And then I wrote… in 2011, the planet Neptune had completed one full orbit around the Sun from the time when it was first discovered, and a small magazine called Argentus, edited by a friend of mine, Steve Silver, invited a number of astronomers to submit articles in its honor. You can see the resulting special issue here, on line. My contribution was to conduct an interview with Dr. Heidi Hammel, who is one of the world’s leading experts on the outer planets… and someone I have known since we were at MIT together, a… few… years ago.
Here’s the interview:
Planet Neptune was discovered on September 23, 1846, by the Berlin Observatory astronomers Johann Galle and Heinrich D’Arrest. They had famously been informed by the French astronomer Urbain Le Verrier that calculations of the perturbations in the orbit of Uranus suggested a planet could be found in a particular spot of the sky; when Galle and D’Arrest pointed their telescope at that spot, they found the planet, first try out. (John Couch Adams in Cambridge had made similar calculations, but couldn’t find any observer to listen to him and take a look.)
Once Neptune was found, people started looking at earlier observations when it should have been visible and discovered that it had actually been seen, but not recognized as a planet, by Jérôme Lalande in 1795 and John Herschel (son of the discoverer of Uranus) in 1835. In fact, one observation of Jupiter and its moons made by Galileo in 1613 also includes a star that is now thought to be Neptune!
With all these observations, it was quite easy to trace out the orbit of Neptune and calculate its period to be 164.79 years, or 60,190 days. On July 12, 2011, exactly 60,190 days will have passed since Neptune was discovered. Thus on that night it celebrates its first Neptune-year since it was recognized by fellow dwellers in its solar system.
But what’s happening today with Neptune? Who better to ask than the world’s leading expert on the planet, Heidi Hammel.
Dr. Hammel achieved world-wide fame in 1994 as the lead scientist on the team using the Hubble Space Telescope to observe the impact of Comet Shoemaker-Levy into Jupiter. Her images not only were published in newspapers and television world-wide, her gracious presence and clear explanations to the reporters made her an instant media star… one that was recognized by the world’s largest association of planetary astronomers, the Division for Planetary Sciences of the American Astronomical Society, who awarded her the Sagan Medal in 2002 for her contributions to the public understanding and enthusiasm for planetary science. She even was the subject of a biography, Beyond Jupiter, by Fred Bortz, written to inspire school children (especially girls aged 9-12) to consider a career in science.
After graduating from MIT in 1982, she went to the University of Hawaii where she earned her PhD in physics and astronomy in 1988. After that she worked at the Jet Propulsion Laboratory and MIT. In 2005 she joined the board of directors of The Planetary Society. Now a senior research scientist with the Space Science Institute, her work concentrates on observing and understanding the outer planets. She has been an author on more than 300 papers, going back to her undergraduate days at MIT.
It was at MIT when I first met Heidi, while she was a student and I was a research post-doctoral fellow. But it wasn’t in the planetary sciences department where we crossed paths… in fact, I was also trying out a future career by appearing in the MIT Musical Theatre Guild’s production of Fiddler on the Roof. While I was on stage as the Rabbi (blame my beard!) Heidi was in the orchestra pit, playing the drums.
We’ve stayed in touch over the years, meeting up at various scientific meetings around the world. This conversation occurred via Skype from my office in the Vatican Observatory south of Rome, and her home in Connecticut.
Were you aware that this is the first anniversary of the discovery of Neptune – in Neptune years?
If I had not been aware, then the number of calls from reporters would have alerted me, because you are not the only one to call me! Sometime this week I will also be talking to the BBC, and I have some other emails awaiting in my in-box.
I am happy that you have actually that you responded to me, then! This is what happens when you’re the world’s leading expert on a planet.
I guess so! [Laughs] One of them, anyway.
What’s hot in Neptune? By that, I mean, not only what are the big mysteries, but it seems that in your most recent work you’ve doing a lot talking about hot spots in Neptune. What’s that all about?
We’re able to do thermal imaging of Neptune now, because we finally have big enough telescopes and big enough detectors that are arrays instead of single channel bolometers. Now we can do infrared imaging, and with the large telescopes we have enough spatial resolution to actually map out where some of the heat is coming from on the planet.
What are some of the telescopes that you are using?
Gemini [a pair of 8.1 meter telescopes in Hawaii and Chile], and the VLT [the Very Large Telescope array, a set of four 8.2 meter telescopes in Chile]. Glenn Orton is really the expert on this… he’s been doing the VLT work, and I have been doing some of the Gemini work.
What we see is that down on the south pole of Neptune is a very bright spot, and we really don’t know much about the details of that or why it’s there. But it’s quite evident in the imaging that there’s this tightly confined spot…
We first thought it was the pole itself. When we look in the near-infrared images from Keck – which are reflected light, not thermal emission – there is a very tiny confined spot at the south pole. So when we got our thermal images (which don’t have as much resolution) with a bright polar spot, we initially that maybe that was the pole itself. But in some of the images, the spot seems to shift around, as if it is something that is close to the pole but not quite at the pole. But we’re not really sure, we don’t have enough data.
So it’s a mystery. Why is there a bright polar spot there? What’s causing it and why should it be moving? It’s very hard to answer these questions with the limited data that we have.
Why does it matter?
The specific question of whether Neptune has a hot spot on its pole or not…
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…is really arcana that only people like me or Glenn Orton care about.
The broader question is really, what drives atmospheric circulation on a planet that gives rise to that structure? We don’t have a good explanation for that yet. What drives atmospheric circulation on any planet? It’s a complex interplay of the sunlight falling on the planet, with whatever heat might be generated within a planet, along with (in the case of Earth and Venus and Mars) the surface interactions with the atmosphere.
So to have a planet that is doing something that you can’t explain yet, really points to knowledge that still needs to be understood about atmospheric circulation, something we don’t really have in our arsenal yet. We don’t understand planetary atmospheres well enough to really explain this yet. If you don’t understand the physics and chemistry of what’s going on there, why not?
And to me, that’s important. We certainly care about the planetary atmosphere in which we live and are enveloped!
Is there anything else going on in Neptune that’s really interesting right now?
The thing about Neptune is that, every time we look at it, it looks different, and we don’t have a clue why. You know, when Voyager flew by, it had the big dark spot. OK… so planets have big spots. Jupiter has this big red spot, it’s been there for 400 years. Neptune has a big dark spot, right? Except… five years later, that big dark spot was gone.
I remember the day it disappeared. Tim Dowling, who does modeling of planetary circulation, came up to me and said, “I am so excited, I can finally make the great dark spot on Neptune!” And I said, “I’ve got bad news for you, buddy… it’s not there anymore.” And he looked absolutely crestfallen.
It’s easy to make spots. It’s not easy to make them go away. And there’s no good explanation for that yet.
It’s also not easy to make them drift around, and in fact Neptune’s great dark spots do that. They drift around, they come, they go, it’s just… they’re very complex and there’s very little understanding at this point of what drives that complexity.
Neptune right now is one of those planets where, the more you look, the more confused you get. I am sure at some point the confusion will give way to solutions. But we’re very much still in the confused stage.
When I was young, we learned about rocky planets and gas giant planets. Somewhere along the line that got shifted to rocky planets, gas giants, and ice giants. What was the logic behind that?
What happened is, people actually started to learn something about Uranus and Neptune!
When you and I went to MIT, there had never been a flyby of these planets, we didn’t have modern telescopes, so we just assumed that they were smaller versions of Jupiter and Saturn.
Well, guess what – they’re not. Their chemistry is quite different, their internal structure is quite different, the dynamics in their atmospheres is quite different, and so basically it became not sensible to call them gas giants; because they’re not.
They have thick mantles of briny fluid; water, we believe. There’s an outer envelope of hydrogen and helium; but the bulk composition, the main components that these planets are made of, is not hydrogen and helium. Not like Jupiter or Saturn. They aren’t gas balls.
Now, if you talk to Fran Bagenal [a fellow planetary scientist who was a student at MIT while Heidi and I were there], she’ll give you a big earful about how it’s wrong to call them ice giants, because it’s not ice really. And she’s right. It’s liquids, they’re water balls. But then, if you talk about water balls, people get all confused, because they think that then you’re talking about oceans, and it really isn’t oceans unless you consider oceans under very, very high pressures, once you’re down deep inside these planets.
The bottom line is that we’ve learned enough to actually distinguish this class of planets from the gas giant class.
What’s your opinion of the model developed by a team of celestial mechanics experts in Nice, commonly called the Nice Model, that suggests Neptune and Uranus were formed much closer to the Sun, and that in planetary migration Neptune moved from being inside, to being outside, the orbit of Uranus?
I think it’s a really cool idea! What a neat concept! It explains a lot of what we see in planetary science. It explains the whole idea of this late heavy bombardment where all of a sudden everything was stirred up and that’s why we have a whole bunch of delivery of volatiles to the inner part of the solar system. That’s where all the water in the inner solar system came from, this period of time where these two planets were swapped around and stirred things up a lot. So you could say that Uranus and Neptune, and Neptune in particular, is responsible for life on Earth!
Is there a visible chemical difference between Uranus and Neptune (beyond the different states of their upper atmospheres that might be attributed to their relative distances from the Sun) that could test this theory one way or another?
That’s one reason we want to send probes into these atmospheres. There’s really nothing you can tell by just looking at them remotely. You need to do in-situ measurements of things that don’t chemically react to understand the isotopic abundances. The stuff that chemically reacts – basically everything you see – that stuff is not going to tell you about primordial conditions. What you need is the isotopes of things that don’t chemically react. I’m talking about noble gases, like xenon and neon. But because they don’t chemically react, you can’t see any chemical signature of them, remotely. So you need to have an in-situ measurement. That’s why we want to send a probe into one of these ice giants.
For the kind of chemistry that we’re talking about, you absolutely need to have a gas-chromatograph mass spectrometer in the planet’s atmosphere itself. You need a probe like the Galileo probe.
What are the odds we’ll be able to do that in the near future?
The most recent  Decadal Survey [a regular ten-year survey of planetary scientists to set priorities for future research] looked closely at the issue of exploring ice giants and identified ice giant in-situ measurements as a very high priority.
They did also talk about an in-situ measurement of Saturn as well. We’ve measured Jupiter with the Galileo probe [launched from the Galileo orbiter, it plunged into Jupiter’s atmosphere in 1995], and we’d like to measure Saturn next because we have so much remote information with the Cassini spacecraft. However, the next priority after that is an ice giant, Uranus or Neptune.
Now, if you’re going to go to all the effort to get out to an ice giant – and it’s not easy, it takes a lot of effort – if you’re going to go to all that effort, would you want to do it as a flyby like Voyager did, or would you want to actually try to do an orbiter where you can go into orbit, study the atmosphere for longer than a couple of days, and study the rings and the moons and all that? The feeling of the Decadal Survey is that an orbiter is really preferred over a flyby.
So which is more feasible to do in the next decade? There is an opportunity to do a Uranus orbiter in the next decade, it’s not technologically feasible to do a Neptune flyby. Basically, to go into orbit around Neptune, you’d need aero-capture technology that we don’t have right now. And so, it would have to be developed, and it’s just not feasible to do that in the next decade.
So at this point, the planetary community has recommended that a Uranus mission have a higher priority than a Neptune mission. But, that said, the planetary community has a lot of very clever people, and a lot of those clever people are very interested in Neptune. So I wouldn’t rule anything out. [Laughs]
What does Neptune tell us about exoplanets?
Many of the exoplanets that we are finding are Neptune-sized bodies. So Neptune is even more relevant in today’s world of 1200 [many more now!] exoplanet candidates. Neptune itself represents a class of planets that we have not fully explored in our solar system. And so it is just out there waiting to have its secrets unveiled.