be life on Europa and that it might even be habitable in the future for humans, but a lot more data is needed before we make any plans. Calisto also has quite a few signs of water, including surface ice, but as it doesn’t seem to be volcanically active scientists think it is much less likely to have any lifeforms present. Last of these four moons is Io, the closest to Jupiter of the four. Io has plenty of volcanic activity, in fact it’s the most volcanically active object in the whole solar system. Unfortunately, it doesn’t have any water to speak of and it’s surface is constantly changing as it’s 400+ active volcanoes spew out lava and sulphur, giving it a distinct yellow colour. So probably not the best place for us to land.


We know so much about Jupiter due to quite a few space probes that have flown by but I thought I’d talk about the two that gave the most information, appropriately named Galileo and Juno who was Jupiter’s wife and is also the name of my dog. Pretty sure its named after the goddess though. The Galileo probe approached Jupiter in the mid 1990s and was able to observe a comet striking the planet in 1994. It entered orbit in 1995 and sent loads of data back before being crashed into the surface in 2003. This was done purposely to prevent the probe landing on Europa and potentially contaminating the surface that may contain life. Juno, the probe not my dog, Juno went into orbit in 2016 and finished its planned observations last year. The mission has been extended until 2025 to include some flybys of Jupiters moons before it will follow Galileo and crash into the planet and be destroyed. Wow, well that’s a lot on Jupiter so let’s have a summary


  • Jupiter is really really big, bigger than all the other planets added together.
  • It’s a gas giant with a surface covered in huge storms
  • The biggest storm is at least 350 years old and is wider across than the earth
  • Jupiter has 80 moons and some of these are thought to be good candidates for extra-terrestrial life.



Ok, so that was Jupiter. Now we’re on to Saturn, much smaller than its neighbour but still a really big beast of a planet. At over 100,000km across is it about 9 times wider than earth and over 750 times bigger by volume. Saturn is pretty far out in the solar system, 10 times more than Earth at 1.5 billion km. Like Jupiter it is a gas giant with a stormy atmosphere and wind speeds of over 1,800 km per hour and lightning strikes more than 1000 times more powerful than those on Earth . But the main thing everyone knows about Saturn is its ring system. Jupiter and Uranus have rings around them, but Saturns rings are so much more prominent, so prominent in fact that they can be seen from Earth. They were first recorded by, you guessed it, Galileo in the early 17th century though he did not know what they were thinking they may be other planets and even recorded them as ears in one account. Later work by the astronomers Huygens and LaPlace described them as a disk and then finally as rings. They extend out from the planet surface to about 80,000km and are extremely thin, as little as a few metres in places. The rings have distinct gaps between them, some caused by orbiting moons that are moving around within the ring system.

They are mostly composed of ice and some rocky particles and no one is really sure how they were formed. One theory is that they are composed of debris made when a moon was destroyed by a collision with another moon or a comet. It’s also not really known how long they’ve been there or how long they will last. Current thinking is that they are not all that old and will likely be gone fairly soon. It should be said though, that this is in solar system terms, so they’re likely tens of millions of years old and are expected to be gone in less than one hundred million years.


Back to the surface of Saturn now. Like Jupiter there is no solid surface as such, just a very stormy atmosphere. Saturn is tilted on it’s axis like Earth so it does experience seasons. Due to its distance from the Sun I don’t think summer time is all that much better than winter, but that wouldn’t matter too much as anyone landing on the surface would be subject to immense pressure as they sank into the planet. Like Jupiter it’s to the moons we need to look if we ever plan to land near Saturn. Saturn has 83 moons, though most are quite tiny so for this episode we will talk about just two of the biggest, Titan and Enceladus. Like Ganymede, Titan is larger than mercury with a diameter of about 5,000km but it’s not its size that makes it extraordinary, it is its atmosphere. Titan has a dense atmosphere and strong evidence of large amounts of liquid water on its surface. The surface of the moon has weather patterns very similar to Earth’s with a key difference or two. It is much much colder at -180C and large amounts of the liquid on the surface are not water but methane, what we call natural gas. This along with suspected volcanic activity and underground ice and water reserves make Titan a good candidate for possible evolution of life, though due to the low temperatures any life would likely be underneath the surface. Another good candidate for extra-terrestrial life is Enceladus. At 500km across it is much smaller than Titan but don’t think that means there’s nothing going on. Enceladus is covered in ice but it’s not a completely frozen world. Volcanic activity means that this small moon has huge plumes of water being sprayed into its atmosphere. Over 100 of these geysers have been identified and a 10km deep ocean identified under the ice at its southern pole.


Space probes to Saturn are a little less frequent than to the closer planets, with both Pioneer and the Voyager programmes performing flybys. Today we’ll talk a bit about a recent probe sent to study Saturn, the Cassini-Huygens probe. Made of two separate probes, they entered orbit in June 2004 before the Huygens probe split away and descended down to the surface of Titan in early 2005.  Cassini continued to orbit Saturn and observe the planet and its moons for 13 years, then it took final observations of the closest rings and Saturns atmosphere before crashing into the planet in 2017. Ok, so that’s Saturn, time for a quick recap.


  • Saturn is another massive gas giant, over 750 times bigger than earth by volume
  • It’s got a very impressive ring system, that can be seen readily from earth with a decent telescope
  • Saturn has loads of moons, including Titan that has an atmosphere and water and Enceladus an ice moon with over 100 geysers shooting water into the atmosphere.


Now we are going even further out into the cold depths of space and there we find the first of the ice giants, Uranus. Now, a little diversion into my other life as a quizzer. The planets are named after gods and sometimes can be hard to remember the family tree of the gods but the planets can help us. Mars was the son of Jupiter, who was the son of Saturn who was the son of Uranus. So moving out from Earth to Uranus is going through the ancient gods generations. Anyway, back to the ice giants. Uranus is about 50,000km across so about 4 times the diameter of earth and about 60 times bigger by volume. It is also much further out than the other planets, almost 3 billion km from the sun or 20 times further than earth. From this distance the sun’s heat is very faint and the earth is just a bright speck in the sky. Uranus is the first of the planets that can’t be seen by the naked eye from Earth and it was thought a star or a comet until the 18th century. John Flamsteed, the first astronomer royal,  was the first to record it as a planet and he tried to name it George. Really, he did, after George III, but the scientific community settled on a more traditional planetary name soon afterwards. Uranus is understandably quite a cold planet, with temperatures as low as -224C on the surface. Like the gas giants, the surface is subject to quite devastating storms with wind speed up to 900 km/hr. Unlike the gas giant though, Uranus is believed to have a partially solid surface made up of rocky and icy material. It’s atmosphere is largely made up of methane and some water vapour and is quite featureless in the photographs that we have of it, looking a pale blue ball under natural light. Uranus also has a ring system, but it is very faint and frankly unimpressive compared to Saturn’s. It has 27 moons that are named after characters from Shakespeare and Alexander Pope, most of which are quite small and the main unique thing about Uranus is that it is the only planet lying on its side. The polar regions of Uranus are where you would expect to find the equator, tilted almost into the same plane as it’s orbit. It’s believed to be like this because when the planets were forming, something massive smashed into Uranus and flipped it over. It must have been absolutely huge to tip a proto planet on its side, but to this day the third largest planet in our solar system is having a lie down to recover. I know I’ve been sketchy on the details of this planet, but that’s because we don’t have too much information. Only one probe has come close to Uranus and that was Voyager 2 that was launched in 1976. It took ten years to reach Uranus and performed a flyby on its way to Neptune. So, without further ado, let’s visit the other ice giant named after the god of the seas, Neptune.


Neptune is even more distant from the sun at 4.5 billion km, about 30 times further than the earth. It’s about 50,000 km across, about 3.8 times wider than earth and 57 times bigger by volume. It is the only one of the planets to be found not by direct observation but instead by mathematical modelling. A French scientist Alexis Bouvard noticed irregularities in the orbit of Uranus and hypothesised that another planet outside of the orbit of Uranus was affecting it. The precise location was then figured out mathematically by two scientists John Couch Adams and Urban Le Verrier long before telescopes were capable of seeing it. It was finally observed almost precisely where the mathematics said it would be in 1846 by Johan Galle. It should be noted at this point, that no matter what the science is eventually it comes down to maths, numbers are the universal language of the universe. Neptune is similar to Uranus in many ways, though it has no well defined ring system (it’s rings are fragmented and usually called arcs) and its surface is less solid than the other ice giant. Like the other giants, Neptune has very high winds and in fact the fastest in the whole solar system are here with speeds over 2,000 km/hr recorded. For context a hurricane on earth has wind speeds over 118 km/hr so these are storms of the most vicious kind. Neptune has a more obvious series of weather systems than Uranus, with the Great Dark Spot being about the size of Earth. Neptune has 14 known moons that are all named after various water gods. The largest of these by far is Triton which makes up over 99% of the mass of Neptunes moons. Triton is 2710km across and the seventh largest moon in the solar system. Its orbit travels in the opposite direction to the planets, known as retrograde, and because of this it is believed to have originally been a planet that was captured by the much larger Neptune’s gravity. It is volcanically active and it’s smooth surface suggests regular lava flows are still in progress on Triton. Again only voyager 2 has ever visited Neptune and it’s moons, so detailed information is comparatively sparse, though the use of advanced telescopes on and in orbit around earth continues to add to what we know about the distant ice giants.


So a quick summary of the ice giants:


  • Both Uranus and Neptune are much further out into the solar system and so they are very cold.
  • Uranus has a relatively featureless atmosphere and a partially solid surface
  • Neptune has more complicated weather systems including a planet earth sized storm called the Great Dark Spot.
  • Uranus is lying on its side probably due to a collision while it was forming
  • Voyager 2 is the only probe to visit both of these planets so far



Ok, that is it for our tour of the planets in our solar system. I thought I would end by talking a bit about some current cool stuff in space exploration and for this episode that will be the recently launched James Webb Space Telescope. Space telescopes have a lot of advantages over ground ones, the most obvious being escaping the scourge of astronomy enthusiasts everywhere, light pollution. In my lab we work on advanced microscopy techniques that have many of the same issues as astronomers do with their telescopes. We aim to have resolution,  so magnifying the things we want to see in a clear way, but that is useless without contrast. If we can’t tell things apart from their background then magnifying is a waste of time. By launching telescopes into space, astronomers can escape all of the light pollution and optical interference caused by looking through our atmosphere. The JWST was launched on Christmas Day last year and is a collaboration between NASA, the European Space Agency and the Canadian Space Agency. It is intended to take over as the most advanced space telescope from the Hubble Space telescope which has been in space for over 30 years. Most high powered telescopes use mirrors to collect their light and as a rule of thumb, more mirror are means it is better able to collect more light. The JWST has about 5 times larger mirrored area than Hubble and along with imaging different wavelengths of light this means it should be able to collect data from objects that are too far away and faint for Hubble to detect. Since it was launched the JWST has travelled to it’s orbit about 1.5m km from earth and all of its components have unfolded into position. As I record this the JWST is cooling down to -223C when it can operate properly. But why does it need to be so cold? Well the JWST will image things at the infra-red end of the spectrum. This happens to be very susceptible to interference from heat radiation, so a lot of the design has been dedicated to stopping heat from the sun, moon and earth reaching the telescope’s sensitive detectors. It is hoped that in the coming months it can start collecting data on how stars and galaxies are formed and we can use this data to better understand how the universe was formed after the big bang.


Well that’s it for this episode. I’d like to end by thanking Neal from PodKnows productions for editing the episode and all his advice on how to do a podcast and thanks also to Paul Farrer for his amazing original theme tune. The next episode will be on fungi, a very interesting group of organisms and will likely be another two parter as they are really fascinating with lots to talk about. Please remember to subscribe and share the podcast if you’ve enjoyed it and leave comments and ratings wherever you get your podcasts from. Thanks for listening.