Sarah Al-Ahmed:
A long-standing Uranian mystery gets an update, this week on Planetary Radio. I’m Sarah Al-Ahmed of The Planetary Society with more of the human adventure across our solar system and beyond. For decades, Uranus has baffled scientists. Voyager 2’s 1986 flyby suggested that the ice giant wasn’t radiating any extra heat, but new research has finally cracked the case. I’ll talk with Michael Roman, assistant professor at the Universidad Adolfo Ibáñez in Chile, and co-author on a new paper showing that Uranus really is giving off more heat than it receives from the sun. We’ll also celebrate the discovery of a brand new moon around Uranus, in this week’s What’s Up, with our chief scientist, Bruce Betts. If you love Planetary Radio and want to stay informed about the latest space discoveries, make sure you hit that subscribe button on your favorite podcasting platform. By subscribing, you’ll never miss an episode filled with new and awe-inspiring ways to know the cosmos and our place within it.
Uranus has a reputation, partly for its name that’s the butt of endless jokes, but mostly because it’s just plain weird. This ice giant spins on its side, tilted nearly 98 degrees. That tilt gives it some of the most extreme seasons in the solar system, with decades of daylight at one pole, while the other one is locked in darkness. Its atmosphere is strangely bland compared to Neptune’s stormy skies, and yet it hides mysteries that we still don’t fully understand. One of the strangest puzzles was uncovered by Voyager 2 in 1986. Unlike Jupiter, Saturn and Neptune, which all radiate more heat than they received from the sun, Uranus appeared to be in perfect balance, giving off no excess heat at all. We would say that this world seemed to be in thermal equilibrium. That made it a definite oddball among the giant planets raising big questions about its history and interior.
We expect our giant planets to radiate more energy than they receive from the sun because they’re still slowly cooling off from their formation, leaking leftover heat from their interiors into space. Now, new research has finally rewritten this story, the paper called Internal Heat Flux and Energy Imbalance of Uranus, which was published in the Geophysical Research Letters on July 14th, 2025, by lead authors, Xinyue Wang and Liming Li, confirms that Uranus is indeed radiating more heat than it receives from the sun. It’s still less than the other giant planets, but definitely more in line with what we would expect from this world. Their work is also reinforced by other recent studies, including by a team led by Patrick Irwin at the University of Oxford.
Joining me now to explain what this means for Uranus, for ice giants in general, and even for our understanding of exoplanets, is Dr. Michael Roman, assistant professor of physics and astronomy at the Universidad Adolfo Ibáñez in Santiago, Chile. He’s one of the co-authors on this study, and an expert on planetary atmospheres, especially when it comes to ice giants like Uranus and Neptune. Hey Michael, thanks for joining me.
Michael Roman:
Hi. Yeah, it’s a pleasure to be here.
Sarah Al-Ahmed:
Or should I say bienvenido, you’re coming to us from Chile, right?
Michael Roman:
Yes, from Santiago. I started just recently here at the university on the eastern side of the city, Universidad Adolfo Ibáñez, where I’m an assistant professor.
Sarah Al-Ahmed:
So, you spent most of your career studying the atmospheres of Uranus and Neptune, what first drew you to these ice giants?
Michael Roman:
I suppose, Uranus and Neptune, they kind of occupy a space where they’re at the very edge of our solar system as far as the giant planets are concerned, and they are far less studied than Jupiter and Saturn, so there’s a lot of mysteries that remain about them. They’re sort of enigmatic in that case. But they’re close enough where we can actually see them through telescopes and resolve clouds on their atmospheres. So, yeah, I think they’re just sort of mysterious outer worlds, that we’ve had some idea about, my entire life, and I remember first seeing those Voyager images as a child, and we’re at a position, I suppose, where we really can start to learn a lot about them just due to technology advancing, and telescopes advancing in the last 20 years. So, for me, it’s an exciting place to study, where there’s opportunity to learn a lot about some worlds we’ve known about for a while, but haven’t been able to really explore as much as we would like.
Sarah Al-Ahmed:
It just kills me that we’ve only ever flown by these worlds once with the Voyager missions. It was so long ago, and I’m really grateful that now we have instruments like JWST to give us a closer look. What was it like to finally see those images of Uranus and Neptune through JWST’s eyes?
Michael Roman:
Oh yeah, that was incredible. I remember some of my older professors talking about their time in the Voyager days, and the excitement of seeing these planets for the first time, and I don’t think anything will ever really catch that same sort of initial excitement of seeing volcanism on Io, and the close-up look of Neptune and its clouds and all that for the first time. But I feel like I got a little hint of it perhaps, of seeing the first spectra from JWST of Uranus and Neptune, because they’re challenging targets, we’ve been trying to observe them from ground-based telescopes for years, and we get some data, but often it can be quite noisy, and to see the precise clean data, that looks almost like a model in some cases, from JWST, given its sort of amazing sensitivity, it really was something I had been waiting years to see. And when it came down, and I was in a room with Lee Fletcher, and some of my other colleagues, and we’re looking at it for the first time, it was exciting. It was really one of those moments I don’t think I’ll forget.
Sarah Al-Ahmed:
Really though, it feels like, depending on how the future plays out, there are a lot more nations that are trying to prioritize potentially missions to Uranus someday, and the results coming out of studies like the one that you and your team have done, but also from all the data coming out of JWST, I hope reinforces that incentive for us to go back to these worlds, because there’s just so much we don’t know.
Michael Roman:
Oh yeah, absolutely. As you say, Voyager was the only time we really got a close look at these planets, and that was 1986, for Uranus. And there’s only so much you can figure out from far away, from 20 times the distance between Earth and the sun. So, you need to get close to really learn a lot about these planets, and it’s a logical next step in our exploration of the solar system I’d say. We got Jupiter studied pretty well, Saturn, we had the amazingly successful Cassini mission, now it feels like it’s Uranus’ turn.
Sarah Al-Ahmed:
Well, can you tell us a little bit about your journey? You’ve gone from Cornell, to Michigan, to Leicester, and now to Chile, how has that shaped your approach to planetary science and international collaboration?
Michael Roman:
I suppose it’s given me different perspectives on the world, just living in different places, in a way that’s sort of a perk of the job, to be able to, as a postdoc, have a reliable income for a few years in a new place, and meet new people. I worked on some different things, in Michigan it was more exoplanets than solar systems, so it gave me that sort of perspective on the astrophysics and just planets in general. There’s a great diversity of planets out there, our solar system is just one subset, and in some ways maybe kind of an odd subset of planets. But so to be able to work with different people on different topics over the years has been a great privilege.
So, yeah, I’ve enjoyed the process of investigating with different collaborators around the world to some extent. At the same time, yes, of course it has… I don’t know where my home is exactly, in essentially [inaudible 00:08:02], I feel a little divided, and it’s sort of a nomadic lifestyle, but yeah, it is what it is. But the planets, they’re there to kind of give me focus as I look from different perspectives.
Sarah Al-Ahmed:
For decades, Uranus has presented this kind of heat mystery. Early Voyager 2 observations, and you said they were in 1986, which is what, 39 years ago? They suggested that Uranus wasn’t emitting this excess heat that we expected from other gas giants. Could you talk a little bit about this long-standing puzzle, and why it’s perplex planetary scientists for so many decades?
Michael Roman:
Yeah, sure. General, Uranus is a bit of an oddball as a planet, it’s the coldest planet in the solar system, and it’s about as cold as any planet we really know of. It gets down to around 50 Kelvin, and so this is colder than the temperatures you see on Neptune, which is farther from the sun. So, there’s already something a little weird there. Uranus, as I guess we’ll talk about in a little more detail, is unlike all the other planets, it’s tilted on its side, essentially… Earth’s axis is tilted about 23 and a half degrees or something, so that it gives you the seasons. Uranus is tilted and it’s something close to 98 degrees, 97.77 or something like this degrees, and as a result, you get these many to high latitudes fall into decades and decades of darkness followed by decades of light.
So, you get these cold, dark winter nights, which last 30, 40 years near the pole, and then on the other side, again, daylight for decades. And so, this is sort of extreme setup in terms of sunlight falling on the planet. And even though that sunlight is 19 astronomical units out there, and the edge of the solar system, that sunlight is something like 370 times weaker than we get on Earth. It still adds up, it still provides energy to the planet. But what’s sort of odd, I guess, about Uranus compared to the other planets, aside from these other facts, is that all the other giant planets, Jupiter, Saturn, and Neptune, if you look at the amount of heat that comes off those planets, the amount of heat that they radiate, it exceeds the amount of heat they actually absorb from the sun.
As I say, they’re not an equilibrium, the amount of energy falling from the sun, enters the atmosphere, and it warms the atmosphere, if it was perfectly an equilibrium, the amount of heat it would give off would be equal to that amount. So, it’s not heating or cooling over time, it’s just getting a certain amount of energy from the sun, and then radiating that same amount of energy off into space. But it turns out Jupiter, Saturn, and Neptune all radiate excess heat. And so, it must come from some internal heat source, some sort of theorized that, essentially, primordial heat from their formation or their contraction, or the way they’re still separating, differentiating over time, they’re still evolving, and as they’re evolving, they’re losing energy still, they’re still losing heat and cooling down. And so, we have determined this for the planets to occur, but for Uranus, it seems the case is somewhat different.
When we did the same sorts of studies, when I say we, scientists back in the Voyager, did the same sort of studies, they realized that the amount of heat that Uranus gave off really didn’t exceed the amount it was receiving from the sun by much, if at all. And so, within [inaudible 00:11:34] , it seemed like it was perhaps in equilibrium with the sun. The amount of energy it emitted seemed to be about equal to the amount it received, which was very different than the other planets as we’ve said. And I think the other planets, in the case of Jupiter, something like 1.67 times as much energy is emitted than it receives, Saturn is 1.78, and Neptune, estimates are something like two and a half times, or 2.7 times as much energy is emitted than what it receives. So, there’s a lot of internal energy coming from its interior that’s escaping and being lost to space.
But for Uranus it’s something close to one, where basically it’s the amount of energy it receives is equal to the amount of energy it emits. To make these measurements is not easy, you can’t do it necessarily from Earth alone, because you need to know how much energy is entering the atmosphere, and how much is being scattered back. And from Earth, if you look at Uranus, you’re only really seeing the side of Uranus that is facing Earth in the sun. And so, you know how much light is being scattered directly back towards you, but you don’t know how much light is being scattered in other directions, unless you’re observing Uranus from those other directions. And so, as a result, from Earth, you can only say something about its geometric albedo as we call it, and you don’t really get a full picture as to how much of that sunlight is actually being scattered in other directions.
And so, to get that measurement, you need to observe the planet from a different range of observing geometries, different phases as we say, and the only way to do that is to have something like a spacecraft go out and observe it at these different angles as it’s, in the case of Voyager, flying past Uranus. So, what the Voyager mission did was allow us to get these observations of all the giant planets at different phase angles, over a range of angles, to see how this light from the sun is scattered off into different directions, basically just an accounting problem, the amount of energy in versus the amount of energy out.
Sarah Al-Ahmed:
But what did Voyager actually tell us about how much heat was escaping the planet into space?
Michael Roman:
Well, you need to be able to look at the thermal emission from the planet, from all positions on the planet, and so again, with the same problem, if you’re looking at Uranus from Earth, you’re only seeing the heat that’s escaping from Uranus in the infrared directed towards Earth. And in order to know how much heat’s being lost all around the planet, you need to be able to look at it, what’s going on the other side. From Earth, you can’t tell how much heat is escaping on the dark, cold night side of Uranus out into space, you just have no idea from looking at Earth alone. And when Uranus is being tilted on its side, when it’s rotating, you’re not getting any of that night side into your field of view over the course of a day, it’s blocked from your view for 30, 40 years.
So, it’s really, it’s a mystery is what’s going on back there without having a spacecraft go right behind and have a look. And so, what Voyager did flying by, gave us those measurements of how the light is scattered as a function of angle going past the planet, but also how much heat was actually escaping from the night side of the planet. And so, Voyager, with this infrared spectrometer was able to measure the thermal emission… I should say, when it flew by in January 1986, I think so we are coming up on just the short of 40 years, it was summer solstice, near summer solstice on Uranus for the hemisphere facing Earth. You had half the planet was in winter solstice, and that winter solstice meant that we couldn’t really see what’s going on there without Voyager. And so, Voyager was able to measure the planet at this time of solstice, for one side is summer, one side is winter.
And what it found was the temperatures were not all that different between the two hemispheres, roughly it was symmetric, or you had sort of warm at the equator and then colder at mid-latitudes, and there was a little difference between the hemispheres, maybe something on order of just a few Kelvin. But it wasn’t dramatic, it wasn’t extreme, which is sort of surprising when you think about… You might expect that that side that hasn’t seen the sun in 30 years, just cooling off, radiating heat, and would be much colder than the day side which is getting baked by sunlight. Weak sunlight out, at that distance in the solar system, but is still getting irradiated, you might expect a significant difference, but in fact it really didn’t seem to be much of a difference. Which has implications, it means that energy is being deposited on one side, not on the other, by the sun, and the fact that they don’t differ by that much in temperature means that that energy is being redistributed somehow, through winds at some height in the atmosphere to equilibrate and give it sort of an average temperature.
But back to the point, is that once you know how much heat is… How much solar energy is coming into the planet, taking into account that that’s scattered off in all different angles, versus that which is absorbed, you can come up with what is called a bond albedo, and that’s what these scientists guys, like Pearl back in the Voyager era, did to determine what the energy balance was of the planet. So, when they do that for Jupiter, and for Saturn, and for Neptune, you find the amount of energy greatly exceeds that which it receives from the sun. So, there’s an internal source of energy that’s quite significant, that’s contributing to all the energy lost into the space.
And that is to say that these planets are losing energy, you can think about maybe in terms of evolution, if the planet’s still cooling down and contracting and it’s not yet reached its sort of steady state, but when you do the same one for Uranus, they found that, well, unlike the others, it doesn’t seem to have much excess energy. It seems that within error bars basically, from those original Pearl paper back in the 90s, that the amount of energy escaping from Uranus was within error bars consistent with the amount that’s being received, statistically significant with it being in equilibrium. And so, that was weird because Neptune, which is a lot ways similar to Uranus, in size, roughly similar size, roughly similar mass, roughly similar composition, out there on the edge, its sister ice giant is giving off more than two and a half times as much energy.
So, something’s weird about Uranus, and people over the years have speculated, well, what’s going on? Did we catch Uranus at some strange time in its history? Is it a clue the fact that Uranus is tilted on side? Perhaps this all is due to one very violent, dramatic collision early in the history of Uranus, that knocked it on its side, but also maybe stirred it up, or mixed in a way, or caused it to dispel some of this internal heat that the others are now just slowly radiating away. So, it’s been a mystery for a while.
Sarah Al-Ahmed:
How does that impact the level of mixing in the atmosphere?
Michael Roman:
It contributes to sort of Uranus as being this kind of oddball strange planet. Without that internal heat escaping, the atmosphere then becomes very… You don’t have that extra source of heat from below, and that’s part of the reason Uranus is very cold, and you don’t get the same sort of mixing that we see on Neptune. Neptune, if you look at the atmosphere on Neptune, there’s a lot more methane up high in the atmosphere, because it seems like it’s being mixed up higher. It’s there, therefore it must be getting mixed up higher, whereas on Uranus, you don’t have that same amount of methane up high, which is consistent with it essentially not being mixed upwards.
Sarah Al-Ahmed:
But what role does methane actually play in Uranus’s energy balance, or its photochemistry?
Michael Roman:
If you don’t have that internal heat leading to that mixing, then Uranus ends up being drier up high, and you don’t have this as rich photochemistry that occurs when methane interacts with sunlight, produces all these other hydrocarbons, the methane gets [inaudible 00:19:41], you end up with carbons and hydrogen floating around, and they recombine, they form all these different hydrocarbons, like ethane, and acetylene, and benzene, and these sorts of things, that then, themselves, are very effective at radiating heat. And so, they affect energy [inaudible 00:19:59] planet too. So, it comes to this complicated picture where an interaction between sunlight, and chemistry, and heating, and where gases are, and internal mixing all comes together to give you this complicated picture as to how the planet evolves, and what sort of composition and temperatures it has. But Uranus, for whatever reason, just has this different tilt, has this lack of internal heat, and it leads to it being cold, and maybe less vigorously mixed, and maybe from an observational point of view, a little more quiescent than the other planets.
Neptune, you’re probably well familiar, you see these pictures of clouds moving along Neptune very quickly. In Uranus you have some clouds, but you don’t have the same dynamic, rapidly changing, and frequently seen clouds as you see on Neptune. And we’re starting to unravel this, learn about this, JWST is providing some insight into this. But numerous studies over the last 20 years have really been helping to give us more information on these planets. They’re challenging targets because they are far away compared to Jupiter and Saturn, Neptune is smaller than the great red spot in the sky, than Jupiter. So, it’s a tiny thing. And so, you need a big telescope in order to observe it, and really, we only had telescopes that were big enough to observe it well, with adaptive optics, since maybe 2000, around year 2000, late-1990s, when you start getting these adaptive optics.
The data I worked on in graduate school for my thesis was data from the Palomar 200-inch, the five-meter telescope, which is a telescope from the 1940s and 50s, but adaptive optics, that allows you to correct for the seeing in the atmosphere, was a game changer. It allowed us to really see details on the planet from the ground for the first time. And that only occurred, yeah, in the early 2000s really, you had data like that. In the 1990s you had Hubble, and those gave us some nice views of a Uranus and Neptune for the first time, that you can see the structure in the atmosphere. But those JWST images show for the first time, and if you look at Uranus over time, you start to see there’s a very seasonal cycle to it, because it goes around the sun every 80, 84 years, which means that each season is around 20, 21 years long on Uranus.
Sarah Al-Ahmed:
God, that’s so long.
Michael Roman:
Yeah, it’s a long time. But don’t get me started on Neptune. Since that tilt, you get these very extreme seasons, again where you are having 20 years where it’s summer, and the pole is basically facing towards the sun, as we were talking about before. But what we found is when you observe Uranus over time, there is a cyclic pattern to its brightness. There’s a great set of data, a guy named Lockwood was observing Uranus and Neptune from Flagstaff, from the Lowell Observatory since the 1950s, I think, and gave this long period of just annual observations looking at Uranus and Neptune, how they looked each year near their opposition, near where they’re highest in the sky, just recorded their brightness, and when you looked at this over time, you found trends where cyclic almost, where Uranus would get darker and brighter, and darker and brighter.
And it really wasn’t clear why this was happening until we really were able to resolve the planet with these bigger telescopes, with Hubble, and then things like Palomar, and the big Keck, and all these telescopes that came on in the last 20 years, that showed that there’s a big difference between Uranus’s low latitudes and its high latitudes in visible light. You see towards the high latitudes in Uranus, which I mean latitudes 45 degrees and north, or 40 degrees and north, it is brighter, it is more reflective than it is near the low latitudes. For reasons we have come to understand, it’s due to the combination of the high latitudes, they seem to have more cloud, there’s clouds around one and a half to two bars, that just seem to be thicker at those high latitudes, and they just reflect more light.
And secondly, very interestingly, if you look at the amount of methane in the atmosphere, and methane is a sort of dominant absorber in Uranus’s atmosphere, you look at the amount of methane, it varies significantly from the equator to the pole. Near the equator you have, when you go down to around a bar level… So, in terms atmospheric pressure, you talk about sort of stratosphere up high, these millibar pressures, and then you go down to around a bar, which is basically around the surface pressure on Earth. And then you’re in the tropopause, where the weather layer, where things are mixing in the atmosphere, in general, and that’s where the thickest clouds are, or hazes are, on these planets. And Uranus… It’s also where methane becomes more abundant. And this is because on Uranus, and the very cold region at the tropopause, where temperatures kind of reach their minimum, the amount of methane just condenses out. You just can’t have a lot of it.
And so, if you look at the amount of methane in the atmosphere of Uranus, it’s some small trace amount, 10 to the -5th, something like this, order of magnitude. As they go down and it gets warmer, you can have more methane, and then you get down to several percent of the atmosphere by mixing ratio, by volume, is methane. And near the equator it seems, there was a paper, I guess in the mid-2000s, 2000 maybe 11 or 2009, I forget. But Erich Karkoschka, one of the great scientists in our field, using some data from Hubble Spectra was able to determine that the amount of methane near the pole differed from the amount of methane near the equator in the tropopause, and the deeper party atmosphere. Something like three or 4% near the equator, and down to 1% or 1.5% near the pole.
So, a factor of two, a significant difference. And what means though is that since methane is the strongest absorber in this atmosphere, the high latitudes have less methane and quite dramatically, changes quite abruptly at these mid-latitudes, and as a result, high latitudes, they’re cloudier and there’s less absorption, so they appear brighter. And the low latitudes in your equator, they have more methane, more of the sunlight’s absorbed, and there’s less clouds, so they appear darker, less albedo is the term we’d sort of use, less reflective. And so, Uranus has reflective poles and less reflective equator, low latitudes. And so, when Uranus, since it’s tilted on its side, there’s points where it’s poles are facing towards you, and points where it’s equator is more or less facing towards you, and then the other pole, and this cycles back and forth over the course of its 84-year orbit.
And so, that leads to some variation in how bright Uranus appears from Earth. And so, what that means is that the amount of sunlight Uranus receives depends on how far Uranus is from the sun, but the amount it absorbs going to depend on how much it’s reflecting back. And so, sometimes in its solstices, it’s going to be reflecting more, and near the Equinox it’s going to be absorbing more. And so, you end up with this change in the amount of heat, that’s sunlight that’s being absorbed in the planet. Also, it turns out, Uranus has an eccentric orbit, it’s not perfectly circular orbit, and so as a result, when it’s at its closest to the sun, the planet’s larger in the sky, it’s subtending a larger arc, and it’s therefore intercepting more sunlight and it’s absorbing more sunlight.
And when Uranus is at this apogee, at the farthest point away of its orbit, then it’s receiving less sunlight. It turns out this is quite a dominant factor, this is actually maybe more important than the change in albedo, just due to the clouds and methane and the orientation of the planet. So, then the question is, how does the amount of energy that it emits over time vary over time? So, that’s what the lead authors, Xinyue Wang and Liming Li looked at in this paper, was basically how the energy balance of Uranus, given the amount of sunlight falling on it, the amount of sunlight being absorbed by it, and the amount of heat it’s radiating away, how it all balances and how that balance changes over time, and that was sort of the crux of this paper.
Sarah Al-Ahmed:
Was it the fact that you had so much time to look at this world over the decades that finally allowed us to realize that it wasn’t in thermal equilibrium? Was it just a matter of getting more data?
Michael Roman:
Right, I think that’s precisely the case. And I guess also critically is just in recent years they’ve recalibrated, one of the other authors, Daniel Wenkert, he went ahead and reanalyzed some of the Voyager data, and had found that some of the original estimates for the energy is a function of different phases, how it scattered over different angles, could be improved. And he came up with a revised number, and never revised number was significant, and that sort of changed things to push in favor that maybe it wasn’t quite so close to equilibrium as previously thought. So, the question is, now have a better measurement, a better idea of how sunlight falling on Uranus heats it up over time, how much of that energy is absorbed versus how much is scattered as a function of Uranus’s orbital period and season. So, better accounting, the simple accounting, adding up photons absorbed by the atmosphere over time to give us the energy coming in.
And so, now, we know how the input energy from the sun changes in time, how does the output energy from the atmosphere change in time? It turns out that in the last, only in the last 20 years, technology from the ground has allowed us to make infrared measurements of Uranus with some accuracy that was just not possible before the early 2000s. And so guys like Glenn Orton, over at JPL, have been making infrared measurements of the giant planets using big telescopes to measure the thermal emission in the mid-infrared. Most of the energy emitted from Uranus and Neptune, given your cold temperature, is going to be in the far infrared, but the mid-infrared is more easily accessible from the ground, and we can get a sense of, at least sample how much heat’s escaping in some of these wavelengths.
And from that we can relate it to the amount of energy given off in total, just through some careful relationships between that and this paper, Liming and Xinyue have looked at, to say how you can relate the brightness temperatures we call it, these observations of thermal emission from the Earth, to sort of total amount of energy just getting from the planet. Over the years, we had some observations collaborating with Glenn Orton, Lee Fletcher, myself, did infrared observations of Uranus, during the 2000s and again in 2018, and again in a few months time I’ll have some new observations from the VLT, these are using telescopes that are eight and a half meters or so, and diameters are large and they can resolve Uranus, and they’re up high in mountains, and here in Chile, and they can get some pretty good measurements of the thermal heat escaping Uranus reaching Earth.
And with these measurements, with some extrapolating, using this sort of relationship between brightness temperatures observed from Earth and what we saw, for example, Cassini in Saturn, the authors of this paper, we were able to infer what the global thermal emission coming from Uranus was over time. And what we had found, well, there’s a couple things we found. [inaudible 00:32:04] in a paper in 2015, and then a paper again in 2020, Glenn and I looked, Glenn Orton and I looked at the thermal emission we sensed from Earth, of Uranus, and we compared that to what Voyagers gave off, and if you put into, say, the same circumstances, the same geometry, if you’re looking at them, I’d say comparable, it turns out they were pretty much exactly the same. They didn’t vary at all. Which is to say over 30 something years, and a whole season on Uranus from solstice to spring, the atmosphere didn’t really change much in temperature. Let’s just say it seems to be rather invariant over time, at least unto the measurement uncertainties.
Sarah Al-Ahmed:
We’ll be right back with the rest of my interview with Michael Roman after this short break.
Jack Kiraly:
This October, NASA needs you. Hi, I’m Jack Kiraly, director of government relations at The Planetary Society. In response to unprecedented proposed budget cuts to NASA Science programs, The Planetary Society and a coalition of our allies and partners are organizing a special day of action to save NASA Science. Join us in person on October 5th and 6th in Washington D.C. You’ll receive training on effective advocacy from our team of space policy experts, then head to the Hill to meet directly with your representatives in Congress to advocate for protecting NASA’s science budget and ongoing missions. If you can’t come to Washington D.C. you can still pledge to take action online. We’ll give you the resources you need to be part of the movement to save NASA Science. This event is open to any US resident, no experience required. Space science benefits all of humanity, let’s stand together to protect it. Registration is open now at planetary.org/dayofaction. We’ll see you in Washington.
Sarah Al-Ahmed:
Well, that sounds like a little bit of a paradox, right? Uranus has these huge seasonal swings and sunlight, but the temperature hardly budges. What does that tell us about how the atmosphere works?
Michael Roman:
This implies that the time scales for the atmosphere to respond to changes in the amount of sunlight coming in are really long and maybe longer than the time scales of a season and an orbital period of Uranus. So, that even though you’re heating it up one time and then putting it in complete darkness for a while, that doesn’t happen for a long enough period of time to actually cause a change on the planet in terms of its temperature. It seems that there’s very little, despite the great seasonal variation on Uranus, where you have decades of constant sunlight and decades of constant darkness, doesn’t seem to have much effect on the temperatures, at least at these heights in the atmosphere, at these pressures that are most strongly radiating, emitting their heat to space.
So, the planet just doesn’t seem to have much in terms of seasons, in terms of temperature swings, despite our sort of… And I say, intuitive, naive expectation, is that you have this planet and it’s going to be pretty extreme, right? You expect it to be freezing, and then getting cooked on one side, but it just doesn’t seem to be the case. It seems that the amount of sunlight falling on it varies greatly over the course of the year. Complete darkness, complete daylight, a eccentric orbit leading to changes over the course of its orbit of the amount of sunlight it’s receiving.
Sarah Al-Ahmed:
Different albedo even.
Michael Roman:
Yeah. So, the albedo and its relative distance from the sun changing, that’s leading to a variable amount of solar energy being deposited into the atmosphere of Uranus. Yet, from the thermal point of view, from the amount of energy escaping, it doesn’t seem to be any statistical variation in the amount of energy that actually escapes from the planet. So, the amount coming in varies, the amount going out doesn’t seem to vary much, and so that implies that energy being deposited from the sun is being redistributed in a way, such that you’re ending up with a structure that is insensitive to the seasonal swings in sunlight.
Sarah Al-Ahmed:
That seems really weird when you think about it, because what we’re saying here, essentially, there’s not a huge amount of mixing going on because while it’s not in thermal equilibrium, it’s close, and now it’s taking a long time for that heat to go around the world, but in such a way that it’s just keeping static between seasons. That is so weird.
Michael Roman:
It is weird. And I guess to also to point that this newest analysis, these new studies with these new measurements and the things that Pearl, that weren’t available back when this first study came out, the revised quantities of this phase function, revised quantity of albedo, and that has allowed for us to revise this energy balance. And what we found is that with smaller error bars also is that the amount of energy that’s emitting is actually in excess of the amount it receives. So, whereas Pearl’s measurement was maybe slightly in excess, but error bars were indicating that it is still statistically consistent with being perfectly in equilibrium with [inaudible 00:37:45], with the amount of energy receives, this new study says no, actually when you look at this a little more carefully, with this new data, in fact, Uranus is giving off more energy than it receives. So, it is closer to, more like the other planets, in that has an internal heat flux that is escaping, and the planet is in fact cooling off still over time.
And so, ends up becoming an interesting thermodynamic problem because it’s basically, two things. Now, we know there is some source of internal heat that’s escaping from the planet, that internal heat we can maybe presume is constant over time, but we don’t really know, and the amount of heat that is falling on a planet we know is not constant over time, but it’s being distributed very efficiently, such that you’re not getting these extreme variations in temperature over time, which is all pretty weird. I should also just shout out, there’s another paper that came out basically at the same time, by some of my other co-authors, Pat Irwin leading this one.
He did a very similar analysis, in that case they did some modeling of what the scattering would look like, using a theoretical model, a ready to transfer model, of given what we know the clouds are like, the hazes are like, and the gases are like in the atmosphere, constrained by observations, given that if you take this model of the atmosphere, and its gases and its clouds and hazes, and you shine light on it and you look at how it’s scattered over time, it gives you a numerical model simulation as to how this light scatters over time.
As opposed to what we kind of did in this study, was to look at observational data and extrapolate [inaudible 00:39:31]. But what they found is very consistent, pretty much the same thing. Within the error bars, more or less agreeing, there is an excess heat flux, internal heat flux from Uranus, contrary to what we for the last 30 years have been saying to be the case, based on these original Voyager studies. So, it’s revised our understanding of the heat flux. It’s still really small, and it still needs an explanation, so it doesn’t sort of make everything right and place Uranus right in line with the other planets, it’s still a weirdo planet that needs some explanation. So, it’s a privilege to have been involved in this work.
Sarah Al-Ahmed:
And really wonderful that we now have a long enough timeline on this world that we can try to piece some of this together. But I am wondering, given what we know now about the seasonal variation, is there an optimal seasonal configuration that you would like to see for us to then actually have an orbiter around Uranus during that time? Because we could conceivably go back during same configuration that Voyager had for us and maybe not learn as much.
Michael Roman:
Yeah. So, Voyager looked at it was near solstice, it would be great to look at it near equinox, and to be honest, as far as our options, solstice is coming up, the next solstice is coming up pretty soon on Uranus, in 2030. And so, we are not going to get a spacecraft there by then, so a good starting point or a good target would be to observe around equinox. That way we can see both hemispheres in daylight and get a good view of the planet illuminated at all latitudes, as opposed to not being able to see invisible light, what’s going on on the night side, for example. So, yeah, I think these sorts of seasonal milestones, the solstices, the equinoxes are a nice time because you can learn a lot about the planet at these periods. And so, having a spacecraft that, say, launches sometime in the next 10 years, gets to Uranus near the solstice, near 2050, so we can cross our fingers on something like that, but…
Sarah Al-Ahmed:
Yeah, it gives us a good reason to be patient for it, other than just waiting for budgets to clear up, right? But also we’re in this interesting phase in our exploration of the universe, where we’re right on the board of almost 6,000 exoplanets at this point, confirmed. We’re getting there, we’re not there yet, but we’re going to be there soon. And so, in some ways I’m really glad that we have this world as an outlier. For whatever reason, maybe it’s tilt, maybe… Who knows what’s actually creating the situation. But that gives us another world that we can look at to compare against some of these other potential ice giants in other exoplanetary systems.
Michael Roman:
Yeah. A lot has been said, when you look at these statistical studies, these population studies of exoplanets, the most common size planet is something in the neighborhood of Uranus and Neptune. Basically, it’s something close to that, several Earth masses, that Uranus and Neptune are basically are the closest things in our solar system to the size of the maybe most common size of exoplanet. So, to some of us it makes Uranus and Neptune even more compelling targets because they may be in some way representative of a class of planet that is extremely common across the galaxy, and here they are right in our own backyard. And so, Uranus and Neptune have the potential to give us a greater insight that could be extrapolated, or greater insight into what may be going on on these other planets out there, that are, I want to say, hopelessly far away for this type of detailed studies. So, yeah, I’m optimistic, I’m hoping that I’ll see a Uranus mission someday, because that would be a culmination of a lot of work, and something that I just would love to behold.
Sarah Al-Ahmed:
Well, I know the United States is talking about it because it is one of the priorities in the Planetary Science Decadal Survey. But even if that doesn’t pan out, there are some plans from the European Space Agency, there’s some plans from the China National Space Administration, so fingers crossed, I believe it, we can hold out, we’re going to see this mission sooner or later.
Michael Roman:
Yeah. Humanity will get to Uranus someday, hopefully sooner than later, and hopefully by equinox at least, because yeah, that’d be great to see. These planets are dynamic, things are changing, seasons in Uranus are changing as we’re approaching solstice. I think it’s a wonderful time to be studying Uranus and Neptune science, which is why I sort of have been obsessively working at it over the past few years, and I hope to continue in the years ahead. But yeah, it’s a privilege. I hope the community agrees, and Congress agrees, and we can start gear up towards the next big mission to look and discover things about it that we’ll never learn any other way.
Sarah Al-Ahmed:
Well, for the people who want to learn more, I’ll be sharing this paper and all of the other papers mentioned in this conversation on the website for this episode at planetary.org/radio. Thank you so much, Michael.
Michael Roman:
Take care. Thank you.
Sarah Al-Ahmed:
See, now I’m doubly convinced that we need a dedicated mission to Uranus. There’s so much left to learn. For example, we just discovered a new Uranian moon. Using the James Webb Space Telescope, a team led by Maryame El Moutamid at the Southwest Research Institute has spotted a previously unknown moon orbiting the ice giant, designated S/2025 U 1. It’s the tiniest and faintest Uranian moon that we’ve discovered so far. The object was detected in February 2025, in a series of long exposure images from Webb’s near infrared camera. Even Voyager 2, which gave us our first close up look of Uranus nearly 40 years ago, completely missed it. This brings Uranus’ known moon count to 29. We’ll talk more about that next, in What’s Up, with our chief scientist Dr. Bruce Betts. Hey, Bruce.
Bruce Betts:
Hello, Sarah.
Sarah Al-Ahmed:
Man, this is the first time I’ve had a true occasion to talk about Uranus on the show, and I cannot wait to see how many jokey joke emails people send me about it.
Bruce Betts:
I used all my really good random space facts years ago about Uranus, but I got some good ones. Yeah, it’s discovering moons, that’s why every time I write about moons in the outer solar system, like in kids’ books, like, hey, don’t worry about memorizing those numbers too closely because we keep discovering them, and it’s impressive. This is a really far away tiny object, that just we, people, discovered using James Webb Space Telescope. Go on, Sarah, tell me more.
Sarah Al-Ahmed:
It’s really cool watching the way that we’re progressing in finding new moons, like the war between Jupiter and Saturn, who has more moons I think is really interesting. But because we don’t have these dedicated missions up to Uranus and Neptune, there’s so much that we don’t know. So, in the midst of learning more about this world, and setting up for this interview, suddenly we get the story dropping that we found a new moon of Uranus, which is all the more reason to love these JWST images.
Bruce Betts:
Oh, they’re amazing. We’ve had images, but it’s the usual. We got a better telescope, it’s super cool, we can see things better, you can see the rings, you can see everything better, that’s the nutshell.
Sarah Al-Ahmed:
What do we know about this moon so far?
Bruce Betts:
It is, I think believed to be about 10 kilometers in diameter. So, compared to a city, pretty good size, compared to moons… It’s one of the closer ones to the planet that have been discovered, it’s in a prograde orbit, so it’s gone the same direction as the planet is spinning, is in an equatorial plane along with the five big moons, bigger moons… They’re small for the solar system, but they’re big enough to be round. And then, you got little guys like this. So, moon number 29, and fits in now they’re 14 of the small moons hanging out, closer in equatorial plane, going prograde, and then there are ones farther out that are just wacky, zany, retrograde, and all sorts of craziness. And they usually form in different ways. But at least that’s the guess. It needs a name, but it gets the official designation, S/2025 U 1, which is actually one of the simpler designations since its satellite year was discovered. U for Uranus, and first one discovered in 2025.
Sarah Al-Ahmed:
Wow. Yeah, I was wondering why that name was so simple. But really though, I think one of my favorite little random space facts when I was a kid was about the naming of the moons around Uranus. Because I was one of those kids that just loved Shakespeare, not to put aside Alexander Pope, the other author who gets some names around Uranus, but I just love that as a naming mechanic. What would you name this moon? Do you have a favorite Shakespeare character you would name it for?
Bruce Betts:
I wouldn’t describe it as a favorite Shakespeare character, but I would describe it as one that I would just enjoy people doing science and having to use the word Bottom. Midsummer Night’s Dream, I’d name it after Nick Bottom, just name it Bottom.
Sarah Al-Ahmed:
Name it Bottom.
Bruce Betts:
It’s just funny, just stupid. What about you?
Sarah Al-Ahmed:
Oh, my favorite Shakespeare play is Much Ado About Nothing. So, if I was being serious about it, I would name it Beatrice, because I love that character. But if I was being funny about it, I would go for Dogberry.
Bruce Betts:
Dogberry, that’s awesome.
Sarah Al-Ahmed:
It’s a great name and a great character in that play, he is so freaking funny.
Bruce Betts:
Somehow I’ve never seen that one or read that one. All my teachers, we always had to read the tragedies, it was a real bummer.
Sarah Al-Ahmed:
Oh man. Highly recommend Much Ado About Nothing, if you love shenanigans falling out from totally silly reasons.
Bruce Betts:
Shenanigans are one of my favorite things.
Sarah Al-Ahmed:
You would love Dogberry.
Bruce Betts:
Dogberry. I change my vote, I want to go with Dogberry.
Sarah Al-Ahmed:
We’re submitting it to the IAU right now.
Bruce Betts:
Yeah, and I’m sure they’ll discover more. Poor Neptune, maybe we can start cranking up the Neptune numbers. Saturn’s just gotten ridiculous, Saturn is just, it decided to just leap forward in the standings. Yeah, I think it has about a billion. Oh wait, that’s only if you count the ring particles, no, never mind.
Sarah Al-Ahmed:
I was going to say. But yeah, the moons of Saturn are interesting, I think for me, in that some of them are being created from the ring around it. So, in my brain, something smashed up some moons or something, created this ring around Saturn, and now it’s reforming cute little potato-esque chaos demons inside of the rings.
Bruce Betts:
Chaos demon. And don’t forget the ravioli moon, that’s… Anyway, yeah, we like moons. We like moons.
Sarah Al-Ahmed:
So, what is our random space fact this week?
Bruce Betts:
So, speaking of traveling… Desperate attempt at a segue. So, I was curious, going out to Neptune being way, way out there, it’s 30 AU, with AU being astronomical unit, average Earth-Sun distance, but you don’t go straight to any of those places. So, I was wondering how far did Voyager 2 go to get there, and it went about over 7 billion kilometers. But it got there in a wonderful 12 years, which is one indication of just how very far this is. But what’s really interesting to me is that by using the gravity assists of doing the grand tour, Jupiter, Saturn, Uranus gravity assist, it cut, at the time for what the estimate was, at least with that configuration, 20 years, 20 years off the… It still took 12 years, without the gravity assist, it would’ve taken 32, I guess.
Sarah Al-Ahmed:
Oh my gosh.
Bruce Betts:
At least that’s what NASA reports.
Sarah Al-Ahmed:
That’s crazy. Okay, so I’m in Uranus brain, so it flew by Uranus in 1986.
Bruce Betts:
’86, yeah. 89, Neptune-
Sarah Al-Ahmed:
When did it go by Neptune? ’89.
Bruce Betts:
’89.
Sarah Al-Ahmed:
So, if it had taken 20 more years… Whoa. I would’ve been well alive for one, but also… No, that’s just crazy. 2000s. Whoa, whoa.
Bruce Betts:
Whoa.
Sarah Al-Ahmed:
Whoa.
Bruce Betts:
You probably wouldn’t have launched it frankly if you didn’t… And you can do faster, with bigger rockets and with smaller spacecraft. And so the New Horizons was sent out of the Earth-Moon system, going faster than anything else had been at that time anyway, although it’s been slowed now by the pesky Sun’s gravity. So, they flew a small spacecraft and they got it out, they went to Pluto in, what, nine years or so? So, they’re booking it, to use a technical term.
Sarah Al-Ahmed:
Yeah. Technically. But no, really, it’s just a great example of the advancement of our technology over time. And yeah, here we are, we haven’t sent one out there yet in a long time, but we can look at it with JWST, this crazy telescope, and learn more like the fact that there are new moons out there that we just didn’t know existed.
Bruce Betts:
Yes, they’re new to us, but not to Neptune.
Sarah Al-Ahmed:
Yeah, they’ve been out there for a long time.
Bruce Betts:
Or Uranus, sorry. Whatever. I’m sure, there got to be more hanging out at Neptune, the Voyager picked up, what, 14, I think? And so, we’re looking for more. Let’s do it. We’ll go out, we can get a three or four inch telescope, I’m sure we’ll do it.
Sarah Al-Ahmed:
It’ll be fun.
Bruce Betts:
We can hallucinate. All right, everybody, go out there, look up the night sky and think about how long you would take to go to the grocery store if it were on Neptune. Thank you and good night.
Sarah Al-Ahmed:
We’ve reached the end of this week’s episode of Planetary Radio, but we’ll be back next week to give you a peek at the upcoming International Observe the Moon Night festivities. We’ll let you know how you can join in no matter where you live or how the weather shakes out in early October. And speaking of celebrating, next week I’m going to be flying off to Philadelphia, Pennsylvania in the United States to host the webcast for NASA’s Innovative Advanced Concept Symposium. This is going to be my third year doing it, and I’m really looking forward to it. So, if you want to watch the webcast, I’m also going to provide a link for that on this webpage, at planetary.org/radio. If you love the show, you can get Planetary Radio t-shirts at planetary.org/shop, along with lots of other cool spacey merchandise. Help others discover the passion, beauty, and joy of space science and exploration by leaving a review or a rating on platforms like Apple Podcasts and Spotify.
Your feedback not only brightens our day, but helps other curious minds find their place in space through Planetary Radio. You can send us your space, thoughts, questions, and poetry at our email, [email protected]. Or if you’re a Planetary Society member, leave a comment in the Planetary Radio Space in our member community app. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible by our members who love a good planetary mystery. You can join us as we advocate for future missions to the ice giants at planetary.org/join. Mark Hilverda and Rae Paoletta are our associate producers. Casey Dreier is the host of our monthly space policy edition, and Mat Kaplan hosts our monthly book club edition. Andrew Lucas is our audio editor, Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. My name is Sarah Al-Ahmed, the host and producer of Planetary Radio, and until next week, ad astra.
