Exploring Neptune’s Unusual Seasons

9:57 minutes

Neptune. Credit: Shutterstock

Planetary scientists monitoring how the outer planets change over time have made a surprising observation of springtime on the planet Neptune. As the planet moves towards summer in its southern hemisphere, one might expect it to get warmer—but in data taken over 17 years, researchers observed that the average temperature actually seems to be declining. One theory involves the conversion of atmospheric methane, which traps heat, to ethane or other hydrocarbon compounds that release heat more readily, but more research is needed.

The researchers also spotted the rapid formation of a hot-spot at the south pole of Neptune, with an increase of some 11 degrees C over just two Earth years. Models had predicted a temperature swing of perhaps 15 degrees over the entire seasonal cycle. 

These findings were reported this week in the Planetary Science Journal. Scientists don’t know very much about Neptune—it’s over 30 times Earth’s distance from the sun, and gets only one nine-hundredth of the sunlight. It takes around 165 Earth years to complete an orbit, meaning that the researchers’ 17 years of data account for only a small fraction of one season. Because of the planet’s tilt and its long orbit, the last time the planet’s north pole was visible from Earth was in the 1960s. And we’ve only visited once, via the Voyager spacecraft, over 30 years ago. 

Michael Roman, a planetary scientist at the University of Leicester in the UK, and one of the authors of the report, joins Ira to talk about the strange springtime on Neptune—and the planet’s many remaining mysteries. 

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Segment Guests

Michael Roman

Michael Roman is a planetary scientist at the University of Leicester in Leicester, UK.

Segment Transcript

IRA FLATOW: For the rest of the hour, maybe you’ve been enjoying the changing seasons, spring getting underway, looking ahead to summer. But what would that look like on the planet Neptune? Neptune is about 30 times further away from the Sun than the Earth. Having visited it once with Voyager 2, over 30 years ago, Voyager showed the planet to be a vibrant shade of azure blue.

But Neptune does have a planetary tilt, like Earth, meaning it has seasons. And despite being cold and dark, it does have active weather. Researchers have been trying to track how Neptune and other outer system planets change over time. And this week they published some unusual findings about Neptune. For example, while you might think approaching summer would lead to warmer temperatures, it appears that the planet is actually cooling as it moves toward the summer in the southern hemisphere. Pretty unexpected.

Joining me now is one of the researchers on that project. Michael Roman is a Planetary Scientist at the University of Leicester, in England.

Welcome to Science Friday.

MICHAEL ROMAN: Hi. Thank you. It’s a pleasure to be here.

IRA FLATOW: Nice to have you. Let’s talk about this unusual finding about Neptune’s seasons. First, explain how you take its temperature. I mean, Neptune is very far away, is it not?

MICHAEL ROMAN: Yes. So it’s very far away and very cold. So measuring its temperatures is rather difficult. What you need is a very large telescope– typically, the ones we’re using are something like 8 meters in diameter– and a very sensitive instrument– basically it’s a thermal camera– hooked up to this telescope to take images of it. So using these infrared images, you can infer how hot the planet is or how cold the planet is.

IRA FLATOW: So your data on Neptune’s seasons, looking as summer starts in its southern hemisphere, would be sort of like trying to say something about seasons on Earth, when you’ve only been observing January and part of February. It’s not very complete is what I’m saying.

MICHAEL ROMAN: Yeah. No. Exactly right. Since it takes 165 years to go around, and we’ve only really been observing at these infrared wavelengths for the last 20 years or so, we’ve only seen a fraction of a season on Neptune. And so we were not exactly sure what we were going to see, because no one has really detected this before.

IRA FLATOW: All right. So let’s get into the details of what you actually saw about the temperature changes on Neptune. What did you find?

MICHAEL ROMAN: Well, basically, we found, when you take all the data– all the data in existence that goes back to about 2003– and you look at its brightness as a function of time, we found that it declined somewhat steadily in brightness– which we interpreted as a change in the temperature– since 2003.

IRA FLATOW: So how much decline in temperature did you see?

MICHAEL ROMAN: So since 2003, we saw a drop of about 8 Kelvin, or 8 degrees Celsius, in the planet’s stratosphere. So this is the layer of the atmosphere that is above the active weather layer. We have a stratosphere on Earth, for example. It’s where ozone is being produced.

IRA FLATOW: Yeah. So that would be unusual, right? Shouldn’t you be seeing an increase if it’s summertime?

MICHAEL ROMAN: Yes. And really, you would expect, with increasing sunlight that’s happening in the summer season, you would expect the atmosphere to be absorbing this sunlight and warming up over time. But in fact, what the data shows is exactly the opposite. The planet seems to be getting cooler in the stratosphere over the observed period. And this was definitely a surprise to us. Because at first glance, it doesn’t really make sense.

IRA FLATOW: And at second glance, does it make sense?

MICHAEL ROMAN: Well, we can come up with theories to try to explain what we’re seeing. All we can really do is speculate as to what’s happening. What we may be seeing is this complex interplay between solar heating and the sun’s effect on the chemistry of the atmosphere. Basically, sunlight is absorbed in the atmosphere by methane that’s in Neptune’s atmosphere. Its atmosphere is mostly hydrogen and helium, but there’s a small amount of methane. And methane very effectively absorbs sunlight, and that heats the atmosphere.

But also the sun can damage these methane molecules– you get high energy photons, UV light, from the sun– can break up these little methane molecules into other hydrocarbons, mostly ethane. And these other hydrocarbons tend to not absorb sunlight, but they tend to radiate heat very effectively. They’re very efficient coolers.

And so what we might be seeing is, as the sunlight increased over time on Neptune, there was a heating due to the sun’s energy directly falling on the atmosphere, but there was also production of these hydrocarbons that effectively cooled the atmosphere. And so perhaps the production of ethane offset the amount of heating due to direct sunlight, and overall that would lead to a cooling of the atmosphere.

IRA FLATOW: Now, I understand that in the images, in addition to seeing this cooling effect, you noticed a bright hot spot by the south pole. Any idea what’s going on there?

MICHAEL ROMAN: Yes. So this is really interesting. So as the whole planet, on average, was cooling down, we saw this brightening that’s localized right at the south pole, this little hot polar vortex. What’s going on there is not exactly clear. Because, in general, we thought that the atmosphere at the pole would be warmer, because air tends to converge and sink there. And when air sinks, it warms. And you get this on Earth, for example, when wind blows down mountainsides. You get these Santa Ana winds, for example.

And we thought perhaps that’s what we were seeing there at the pole, in combination with perhaps a seasonal effect, where just as the pole was now pointing more towards the sun, it’s getting more sunlight and it should warm up. The problem is that we saw these changes very abruptly occurring. Just between 2018 and 2020, the whole southern pole region warmed by 11 Kelvin on average, or 11 degrees Celsius. That was unexpected, to have such a rapid response on Neptune. Given how slowly the seasons change, it was surprising.

Over the course of a full orbital cycle– or the difference between summer and winter on Neptune– you might expect a change as large as maybe 15 degrees. And here we’re seeing a change of 11 degrees in just a mere fraction of a season. So it’s really quite surprising to see.

IRA FLATOW: I’m Ira Flatow, and this is Science Friday, from WNYC Studios.

This all says to me that our existing models of how Neptune’s atmosphere behaves– or it should behave– well, those models are all wrong, or need to be changed.

MICHAEL ROMAN: Yeah. No. Exactly. And this happens all the time in science. This points to a picture of seasonal response, or at least temporal variability, on Neptune that is just more complicated than we previously expected.

IRA FLATOW: You mentioned that you don’t have enough data about Neptune. Will the new James Webb Telescope help you collect more data?

MICHAEL ROMAN: Oh, absolutely. So the James Webb Telescope is really going to be a game changer in a lot of ways. Because it is so much more sensitive to the atmosphere of Neptune than anything we had before and anything we can get from the ground at the moment. Also, it’s a lot bigger than the Spitzer Space Telescope, which in the past observed Neptune. The Spitzer was only really able to resolve Neptune as 1 or 2 pixels on its chip, whereas the James Webb Space Telescope is large enough to be able to resolve the disk. And so we’re going to be able to make maps of the temperature and the chemistry over the entire disk.

And this will be unprecedented, the amount of data we’ll get. It will allow us to better understand what exactly it is we’re seeing on Neptune at this moment in time.

IRA FLATOW: Yeah. You sound very excited about Neptune.

MICHAEL ROMAN: Well, it is an intriguing planet, I find. I mean, the fact that it is so far out there in the solar system. If you look at it in your backyard telescope– I mean, the first time I looked at it at Fuertes Observatory on Cornell’s campus, through a telescope there, and it was just a little tiny blue disk in the sky. And yet, out there on the edge of the solar system, it has some of the most active weather– the strongest winds of any planet in the solar system. And this is all still rather mysterious to us. And it’s really one of the least explored bodies in the entire solar system.

IRA FLATOW: Should we be sending another probe by it?

MICHAEL ROMAN: Oh, absolutely. There’s only so much one can learn about Neptune from here on Earth, over 4 billion kilometers away. Essentially, you’re only seeing the side of Neptune that’s pointed towards us, for example. So we don’t really know what’s going on on the north pole of Neptune. In fact, we haven’t seen the north pole of Neptune from Earth since the 1960s. And so if you want to say something about how the planet overall is changing, you need to also look at the other side of planet we can’t see.

But there’s also a lot more to Neptune than just the atmosphere itself. It has rings. It has moons. Triton, one of its largest moon, is perhaps a captured body– a captured Kuiper Belt object– that’s brought into orbit around Neptune, and in itself is fascinating. It has probably a subsurface ocean covered in a layer of ice, and it seems to have geysers.

In order to observe these, you really have to get up close in some cases. And also just to understand the interior of the planet better, you really need to have a sense of its distribution of mass in the planet. And that can only really be acquired by getting up close to it and detecting how its gravity field is shaped.

In addition, you need to know about its composition a little more carefully. And you really can only say so much about the composition of a planet remotely, from observing it from afar. At the end of the day, you need to get a probe into the atmosphere in order to measure things like the noble gas quantities. And these are important, because they can tell us something about the formation of the solar system, the history of these planets, and how they came to be.

IRA FLATOW: Dr. Roman, I’m with you on this. Neptune is quite fascinating, gorgeous, and deserves more attention. Thank you for taking time to be with us today.

MICHAEL ROMAN: Well, it’s my pleasure. Thank you.

IRA FLATOW: Dr. Michael Roman, Planetary Scientist at the University of Leicester, smack dab in the middle of England.

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