We’ve reached Neptune, the final planet in our tour through the solar system – but don’t worry! The tour’s not over, but after this week we’ll be all out of planets. Neptune has a controversial story about its discovery, some of the strongest winds in the solar system and some weird moons.
Don’t forget to check out the rest of our solar system tour!
- NASA’s Neptune Fact Sheet – just the numbers
- Neptune from Bill Arnett’s NinePlanets.org
- Neptune Background – including history of discovery
- NASA’s Solar System Exploration: Neptune
- The Planetary Society: Neptune
- Neptune: The Magnetic Environment
- Hubble Discovers New Dark Spot on Neptune (April 1995)
- The Dynamics of Dark Spots on Neptune – Sromovsky, Lawrence (07/2000)
- Explanation of the observed irregularities in the motion of Uranus, on the hypothesis of disturbance by a more distant planet – Adams, J.C. (11/1846)
- NASA’s Neptunian Rings Fact Sheet – just the numbers
- The Planetary Society: Neptune’s Rings
- Neptune: Rings
- Rings and Ring Arcs
- NASA’s Neptunian Satellite Fact Sheet – just the numbers
- The Planetary Society: Triton
- Triton – Voyager Mission Website
- Neptune’s Small Satellites
- Voyager Images of Neptune – Hosted by The Planetary Society
- Welcome to the Planets: Neptune – Voyager Image Slideshow
- Hubble Images of Neptune
Fraser Cain: Now weâ€™ve reached Neptune, the final planet in our tour through the solar system â€“ but donâ€™t worry! The tourâ€™s not over, but after this week weâ€™ll be all out of planets. Neptune has a controversial story about its discovery; it has some of the strongest winds in the solar system and some weird moons. Thereâ€™s plenty of interesting stuff to talk about Neptune.
Should we talk about the discovery first?
Dr. Pamela Gay: Seems like as good a place as any to begin.
Fraser: I think itâ€™s a cool story.
Pamela: (laughing) I tend to agree.
So, Neptune is actually the very first planet discovered strictly thanks to math. It was first seen by Galileo, but he totally missed the fact that itâ€™s a planet because it was just entering retrograde when he observed it so it really wasnâ€™t moving relative to the stars.
Fraser: That happened with Uranus too â€“ they saw it, didnâ€™t know what it was and moved on. Or thought it was a star, put it in a catalogue, moved on.
Pamela: Right, so that really doesnâ€™t count. Yeah, he did see it, but he didnâ€™t discover it.
So once Uranus had been discovered, people were looking at its orbit, mapping its orbit, and they realised its orbit made no sense. They could explain its orbit if there was another planet, another large planet, out beyond Uranus.
So there was a British mathematician, Adams, who threw out all sorts of different possible places for Neptune to be. Heâ€™s throwing out ideas, doing calculations, pre-calculator, pre-computer â€“ itâ€™s not easy to do, but you can still do it accurately. H e just didnâ€™t.
Fraser: If he had better math, he couldâ€™ve come up with the right number, he just had bad math or bad observations?
Pamela: Well, Adams was a theorist, so heâ€™s working purely with mathematics. You have to be careful and make sure you take everything into account, and itâ€™s a lot of detailed mathematics. He didnâ€™t always do it perfectly. So he threw out multiple possible locations for where Neptune could be located. There was no planet name at that point â€“ it was the mystery planet pulling on Uranus. It was never found.
Then, Urbain Le Verrier, in 1846, he did very careful mathematics, came through, got an answer, and tried to get an observer to look.
Then one day in September of 1846, he pointed out â€œhey, we have these great hand-drawn maps of this one area of the sky.â€? He got Johann Gottfried Galle to go and look in the area of the sky the maps already existed for, that his mathematics indicated Neptune should be located in. the very first night they went out and looked, they were able to compare the maps with the actual sky and find Neptune as the object that wasnâ€™t there when the maps were drawn.
Fraser: Okay, so because Neptune moves so slowly, if they just looked in the region of the sky, theyâ€™d only see a bunch of stars and wouldnâ€™t be able to tell from one night to the next if something was moving. Especially because it wasnâ€™t like they had photography and a way to capture an image each night. They just had to look and remember and draw some pictures. So they found an old map that showed where all the stars were and then saw something that shouldnâ€™t be there.
Pamela: It was really cool. His calculations were within one degree of the actual location. Considering all the positions of all of the planets were based on visual estimates, which means youâ€™re going to introduce a fair amount of error as youâ€™re looking through the telescope at the sky and drawing pictures of things relative to one another. They did have good etchings on some of the eyepieces that allowed them to calibrate things, but youâ€™re still going to have errors cropping up. One degree is a really good error.
The thing was, one of Adamsâ€™ many calculations was actually within ten degrees of where Neptune was found.
Fraser: Uh oh.
Pamela: So hereâ€™s Adams in England, and Verrier in France – two countries that didnâ€™t exactly like each other in the 1800s. Adams is going, â€œI predicted it too!â€? and stomping up and down and England is stomping up and down with him and thereâ€™s of course all sorts of international intrigue and theyâ€™re arguing over what the thing should be named and theyâ€™re trying to give it nationalistic namingâ€¦ and now, weâ€™re thinking more and more that Adams was not right.
The thing is, all the papers regarding Adamsâ€™ calculations sort of havenâ€™t been around for people to consult for a while. Thereâ€™s an astronomer, Olin J. Eggen â€“ O.J. Eggen, the breakfast astronomer â€“ really good astronomer, did some excellent work, and somehow in the process of his lifetime of research, he scooped up Adamsâ€™ papers. It wasnâ€™t until after he died in 1998 that people were going through his papers and found all of the records for what Adamsâ€™ had done.
When all this intrigue had initially happened, it was finally decided to jointly give credit to Adams and Verrier and allow to both countries to stand there and go, â€œwe discovered this new planet.â€? Now itâ€™s looking like England might need to take a step back and say, â€œokay this time France is actually the country that can claim the person who made the correct calculations.â€?
Fraser: Yeah, Iâ€™m sure thatâ€™ll happen.
Pamela: Yeahâ€¦ no. But at least the history is starting to get it sorted out.
The name Neptune was actually sort of settled upon by a Russian astronomer, so this really was a full international affair to get from the point of starting the mathematics, doing the mathematics correctly, and saying weâ€™re going to name it Neptune. Neptune is another Roman God, and it sort of fits in with the rest of the planets, most of which had been named in ancient times. Whatâ€™s kind of cool is since itâ€™s a modern naming, pretty much all nations have some form of the God of the Sea as the name for this particular planet.
Fraser: Thatâ€™s cool. I didnâ€™t know that.
Fraser: I think itâ€™s funny to hear how, around that time, it was quite an age of scientific discovery. In the 1800â€™s, they put a lot of emphasis on it. There are so many stories on it, about that time. I remember there was the planetary transit of Venus across the surface of the Sun, and the search for elementary particles. A lot of these things are very similar â€“ there were countries competing to get to the bottom of this. In many cases a lot of the stories all worked out kind of the same way â€“ theyâ€™re racing to the same discovery at the same time. Itâ€™s neat to hear Neptune was part of that.
Okay, so what do we know about Neptune?
Pamela: Weâ€™ve finally been able to start getting detailed observations of it thanks to first the Voyager 2 mission in 1989 and now we also have observations from the Hubble Space Telescope and the Very Large Telescope down in the southern hemisphere.
Unlike Uranus, Neptune isnâ€™t just a big blue blob. It actually has some of the most fascinating storms in the entire solar system. Its winds are practically supersonic. Theyâ€™re whipping along at upwards of 2000km/s. so we have these extremely high-speed winds.
Fraser: Hold on. Neptune has weirder storms, higher wind speeds than Uranus, but itâ€™s further away? How is this possible?
Pamela: Thatâ€™s actually one of the things people have struggled with. How do you end up with this crazy set of storms going on? What it appears to be is Neptuneâ€™s core is still much warmer. It has a several thousand-degree core â€“ its centre is about the same temperature as the surface of the sun (which is true of pretty much all the planets except Uranus, which has a cooler core). Itâ€™s the temperature difference between how hot it is in Neptune’s core and how cold it is on Neptuneâ€™s surface that helps to drive all these amazing wind storms that are going on.
Fraser: I actually did an article (this is way out of left field), about a year and a half ago, about the possibility that there are liquid oceans in Neptune or Neptune-type objects in extrasolar planetary systems. You could have the right combination of warm core and pressures that water could end up being oceans. You could move down through the planet, hit ocean, and move further down and hit super hot compressed water that wouldnâ€™t be good for life. But you could actually hit a reasonable spot somewhere in between.
Pamela: Whatâ€™s so neat about these worlds is yeah â€“ we call them ice giants because the type of stuff their inner layers are made of is stuff that for whatever reason (even if itâ€™s not frozen solid) we refer to as ice (so it has water, ammonia, etc). They are, just like you said, heated so much and under so much pressure, that theyâ€™re either acting like liquids or theyâ€™re actually starting to be at that point where theyâ€™re so hot they would be gases if it wasnâ€™t for the amount of pressure theyâ€™re under. So thereâ€™s really weird, cool fluid dynamics going on in the different layers of this object.
Neptuneâ€™s just a funky system geologically, thanks to this neat combination of high internal heat, cold temperatures on the surface, and itâ€™s just out there generating storms. It had a giant dark spot for a while that was likened very much to Jupiter’s Red Spot. Itâ€™s had dark spots. The storms appear to come and go over time. It has seasons, and because it is tilted so much we can see the seasons change from one pole getting more heat to the other pole getting more heat.
All these different changes really cause weird things to happen. In fact, right now the south pole of Neptune is about ten degrees Celsius warmer than the rest of Neptune. That slight difference is just enough to make the methane gas in the atmosphere change from icy crystals to gas. So methane gas is coming out the south pole of Neptune, which is just a cartoon waiting to happen.
Fraser: (laughing) Yeah, you thought you giggled when you talked about Uranus.
Pamela: But this isnâ€™t the pronunciation! This is actually what Neptune is doing. Neptune is blowing gas out its pole.
You have to laugh, itâ€™s real though â€“ itâ€™s what the science is showing us.
Fraser: Well itâ€™s realâ€¦ but itâ€™s what youâ€™re focusing on thatâ€™sâ€¦
Letâ€™s talk about the winds again, because thatâ€™s crazy. Up to 2000km/hr, right?
Pamela: Yeah, near supersonic speeds.
Fraser: The only other place we see winds that strong are around extrasolar planets that are hot Jupiters. Theyâ€™re right up close to their parent star, theyâ€™re tidally locked, theyâ€™re experiencing thousands of degree differences of temperatures from one face to the other, and that severe temperature difference is causing the supersonic winds to whip around the planet to stabilise things out. Neptuneâ€¦ youâ€™ve got a few degree difference from the feeble Sunâ€¦ but how can it be whipped up to those kinds of temperatures?
Pamela: You have a core thatâ€™s 7000 degrees and a surface that is -200 degrees Celsius. So here itâ€™s instead of having strictly winds going from the dark side of the planet to the light side of the planet, instead you have churning that is going from the lower layers of the atmosphere out to the outer layers of the atmosphere. Combined with rotation rates and the day and night cyclesâ€¦ all these different things combine to give these multi-thousand kilometre per hour winds.
Fraser: Wouldnâ€™t it be even? It would be working on all directions at the same time, so it should even out.
Pamela: You end up getting convective cells.
Pamela: Itâ€™s the same sort of bulk motion of the hot stuff in the centre rising to the surface, cools off, and flows back down. This can end up driving entire weather systems.
Fraser: All right. Iâ€™m going to chalk it up as a great big mystery and move on. We donâ€™t know.
Now, letâ€™s talk about observations. Before the 1980â€™s, all we had was a tiny little blue dot.
Fraser: Now, with Voyager 2 and with Hubble, weâ€™ve got a much better view. What did Voyager see?
Pamela: Voyager saw these amazing storms on it. That was the first thing that was noticed as Voyager started to approach Neptune. First it had this giant darker storm. They named a cluster of these little white storms Scooters. Thatâ€™s just kind of cutesy. So there were these little whiter storms moving around faster than the dark storm.
After how boring Uranus had been, it was amazing to see this planet with these high speed storms and all these little wispy features all over its surface.
Fraser: Rings? Does Neptune have rings?
Pamela: It has a few of some of the wussiest rings in the entire solar system, but they actually behave differently than any of the other rings.
In general, dynamically we expect that if you get a chunk of something falling apart in a ring, very quickly it will end up spreading itself evenly out around the ring. When we look at Neptuneâ€™s rings, we actually find these arcs in the rings where when you look at one particular ring, youâ€™ll see little tiny sections of the ring that seem to be much thicker and much brighter than other sections. These arcs are something we havenâ€™t seen anywhere else. We think these arcs are actually due to different resonances with one of Neptuneâ€™s moons. So in this case, itâ€™s Galatea whoâ€™s in a 43:42 resonance (which is a really crazy-small resonance).
Fraser: What do you mean, though? The rings go around 43 times for every 42 orbitsâ€¦?
Pamela: Right. So these random times they line up, one goes around 43 times and the other goes around 42 times and they line up with each otherâ€¦ is just enough to end up creating little acrlets inside the ring where the material is a little bit thicker.
Fraser: Okay, itâ€™s almost like it builds up. Itâ€™s like a wobble on the ring that clumps together and piles up each time Galatea comes around, so weâ€™re able to see them as arcs.
Pamela: Exactly. That leads to a feature that we just havenâ€™t seen anywhere else in the solar system. So it has these rings, theyâ€™re very wussy rings, but they still have some really neat features that we havenâ€™t seen anywhere else in the solar system.
Fraser: Now, I remember, back in the day when I was a kid, there was a sort of historic moment where Neptune became the most distant planet, passing the orbit of Pluto. Now of course, with Pluto not being a planet, that distinction doesnâ€™t matter anymore, but letâ€™s talk about that switching.
Pamela: Back when we thought Pluto was a planet, Pluto and Neptuneâ€™s orbits are such that they intersect each other. Mathematically, theyâ€™re never going to run into each other. Itâ€™s timed just right that the two of them are never in the same place at the same time. Not only that, but because of the tilts of the orbits, their physical orbits donâ€™t actually cross one another either.
Fraser: Right, but if you look at it from up above, it would look like theyâ€™re crossing.
Fraser: But in three dimensions, they donâ€™t actually get that close to each other.
Pamela: Even if weâ€™re only limiting it to looking at the crossing points, they never end up at the crossing point at the same time.
Fraser: They have a resonance.
Pamela: Yeah. This resonance, Pluto and Charon arenâ€™t the only things in that resonance. This leads to a lot of people saying, â€œwaitâ€¦ the International Astronomical Union says one of the definitions of a planet is that youâ€™ve cleared out your orbit in the solar system.â€? Pluto is in Neptuneâ€™s orbit. Charon is in Neptuneâ€™s orbit. All these other things are in Neptuneâ€™s orbitâ€¦ so how has it cleared out is orbit?
The IAU definition actually refers more to has it cleared its orbit of other things the same size. Is it the gravitationally dominant object in its orbit. If you look at how big Neptune is, thereâ€™s nothing near its size in its orbit.
Fraser: There arenâ€™t other Neptunes orbiting in the same place.
Pamela: Right. So Neptune is the biggest thing in its orbit, the same way the Earth and the Moon are the biggest things in their orbit. There are lots of near-Earth objects crossing our orbit all the time. We donâ€™t question if Earth is a planet or if Jupiter (which has lots of things sharing its orbit) is a planet.
Fraser: We already covered whatâ€™s a planet, back in our very first episode, and Iâ€™m sure weâ€™ll bring it back up again with the Pluto conversation â€“ yes, weâ€™ll still go to Pluto on the tour.
But I wonder if there was another Neptune, would that beâ€¦ anyway. We could go on forever.
So does Neptune have a roughly circular orbit, and itâ€™s Pluto that has the really elliptical one?
Pamela: Itâ€™s Pluto that has the crazy-elliptical orbit. The eccentricity of Neptuneâ€™s orbit is 0.01, which basically means itâ€™s a circle.
Fraser: Itâ€™s a circle. Right.
Pamela: So it has this really nice, pretty much circular orbit. The difference between its furthest point and its nearest points is only 100 million kilometres. When itâ€™s 4 billion kilometres away, thatâ€™s not that big a difference. So yeah, itâ€™s basically a circular orbit.
Fraser: Letâ€™s talk about its moons.
Pamela: It actually has one of the cooler moons in the solar system. Its moon Triton (which somehow works well with Neptune) goes the wrong way around the planet.
So you have Neptune happily doing its Iâ€™m-going-to-spin-on-my-axis thing going one way, and thatâ€™s the direction it lines up with the entire rest of the solar system.
Triton is basically fighting against the current in a purely retrograde motion.
Fraser: So if you looked at it from above, the Earth, Jupiter, Saturn, Mars, even Neptune… theyâ€™re all rotating in the same direction. All the moons are all going in the same direction as well, around their parent planets?
Pamela: All the major moons.
Fraser: So there are other retrograde moons out there.
Pamela: But none of them are big! Theyâ€™re little tiny things that you know were captured. Tritonâ€™s a nice, big moon. Itâ€™s actually the 7th largest moon of all the moons in the solar system. So the fact that itâ€™s going in reverse says it didnâ€™t actually form where itâ€™s located. What weâ€™re now thinking is itâ€™s a captured Kuiper Belt Object.
So once upon a time, just like Pluto, Sedna or Charon, it was out orbiting by itself. It was probably actually orbiting with a second Kuiper Belt Object â€“ it was probably in a binary dwarf planet system. Somehow there was a 3-body interaction with Neptune, Triton and this other object (we donâ€™t know where it went). In the interaction, Triton was carefully captured, itâ€™s now in an almost exactly circular orbit, and it was captured such that itâ€™s now going the wrong way around Neptune.
Fraser: Can you explain the concept of the 3-body interaction? I think thatâ€™s pretty important.
Pamela: There are a lot of different ways that things can gravitationally interact. The 3-body problem is perhaps one of the most complicated mathematics to try and sort out that is still solvable in a few cases.
Fraser: All right, I think our listeners are going to want to do the math in front of them â€“ so everyone get a piece of paper and a penâ€¦
Fraser: I was kidding.
Pamela: Yeah â€“ no. I like our listeners too much to do that to them.
Fraser: All right. Weâ€™ll just go withâ€¦ generalities.
Pamela: Thereâ€™s only really one complete solution to a 3-body problem, and that was figured out by Victor Zebehe. You can, with computer modelling, chew through 3-body problems and see if you have these two small objects come in and they get disrupted in this way, one gets flung out violently in one direction and the other can get captured.
Fraser: Itâ€™s almost like how we shoot spacecraft at planet through gravitational assist, but because of the dynamics between the three objects, one gets slingshotted out, and one has to stay.
Pamela: This happens all the time in the universe. We have binary systems in globular clusters where one of the stars gets pulled off of the binary and the other one rejoins another star or the entire binary gets disrupted. It looks like it happened with Triton and another object. We can see different places where it looks like a binary system was disrupted.
In this particular case, the disruption led to Triton being nice and gracefully captured. Since itâ€™s going the wrong way, itâ€™s constantly getting stuff slamming into its face â€“ which makes it somewhat interesting geologically because itâ€™s been resurfaced very recently.
Fraser: So, because itâ€™s going the wrong way, it acts like a vacuum cleaner?
Pamela: Pretty much. If you look at the nose of Triton, for lack of a better way to describe it, the part thatâ€™s leading through its orbit, looking at that area and out in a circular pattern around it, you start finding all these large craters from where things have slammed into it. These craters are concentrated around the nose that itâ€™s putting forward as it orbits. If these craters were randomly distributed over the surface of Triton, we might be able to figure out some where swept up from things orbiting Neptune, others were Kuiper Belt Objects that hit Tritonâ€¦ but it looks like the majority of the craters are coming from things that Triton probably swept up that were orbiting Neptune (in the correct direction) that it slammed face-first into.
Fraser: So what does the future hold for Triton?
Pamela: Itâ€™s actually slowly spiralling in toward its doom.
Fraser: Uh oh.
Pamela: Yeah, yeahâ€¦ this is what happens when you go the wrong way around your planet. Its orbit is actually decaying such that itâ€™s getting a little bit closer in every year, and it looks like in about three and a half billion years (not that we need to worry about it real soon), itâ€™s going to get close enough that it will get tidally disrupted and itâ€™s going to end up adding to the ring system around Neptune.
Fraser: So itâ€™s going to go through the Roche limit.
Pamela: Itâ€™s going to go through the Roche limit. Itâ€™s going to get disrupted. Itâ€™s going to become rings.
Fraser: So it wonâ€™t be able to hold itself together gravitationally, the gravity of Neptune will tear it apart, and it will have a ring. I wonder what kind of ring that would be? It would probably be the biggest ring in the solar system, wouldnâ€™t it? Itâ€™d be crazy!
Pamela: It doesnâ€™t have a whole lot of mass, so Iâ€™d actually need to look up the numbers to see if the mass of Triton is greater than the mass in the rings of Saturn. Iâ€™m not sure how the masses add up. It would be a really shiny ring, because Triton does have a lot of ices, a lot of water ice in it. All of these shiny liquids, once it breaks apartâ€¦ it has frozen nitrogen/dry iceâ€¦ these things reflect light really well. So when it breaks apart, itâ€™s going to create all these shiny ice chunks that are going to very readily reflect sunlight back toward observers, wherever they happen to be observing from.
Fraser: In 3.5 billion years, the Sun wonâ€™t be destroyed yet, but it will be a lot brighter, so it might actually be a really pretty view.
Pamela: It just might be. So in the future, there will be some pretty, shiny, highly reflective rings around Neptune.
Fraser: Are there any spacecraft headed to Neptune for the future?
Pamela: Not right now. There have been people whoâ€™ve put forward the idea of doing a Cassini-style mission â€“ go, explore, see what the moons have to offer, study the storm systemsâ€¦ but itâ€™s far out. Itâ€™s expensive to get to. So right now, thereâ€™s nothing concrete other than firm desire to go out and take a look.
Fraser: Itâ€™s interesting â€“ Neptune is more interesting than Uranus, from a science point of view.
Pamela: Yeah, and that boils down to it has more interesting weather because itâ€™s a lot hotter on the inside.
Fraser: Right. Okay. I think that wraps up all our planets, but thereâ€™s still some more solar system left. The tour isnâ€™t over yet â€“ weâ€™ll keep going.
Pamela: We have a whole lot of ice.
Fraser: Thereâ€™s ice, the Oort Cloud, the Heliopause, Heliosphere and interactions with the rest of our galaxy â€“ thereâ€™s still a little ways to go.
Pamela: So maybe weâ€™ll wrap it up by Christmas. Weâ€™ll see.
Fraser: And then maybe weâ€™ll just tour through the Milky Wayâ€¦ I donâ€™t know.
This transcript is not an exact match to the audio file. It has been edited for clarity.