For the longest time, the only gas giant planets we knew about were Jupiter and Saturn. But now in the age of extrasolar planets, astronomers have discovered thousands of gas giants across almost as many star systems. What new discoveries have been made about gas giants, both here in the Solar System and across the Milky Way?

Download MP3 | Show Notes | Transcript

Show Notes

November 2021 Pacific Northwest floods (Wikipedia)

Russian anti-satellite missile test draws condemnation (BBC News)

What are Gas Giants? (Universe Today)

PODCAST: Ep. 56: Jupiter (Astronomy Cast)

PODCAST: Ep. 59: Saturn (Astronomy Cast)

What are brown dwarfs? (EarthSky)

New ‘Gas Dwarf’ Class of Alien Planets Revealed (

Jupiter’s Great Red Spot: A Swirling Mystery (NASA)

Hubble’s Grand Tour of the Outer Solar System (Hubblesite)

What in the World Is Metallic Hydrogen? (


Hot Jupiter (NASA)

Hertzsprung-Russell Diagram (Swinburne University)

Weird Object: Pulsar Planets (Astronomy)

Exoplanet Anniversary: 1st Alien Worlds Confirmed 25 Years Ago Today (

Simulations provide clue to missing planets mystery (

Atacama Large Millimeter/submillimeter Array (ALMA)

INFOGRAPHIC: Profile of planet 51 Pegasi b (NASA)

The Late Heavy Bombardment: A Violent Assault on Young Earth (

More Jupiter Weirdness: Giant Planet May Have Huge, ‘Fuzzy’ Core (

Hubble Observes a Mystery: ‘Cotton Candy’ Planets (NASA)

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Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy Cast, episode 621, “Gas Giants”. Welcome to Astronomy Cast, our weekly facts-based journey though the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today, with me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmo Quest. Hey, Pamela, how are you doing? 

Dr. Pamela:                 I’m doing okay. I am currently looking forward to the discussion I know I’m going to have this evening with my Canadian husband about how we need to leave the US.

Fraser:                         To a counter point, my part of Canada has been detached from the rest of Canada thanks to brutal weather. So, you’ve probably heard we got hit with an atmospheric river that dropped hundreds of millimeters of rain over the course of 24 hours, washing out every single road, highway, bridge, railway line leading from Vancouver out to the rest of the country. So, we are logistically cut off from the rest of Canada, although everything into the state still works, so, we can still pick up American food, which is great. But yeah, so, it’s pretty weird, and I think it’s going to take us months to get back online.

                                    So, I will give people regular updates as we go through this unfolding disaster. We’re definitely in – We’ve declared an emergency.  I’m fine, it missed me, and it didn’t take my power out, and we got a light smattering of rain, it was fine, but for the rest of my friends on Vancouver Island, my friends in Vancouver, it’s an unmitigated disaster.

Dr. Pamela:                 Yeah, and it’s just one of those weeks, I think, that we’re going to look back on history and go, “Huh, that was a statistical flex of everything in one week.” Because you had all of that going there on the space science front, we had Russia blowing up one of their own satellites and endangering lower Earth orbit rather profoundly. We have –    

Fraser:                         Yeah, it’s been a week.

Dr. Pamela:                 Yeah. And then, all the social justice issues with my own country, it’s just sort of like, “Can we get a re-do, or at least fast forward a little bit, please?”

Fraser:                         I think we’re out of asking for re-dos at this point. I think we have a series of events that have happened so far. I think we’re done; this is our life now. All right. For the longest time, the only gas giants we knew about were Jupiter and Saturn. But now, in the age of extra solar planets, astronomers have discovered thousands of gas giants across almost as many star systems. What new discoveries have been made about gas giants both here in the solar system and across The Milky Way? All right, so, we did a series on the solar system, all of the individual planets in the solar system, Jupiter, Saturn, but like, 600 episodes ago.

Dr. Pamela:                 Yes.

Fraser:                         So, I think we can –

Dr. Pamela:                 That is accurate.

Fraser:                         Yeah, so, I think we can merge the gas giants together and have a quick brief refresher, but also, we’ve learned a tremendous amount of new things about the gas giants, so, I think this is a perfectly reasonably time to take another crack at it. All right, I’ll bite, Pamela, what is a gas giant?

Dr. Pamela:                 So, a gas giant, unlike an ice giant, which we talked about last week, is a world that is majority hydrogen helium in its atmosphere and pretty much in its entirety, this is an object that you could refer to as a failed star. The difference between a Jupiter and a brown dwarf is a factor of 13 in mass. So, here we’re looking at objects that are chemically, very similar to a star, just physically, they don’t have the umph needed to start nuclear reactions.

Fraser:                         So, how do they form? Why do you get a gas giant where and how you do, as opposed to a terrestrial planet or an ice giant?

Dr. Pamela:                 It’s all a matter of being lucky enough to hold onto the entire atmosphere. You can’t form a gas giant next to a hot star. One of the really cool things I learned preparing for this episode, is it’s actually possible to have little, tiny, gas giants defined by their composition of hydrogen and helium, you just have to have them at a healthy distance from a really cold star.

So, the key is, stick a blob of mass, a planetesimal, in an area rich in hydrogen and helium, so at a significant distance from a star, so that solar wind isn’t completely blasting everything out of your vicinity, and grow fast enough that other things don’t have a chance to grab ahold of this material, and then, just keep holding onto it, it’s just that simple.

Fraser:                         And so, in the early solar system, the material that was closer to the sun, some of it would have swirled in and joined the sun, and others would have just been blasted away off into space by the solar wind. And it’s just that Jupiter and Saturn happen to be far enough away that neither of those forces were removing that material, and so, they were able to actually form these planets.

Dr. Pamela:                 And in the end, they ended up with strong magnetic fields that further help them hold on to all of this material. And so, they have that sweet combination of large enough mass to hold on to their atmosphere, help of a magnetic field to hold on to their atmosphere, some place cold enough that their atmosphere isn’t moving too fast, but it’s really trying to escape on its own, and it’s just not a hostile environment.

Fraser:                         Right, right. So, is there much of a difference between Saturn and Jupiter?

Dr. Pamela:                 The biggest difference is, we see such tremendous storm behavior in the much more structured banding of Jupiter. Both of these worlds have these counter circulating bands of winds, but with Jupiter, for reasons that I’m not sure we’re every fully going to understand, it is able to generate things like the more than 400-year-old great red spot. It has these constant chemically evolutions of its bands, such that its equatorial region is currently orangier than it used to be, while that great red spot is beigier than it used to be. We see other storms popping in and out of existence on a regular basis within these counter rotating bands.

And Saturn, while it gets its fair share of storms, especially during its equinoxes’ it’s mostly beige, it’s just hanging out there, being beige, it’s beige.

Fraser:                         So, is that a factor of the mass? As you get a bigger mass of planet, you get more of these bigger, deeper, longer lived, storms?

Dr. Pamela:                 Probably, but understanding weather is one of these things that is difficult. And what we see observationally, is both worlds have these stratified atmospheres, where they have different things going on at different altitudes. And with Jupiter, we have a world that is thermodynamically much more interesting, it is closer to the sun, it has a fast rotation rate, and it has these near by moons, and everything together is interacting and decupling all of these different factors to figure out what is going on, is a special numerical mass that many, many dissertations have been written on, and we still don’t have an understanding everyone agrees on.

Fraser:                         Now, to echo a question that I had last week, if you were to take a big knife and slice Jupiter in half and look inside, what would you see?

Dr. Pamela:                 So, in its very core, you have what, for reasons that annoy and baffle me, is referred to as a rocky core. There is not a single, rocky thing about this core, it is molten metals, people, molten metals.

Fraser:                         Right.

Dr. Pamela:                 We do not call the Earth’s core rocky; we call it molten. Jupiter’s core, higher pressures, higher temperatures, even more molteny than our core. So, okay, very center of Jupiter, as far as we know, hot, heavy metal, molteny stuff, heavy elements all sank down there. Above that, you have metallic hydrogen, and metallic hydrogen is hydrogen that is at such a high pressure, that it becomes capable of conducting electrons, just like a wire. And it is this layer of metallic hydrogen that is likely driving the magnetic dynamo of Jupiter.

And then, above that, you start to get to your normal hydrogen helium outer parts that has ammonia vapor, water vapor, all of these different things, leading to those different color storms that we’re able to see.

Fraser:                         Wow. Now, one of the questions that I get a lot, and I’m sure you get the same thing, is people asking what it would be like to fly into Jupiter, since it’s a gas giant. Couldn’t you just fly into Jupiter and just land on the surface, on that rocky, molten core?

Dr. Pamela:                 So, this reminds me of the question, “Well, why can’t we just go down and explore the bottom of the ocean? It’s just water, you should be able to just swim around, shouldn’t you?” And the problem in both situations, to differing degrees, is as you go lower and lower down, the pressure you have to withstand goes up, and up, and up. And it’s a bit more challenging to engineer things to maintain one atmosphere on the inside, and support tens of atmospheres on the outside, while also maintaining all of your heating and cooling, and everything else. So, sure, you could try, but you’re going to actually get crushed.

Fraser:                         Yeah, you’re looking at 100,000,000 times the atmospheric pressure of the earth inside Jupiter. If you were going to fly down, land on the surface, on that molten surface, which is say, 25,000 degrees Celsius, you would have –

Dr. Pamela:                 You have to go through the metallic hydrogen to do that.

Fraser:                         Through thethe metallic hydrogen, yeah, you’ve got –

Dr. Pamela:                 That’s not happening.

Fraser:                         You’ve got 100,000,000 times the pressure of the earth’s atmosphere baring down on you, which needless to say, is plenty. So, it is not a fluffy, hazy, glass cloud, that you just blow away and land on the rocky surface down below, it is hydrogen helium mashed into a soup, ludicrously hot, and high pressure.

Dr. Pamela:                 We’re looking more at cloud cities, floating cities. Build yourself a nice zeppelin, build a city on top of it, you’re good. It’s –

Fraser:                         Well, except that nothing is lighter than hydrogen, that’s the challenge. What do you float on when you’re floating on hydrogen?

Dr. Pamela:                 The trick becomes one of, as you go deeper through the atmosphere, the hydrogen becomes denser. So, you have to build a zeppelin that is able to withstand the crushing external forces to maintain its lower density hydrogen inside, and this starts to get into the land of crazy stuff. But the physics works, I could assign it as a homework problem, I just wouldn’t do so in an engineering class.

Fraser:                         Right. Now, I mentioned in my intro that we have now discovered literally thousands of extra solar planets, many of them are gas giants. What have we learned about gas giants across The Milky Way that surprised us compared to what we though we knew?

Dr. Pamela:                 I think the biggest surprise is they are everywhere. We originally, when we started finding planets in the 90’s, thought that we’d be finding planets around sun-like stars, that the smaller red dwarfs wouldn’t have planets, because they just weren’t big enough to have collected a disk of material around them that could form planets. We originally thought that the hottest stars out there wouldn’t have planets, because they were blasting their surroundings with so much light that it wouldn’t be possible to form planets, and we were wrong. My very favorite planet is called KELT-9b, this is a hot Jupiter that –

When I say hot Jupiter, I feel like I’m really underselling this world. It’s orbiting a blue giant, and its surface temperature, because it’s getting super-heated by this blue giant, its surface temperature isn’t all that different from the surface temperature of the sun, and it just –

Fraser:                         Right. It reflected light, it absorbed light from the star.

Dr. Pamela:                 Yeah. So, this super Jupiter is –super-hot Jupiter, is not long for this universe, its atmosphere is getting stripped away by the solar pressure baring down upon it. But this tells us that we can find planets around pretty much any kind of star, and that’s amazing.

Fraser:                         Well, the first planets that were discovered…well, apart from the ones around a pole star – Everyone, whenever I say planets, everyone goes, “What about the pole star planets?” Fine, the pole star planets, yes, those are the first ones. The first regular planets around regular stars, was of course a hot Jupiter. And you would have originally said, “Okay, gas giants need to be able to hold onto their material, gather up enough, accumulate enough material that isn’t either swirled into the star, or blown away by the solar winds.” So, how do you get a planet like a hot Jupiter, that is so close to the star, so hot?

Dr. Pamela:                 If I knew the details, I could probably get nominated for a Nobel Prize. But the not ent…

Fraser:                         Take a swing at it anyway.

Dr. Pamela:                 The not entirely detailed answer is, they migrate. And we keep getting hints of how this is happening. There’s actually a new paper that came out just this week, I think, where they have been looking at a series of discs observed by the anatomical large millimeter array, where we can see the rings of material with gaps that we believe are emptied out by planets. And one of the confusing things with all of the ALMA data, is when we see gaps, we expect there to be a planet in that gap, and we do not always find the planets in the gaps. We will periodically see these knots of material in the outskirts of the gaps.

And by doing a whole series of numerical models where they basically built a solar system in a digital box and said, “Okay, gas, dust, evolve, collide, form worlds, show us your stuff.” And in the process of evolving these solar systems in a box, they were able to see these different steps where it would go from having this pattern of gaps, to having this other pattern of gaps that look just like what we’re seeing coming out of ALMA. And they were able to see how over time, through multiple phases, worlds will migrate through these discs, changing the shapes of the disc, and changing their own location, and our ability to find them.

Fraser:                         I love this paper, I know exactly what you’re talking about, where you should see a planet – But we have seen planets in these rings, in these accretion discs around stars, but to then, also see these discs and no planets, you’re like, “Where did the planets go?” That’s got to be these planets migrating around.

Dr. Pamela:                 And so, the next thing that we have to figure out is why do we have solar systems like ours, where we know what there were originally more planets than we have now, where things were originally in different places than they are now. How do we end up with a solar system that ends up stopped in the format we’re currently in? How do we end up with solar systems that are just like, everything goes into the sun, you into the sun, you into the sun, you into the sun? And then, we see these intermediate systems that either have a hot Jupiter right next to its sun –51 Pegasus B, that hot Jupiter, it’s orbiting its star every four days, it’s far closer to its star than Mercury is to the sun.

 But then, we see other worlds where you’ll get three little, tiny, rocky, worlds, and then, a hot Jupiter still within that tens of days orbit. So, how do we end up with all of those systems, and then, have all this wealth of systems we’re finding that have ice giants and Earth like orbits, and we don’t know. It appears that there is this very particular set of initial conditions that allows solar systems to end up with a variety of different configurations.

Fraser:                         The impression that I’m really getting is that there was a lot more mayhem in early solar system history, than I think we have been led to believe. So, when you see the late heavy bombardment, might have been ten times more violent than astronomers have previously predicted. When we see planet forming discs devoid of the planets that formed in those discs, you start to wonder, was the early solar system –? Were there 100 times as many planets in the solar system? Must have a rough – Probably, but they merged and collided, and were kicked out, and were eaten by the sun, and were eaten by Jupiter, and so on and so forth, so who knows.

All right, now, we talked about hot Jupiter as one example of a planet of a gas giant that is different than what we have in the solar system. Are there other examples of gas giants that we just don’t have in the solar system, but still qualify?

Dr. Pamela:                 Probably not.Tens. You’re changing the topic too fast. You had the perfect opportunity, and I’m going to roll us back to it, to introduce Jupiter’s fluffy core.

Fraser:                         What?

Dr. Pamela:                 So, one of my favorite scientific results of the pandemic – All days are blursday, but this is a more recent discovery, it’s the realization that Jupiter’s core, this comes from Juno, its density profile has its core far larger than any of the computer models for a standard built up through the aggregation of materials model would allow you to have. And so, it’s thought that some, perhaps earth-ish, add multiples as needed, sized object, wacked the early Jupiter, and all of the energy of this collision fluffed out Jupiter’s core, and they actually used the word fluffy to describe Jupiter’s core.

And so, as we look around, we’re also seeing evidence that Saturn probably underwent collisions. And so, it’s a world-knock-world solar system out there, and we are able to find out that there were other worlds in our early solar system by the carnage they left behind.

Fraser:                         Well, fine. So, if you’re going to use the word fluffy planets, astronomers have found fluffy exoplanets.

Dr. Pamela:                 Yes.

Fraser:                         That’s a gas giant, something we don’t have in the solar system.

Dr. Pamela:                 There have been some gas giants, again, defined by what we believe they’re composed of, that are less than 10 earth masses in mass, and it’s believed that because they’re orbiting cold stars, they were able to end up – And we can see their radius, they’re like – I want to say they’re three to four Earth radii, so, we can see the radius as they transit in front of their star. We can get at their mass through all sorts of cool calculations, again, things I would assign as homework assignments. And when you put this together, you end up with that’s a gas giant that is measured in multiples of Earth orbiting a cool star, so, this is allowed to happen.

Fraser:                         How small can a gas giant get?

Dr. Pamela:                 So, that starts to get into limits on solar system formation. And what do you define as the limits on something like that? We’re still trying to figure out, “Okay, so, can a fragmenting collapsing molecular cloud just randomly decide this fragment is going to be a planet?” It doesn’t look like that happens; it looks like you need to have more substantial mass to generate the kind of collapse that leads to planets being formed. But then, you start getting down to, “Okay, so, you have systems forming where you have Jupiter’s, and is that planet-like formation, or star-like formation in binary systems where it’s a red dwarf and a Jupiter?”

We don’t know how small the process can go before things stop allowing things to form, because they’re able to just thermodynamically support themselves as not planets.

Fraser:                         Right. But you said a Jupiter, like a gas giant that is 10 times more massive than the earth, could you have one that is two times more massive than the earth, five times? If you have a ball of hydrogen, you can turn it into a gas giant. If you have a disc of hydrogen, and it’s say, around a dwarf star, can you get a fairly low mass, small, gas planet?

Dr Pamela:                  Yes. I don’t know the lower limit on it. It would probably be order of multiples of an earth mass, and the reason that I say that is because you need the rocks to come together to be able to draw all the hydrogen in, because it doesn’t take a whole lot of energy to send a hydrogen particle off at a scape velocity. So, you’re going to have to build up all of the rocks and heavier materials first to hold on to the lighter massed atoms.

Fraser:                         So, there’s a certain recipe that has to come together to get what you need. Still, that’s pretty fascinating. Well, super cool, Pamela. We’ll talk about gas giants again in another 600 episodes, or so.

Dr. Pamela:                 That sounds good Fraser, I will be here for that, and hopefully we’ll have more spacecraft with us.

Fraser:                         Yeah, we will, we will. All right, thanks Pamela.

Dr. Pamela:                 Thank you so much, Fraser. And thank you to all of you who support us through Patreon. This week, our episode is brought to us by Thomas Sepstrup, Burry Gowen, Steven Veit, Jordan Young, Kevin Lyle, Jeanette Wink, Mountain Goat, Andrew Poelstra, Brian Cagle, Venkatesh Chary, Dravid Truog, TheGiantNothing, Aurora Lipper, Joe Hook, David, Gerhard Schwarzer, Bill Hamilton, Jean-François Rajotte, Cacoseraph –

Cacoseraph? It looks like chocolate angel translated, Bill Hamilton, Laura Kittleson, Joshua Pierson, Robert Palsma, Les Howard, Joe Hollstein, Jack Mudge, Gordon Dewis, Adam Annis-Brown, Sean Martz, Helge Bjørkhaug, Frank Tippin, Alexis, Ben Lieberman, William Baker, WandererM101, William Andrews, Jeff Collins, Travis Dalziel, Harald Bardenhagen, Matthew Horstman, David Gates, David, Phillip Walker, Nicky Lynch, Scott Beiber, Alex Cohen, Brian P. Cox, Justin Proctor, and Paul Hayden. Thank you all so very much. We are able to support Beth, Nancy, Richard, Allie, everyone behind the scenes, because of you. And those humans really keep Fraser and I on the straight and narrow, and we need that help.

Fraser:                         Thanks everyone.

Dr. Pamela:                 Bye.

Narrator:                     Astronomy Cast is a joint product of Universe Today and The Planetary Science Institute. Astronomy Cast is released under a creative comment’s attribution license. So, love it, share it, and remix it, but please, credit it to our hosts Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this show going, please consider joining our community at Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. 

                                    We are so grateful to all of you who have joined our patreon community already. Anyways, keep looking up. This has been Astronomy Cast.

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