For almost 200 years humans have been watching the Great Red Spot (GRS) on Jupiter and wondering what’s behind it. Thanks to NASA’s Juno mission, we’ve been getting better and better looks at it. New images from JunoCam reveal some of the deeper detail in our Solar System’s longest-lived storm.
When is a Brown Dwarf star not a star at all, but only a mere Gas Giant? And when is a Gas Giant not a planet, but a celestial object more akin to a Brown Dwarf? These questions have bugged astronomers for years, and they go to the heart of a new definition for the large celestial bodies that populate solar systems.
An astronomer at Johns Hopkins University thinks he has a better way of classifying these objects, and it’s not based only on mass, but on the company the objects keep, and how the objects formed. In a paper published in the Astrophysical Journal, Kevin Schlaufman made his case for a new system of classification that could helps us all get past some of the arguments about which object is a gas giant planet or a brown dwarf. Mass is the easy-to-understand part of this new definition, but it’s not the only factor. How the object formed is also key.
Schlaufman is an assistant professor in the Johns Hopkins Department of Physics and Astronomy. He has set a limit for what we should call a planet, and that limit is between 4 and 10 times the mass of our Solar System’s biggest planet, Jupiter. Above that, you’ve got yourself a Brown Dwarf star. (Brown Dwarfs are also called sub-stellar objects, or failed stars, because they never grew massive enough to become stars.)
“An upper boundary on the masses of planets is one of the most prominent details that was missing.” – Kevin Schlaufman, Johns Hopkins University, Dept. of Physics and Astronomy.
Improvements in observing other solar systems have led to this new definition. Where previously we only had our own Solar System as reference, we now can observe other solar systems with increasing effectiveness. Schlaufman observed 146 solar systems, and that allowed him to fill in some of the blanks in our understanding of brown dwarf and planet formation.
“While we think we know how planets form in a big picture sense, there’s still a lot of detail we need to fill in,” Schlaufman said. “An upper boundary on the masses of planets is one of the most prominent details that was missing.”
Let’s back up a bit and look at how Brown Dwarfs and Gas Giants are related.
Solar systems are formed from clouds of gas and dust. In the early days of a solar system, one or more stars are formed out of this cloud by gravitational collapse. They ignite with fusion and become the stars we see everywhere in the Universe. The leftover gas and dust forms into planets, or brown dwarfs. This is a simplified version of solar system formation, but it serves our purposes.
In our own Solar System, only a single star formed: the Sun. The gas giants Jupiter and Saturn gobbled up most of the rest of the material. Jupiter gobbled up the lion’s share, making it the largest planet. But what if conditions had been different and Jupiter had kept growing? According to Schlaufman, if it had kept growing to over 10 times the size it is now, it would have become a brown dwarf. But that’s not where the new definition ends.
Metallicity and Chemical Makeup
Mass is only part of it. What’s really behind his new classification is the way in which the object formed. This involves the concept of metallicity in stars.
Stars have a metallicity content. In astrophysics, this means the fraction of a star’s mass that is not hydrogen or helium. So any element from lithium on down is considered a metal. These metals are what rocky planets form from. The early Universe had only hydrogen and helium, and almost insignificant amounts of the next two elements, lithium and beryllium. So the first stars had no metallicity, or almost none.
But now, 13.5 billion years after the Big Bang, younger stars like our Sun have more metal in them. That’s because generations of stars have lived and died, and created the metals taken up in subsequent star formation. Our own Sun was formed about 5 billion years ago, and it has the metallicity we expect from a star with its birthdate. It’s still overwhelmingly made of hydrogen and helium, but about 2% of its mass is made of other elements, mostly oxygen, carbon, neon, and iron.
This is where Schlaufman’s study comes in. According to him, we can distinguish between gas giants like Jupiter, and brown dwarfs, by the nature of the star they orbit. The types of planets that form around stars mirror the metallicity of the star itself. Gas giants like Jupiter are usually found orbiting stars with metallicity equal to or greater than our Sun. But brown dwarfs aren’t picky; they form around almost any star. Why?
Brown Dwarfs and Planets Form Differently
Planets like Jupiter are formed by accretion. A rocky core forms, then gas collects around it. Once the process is done, you have a gas giant. For this to happen, you need metals. If metals are present for these rocky cores to form, their presence will be reflected in the metallicity of the host star.
But brown dwarfs aren’t formed by accretion like planets are. They’re formed the same way stars are; by gravitational collapse. They don’t form from an initial rocky core, so metallicity isn’t a factor.
This brings us back to Kevin Schlaufman’s study. He wanted to find out the mass at which point an object doesn’t care about the metallicity of the star they orbit. He concluded that objects above 10 times the mass of Jupiter don’t care if the star has rocky elements, because they don’t form from rocky cores. Hence, they’re not planets akin to Jupiter; they’re brown dwarfs that formed by gravitational collapse.
What Does It Matter What We Call Them?
Let’s look at the Pluto controversy to understand why names are important.
The struggle to accurately classify all the objects we see out there in space is ongoing. Who can forget the plight of poor Pluto? In 2006, the International Astronomical Union (IAU) demoted Pluto, and stripped it of its long-standing status as a planet. Why?
Because the new definition of what a planet is relied on these three criteria:
- a planet is in orbit around a star.
- a planet must have sufficient mass to assume a hydrostatic equilibrium (a nearly round shape.)
- a planet has cleared the neighbourhood around its orbit
The more we looked at Pluto with better telescopes, the more we realized that it did not meet the third criteria, so it was demoted to Dwarf Planet. Sorry Pluto.
Our naming conventions for astronomical objects are important, because they help people understand how everything fits together. But sometimes the debate over names can get tiresome. (The Pluto debate is starting to wear out its welcome, which is why some suggest we just call them all “worlds.”)
Though the Pluto debate is getting tiresome, it’s still important. We need some way of understanding what makes objects different, and names that reflect that difference. And the names have to reflect something fundamental about the objects in question. Should Pluto really be considered the same type of object as Jupiter? Are both really planets in the same sense? The IAU says no.
The same principle holds true with brown dwarfs and gas giants. Giving them names based solely on their mass doesn’t really tell us much. Schlaufman aims to change that.
His new definition makes sense because it relies on how and where these objects form, not simply their size. But not everyone will agree, of course.
Let the debate begin.
The hunt for exoplanets has turned up many fascinating case studies. For example, surveys have turned up many “Hot Jupiters”, gas giants that are similar in size to Jupiter but orbit very close to their suns. This particular type of exoplanet has been a source of interest to astronomers, mainly because their existence challenges conventional thinking about where gas giants can exist in a star system.
Hence why an international team led by researchers from the European Southern Observatory (ESO) used the Very Large Telescope (VLT) to get a better look at WASP-19b, a Hot Jupiter located 815 light-years from Earth. In the course of these observations, they noticed that the planet’s atmosphere contained traces of titanium oxide, making this the first time that this compound has been detected in the atmosphere of a gas giant.
The study which describes their findings, titled “Detection of titanium oxide in the atmosphere of a hot Jupiter“, recently appeared in the science journal Nature. Led by Elyar Sedaghati – a recent graduate from the Technical University of Berlin and a fellow at the European Southern Observatory – the team used data collected by the VLT array over the course of a year to study WASP-19b.
Like all Hot Jupiters, WASP-19b has about the same mass as Jupiter and orbits very close to its sun. In fact, its orbital period is so short – just 19 hours – that temperatures in its atmosphere are estimated to reach as high as 2273 K (2000 °C; 3632 °F). That’s over four times as hot as Venus, where temperatures are hot enough to melt lead! In fact, temperatures on WASP-19b are hot enough to melt silicate minerals and platinum!
The study relied on the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) instrument on the VLT, a multi-mode optical instrument capable of conducting imaging, spectroscopy and the study of polarized light (polarimetry). Using FORS2, the team observing the planet as it passed in front of its star (aka. made a transit), which revealed valuable spectra from its atmosphere.
After carefully analyzing the light that passed through its hazy clouds, the team was surprised to find trace amounts of titanium oxide (as well as sodium and water). As Elyar Sedaghati, who spent 2 years as a student with the ESO to work on this project, said of the discovery in an ES press release:
“Detecting such molecules is, however, no simple feat. Not only do we need data of exceptional quality, but we also need to perform a sophisticated analysis. We used an algorithm that explores many millions of spectra spanning a wide range of chemical compositions, temperatures, and cloud or haze properties in order to draw our conclusions.”
Titanium oxide is a very rare compound which is known to exist in the atmospheres of cool stars. In small quantities, it acts as a heat absorber, and is therefore likely to be partly responsible for WASP-19b experiencing such high temperatures. In large enough quantities, it can prevent heat from entering or escaping an atmosphere, causing what is known as thermal inversion.
This is a phenomena where temperatures are higher in the upper atmosphere and lower further down. On Earth, ozone plays a similar role, causing an inversion of temperatures in the stratosphere. But on gas giants, this is the opposite of what usually happens. Whereas Jupiter, Saturn, Uranus and Neptune experience colder temperatures in their upper atmospheres, temperatures are much hotter closer to the core due to increases in pressure.
The team believes that the presence of this compound could have a substantial effect on the atmosphere’s temperature, structure and circulation. What’s more, the fact that the team was able to detect this compound (a first for exoplanet researchers) is an indication of how exoplanet studies are achieving new levels of detail. All of this is likely to have a profound impact on future studies of exoplanet atmospheres.
The study would also have not been possible were it not for the FORS2 instrument, which was added to the VLT array in recent years. As Henri Boffin, the instrument scientist who led the refurbishment project, commented:
“This important discovery is the outcome of a refurbishment of the FORS2 instrument that was done exactly for this purpose. Since then, FORS2 has become the best instrument to perform this kind of study from the ground.”
Looking ahead, it is clear that the detection of metal oxides and other similar substances in exoplanet atmospheres will also allow for the creation of better atmospheric models. With these in hand, astronomers will be able to conduct far more detailed and accurate studies on exoplanet atmospheres, which will allow them to gauge with greater certainty whether or not any of them are habitable.
So while this latest planet has no chance of supporting life – you’d have better luck finding ice cubes in the Gobi desert! – its discovery could help point the way towards habitable exoplanets in the future. On step closer to finding a world that could support life, or possibly that elusive Earth 2.0!
KIC 8462852 (aka. Tabby’s Star) continues to be a source of both fascination and controversy. Ever since it was first seen to be undergoing strange and sudden dips in brightness (in October of 2015) astronomers have been speculating as to what could be causing this. Since that time, various explanations have been offered, including large asteroids, a large planet, a debris disc or even an alien megastructure.
The latest suggestion for a natural explanation comes from the University of Antioquia in Colombia, where a team of researchers have proposed that both the larger and smaller drops in brightness could be the result of a ringed planet similar to Saturn transiting in front of the star. This, they claim, would explain both the sudden drops in brightness and the more subtle dips seen over time.
The study, titled “Anomalous Lightcurves of Young Tilted Exorings“, recently appeared online. Led by Mario Sucerquia, a postdoctoral student at the University of Antioquia’s Department of Astronomy, the team performed numerical simulations and semi-analytical calculations to determine if a the transits of a ringed gas giant could explain the recent observations made of Tabby’s Star.
Currently, exoplanet-hunters use a number of methods to detect planetary candidates. One of the most popular is known as the Transit Method, where astronomers measure dips in a star’s brightness caused by a planet passing between it and the observer (i.e. transiting in front of a star). How a gas giant with rings would dim a star’s light was of concern here because it would do so in an irregular way.
Basically, the rings would be the first thing to obscure light coming from the star, but only to a small degree. Once the bulk of the gas giant transited the star, a significant drop would occur followed a second smaller drop as the rings on the other side passed by. But since the rings would be at a different angle every time, the smaller dips would be larger or smaller and the only way to know for sure would be to compare multiple transits.
In the past, researchers from the University of Antioquia developed a novel method for detecting rings around exoplanets (“exorings”). Essentially, they showed how an increase in the depth of a transit signal and the so-called “photo-ring” effect (often mistaken for false-positives in previous surveys) could be interpreted as signs of an exoplanet with a Saturn-like ring structure.
The team that devised this method was led by Jorge I. Zuluaga of the Harvard Smithsonian Center for Astrophysics (CfA), who was also a co-author on this study. To test this theory with KIC 8462852, the team simulated a light curve from a ringed planet that was about 0.1 AU from the star. What they found was that a tilted ring structure could explain the dimming effects detected from Tabby’s Star in the past.
They also found that a tilted ring structure would undergo short-term changes in shape and orientation as a result of the star’s gravitational tug on them. These would be apparent due to strong variations of transit depth and contact times even between consecutive transits. This too would likely be interpreted as anomalies in signal data, or lead to miscalculations of a planet’s properties (i.e. radius, semi-major axis, stellar density, etc).
This is not the first time that a ringed-structure has been suggested as an explanation for the mystery that is Tabby’s Star. And the team admits that there are other possible explanations, which include the possibility of an exomoon breaking up around a larger planet (i.e. leaving a debris disk). But as Sucerquia indicated in an interview with New Scientist, this latest study does offer some compelling food for thought:
“The point of this work is to show the community that there are mechanisms that can alter the light curves. These changes can be generated by the dynamics of the moons or the rings, and the changes in these systems can occur in such short scales as to be detected in just a few years.”
Another interesting takeaway from the research study is the fact that oscillating ring structures could also account for the strangeness of some light-curves that are already known. In other words, its possible that astronomers have already found evidence of ringed exoplanets, and simply didn’t know it. Looking ahead, it is possible that future surveys could turn up plenty more of these worlds as well.
Of course, if this study should prove to be correct, it means that what some consider our best hope of finding an alien megastructure has now been lost. Admittedly, this would be a disappointment. If there’s one thing about the mystery of Tabby’s Star that has been consistently intriguing, it’s the fact that a megastructure couldn’t be ruled out. If we have come to that point at last, there’s not much more to say.
Except, perhaps, that’s it’s a big Universe! There’s sure to be a Kardashev Type II civilization out there somewhere!
I look forward to all the future missions that NASA is going to be sending out in the Solar System. Here, check this out. You can use NASA’s website to show you all the future missions. Here’s everything planned for the future, here’s everything going to Mars.
Now, let’s look and see what missions are planned for the outer planets of the Solar System, especially Uranus and Neptune. Oh, that’s so sad… there’s nothing.
It’s been decades since humanity had an up close look at Uranus and Neptune. For Uranus, it was Voyager 2, which swept through the system in 1986. We got just a few tantalizing photographs of the ice giant planet and it’s moons.
What’s going on there?
What are those strange features? Sorry, insufficient data.
And then Voyager 2 did the same, zipping past Neptune in 1989.
Check this out.
What’s going here on Triton? Wouldn’t you like to know more? Well, too bad! You can’t it’s done, that’s all you get.
Don’t get me wrong, I’m glad we’ve studied all these other worlds. I’m glad we’ve had orbiters at Mercury, Venus, everything at Mars, Jupiter, and especially Saturn. We’ve seen Ceres and Vesta, and the Moon up close. We even got a flyby of Pluto and Charon.
It’s time to go back to Uranus and Neptune, this time to stay.
And I’m not the only one who feels this way.
Scientists at NASA recently published a report called the Ice Giant Mission Study, and it’s all about various missions that could be sent to explore Uranus, Neptune and their fascinating moons.
The team of scientists who worked on the study considered a range of potential missions to the ice giants, and in the end settled on four potential missions; three that could go to Uranus, and one headed for Neptune. Each of them would cost roughly $2 billion.
Uranus is closer, easier to get to, and the obvious first destination of a targeted mission. For Uranus, NASA considered three probes.
The first idea is a flyby mission, which will sweep past Uranus gathering as much science as it can. This is what Voyager 2 did, and more recently what NASA’s New Horizons did at Pluto. In addition, it would have a separate probe, like the Cassini and Galileo missions, that would detach and go into the atmosphere to sample the composition below the cloudtops. The mission would be heavy and require an Atlas V rocket with the same configuration that sent Curiosity to Mars. The flight time would take 10 years.
The main science goal of this mission would be to study the composition of Uranus. It would make some other measurements of the system as it passed through, but it would just be a glimpse. Better than Voyager, but nothing like Cassini’s decade plus observations of Saturn.
I like where this is going, but I’m going to hold out for something better.
The next idea is an orbiter. Now we’re talking! It would have all the same instruments as the flyby and the detachable probe. But because it would be an orbiter, it would require much more propellant. It would have triple the launch mass of the flyby mission, which means a heavier Atlas V rocket. And a slightly longer flight time; 12 years instead of 10 for the flyby.
Because it would remain at Uranus for at least 3 years, it would be able to do an extensive analysis of the planet and its rings and moons. But because of the atmospheric probe, it wouldn’t have enough mass for more instruments. It would have more time at Uranus, but not a much better set of tools to study it with.
Okay, let’s keep going. The next idea is an orbiter, but without the detachable probe. Instead, it’ll have the full suite of 15 scientific instruments, to study Uranus from every angle. We’re talking visible, doppler, infrared, ultraviolet, thermal, dust, and a fancy wide angle camera to give us those sweet planetary pictures we like to see.
Study Uranus? Yes please. But while we’re at it, let’s also sent a spacecraft to Neptune.
As part of the Ice Giants Study, the researchers looked at what kind of missions would be possible. In this case, they settled on a single recommended mission. A huge orbiter with an additional atmospheric probe. This mission would be almost twice as massive as the heaviest Uranus mission, so it would need a Delta IV Heavy rocket to even get out to Neptune.
As it approached Neptune, the mission would release an atmospheric probe to descend beneath the cloudtops and sample what’s down there. The orbiter would then spend an additional 2 years in the environment of Neptune, studying the planet and its moons and rings. It would give us a chance to see its fascinating moon Triton up close, which seems to be a captured Kuiper Belt Object.
Unfortunately there’s no perfect grand tour trajectory available to us any more, where a single spacecraft could visit all the large planets in the Solar System. Missions to Uranus and Neptune will have to be separate, however, if NASA’s Space Launch System gets going, it could carry probes for both destinations and launch them together.
The goal of these missions is the science. We want to understand the ice giants of the outer Solar System, which are quite different from both the inner terrestrial planets and the gas giants Jupiter and Saturn.
The gas giants are mostly hydrogen and helium, like the Sun. But the ice giants are 65% water and other ices made from methane and ammonia. But it’s not like they’re big blobs of water, or even frozen water. Because of their huge gravity, the ice giants crush this material with enormous pressure and temperature.
What happens when you crush water under this much pressure? It would all depend on the temperature and pressure. There could be different types of ice down there. At one level, it could be an electrically conductive soup of hydrogen and oxygen, and then further down, you might get crystallized oxygen with hydrogen ions running through it.
Hailstones made of diamond could form out of the carbon-rich methane and fall down through the layers of the planets, settling within a molten carbon core. What I’m saying is, it could be pretty strange down there.
We know that ice giants are common in the galaxy, in fact, they’ve made up the majority of the extrasolar planets discovered so far. By better understanding the ones we have right here in our own Solar System, we can get a sense of the distant extrasolar planets turning up. We’ll be better able to distinguish between the super earths and mini-neptunes.
Another big question is how these planets formed in the first place. In their current models, most planetary astronomers think these planets had very short time windows to form. They needed to have massive enough cores to scoop up all that material before the newly forming Sun’s solar wind blasted it all out into space. And yet, why are these kinds of planets so common in the Universe?
The NASA mission planners developed a total of 12 science objectives for these missions, focusing on the composition of the planets and their atmospheres. And if there’s time, they’d like to know about how heat moves around, their constellations of rings and moons. They’d especially like to investigate Neptune’s moons Triton, which looks like a captured Kuiper Belt Object, as it orbits in the reverse direction from all the other moons in the Solar System.
In terms of science, the two worlds are very similar. But because Neptune has Triton. If I had to choose, I’d go with a Neptune mission.
Are you excited? I’m excited. Here’s the bad news. According to NASA, the best launch windows for these missions would be 2029 or 2034. And that’s just the launch time, the flight time is an additional decade or more on top of that. In other words, the first photos from a Uranus flyby could happen in 2039 or 2035, while orbiters could arrive at either planet in the 2040s. I’m sure my future grandchildren will enjoy watching these missions arrive.
But then, we have to keep everything in perspective. NASA’s Cassini mission was under development in the 1980s. It didn’t launch until 1997, and it didn’t get to Saturn until 2004. It’s been almost 20 years since that launch, and almost 40 years since they started working on it.
I guess we need to be more patient. I can be patient.
Beyond the Solar System’s Main Asteroid Belt lies the realm of the giants. It is here, staring with Jupiter and extending to Neptune, that the largest planets in the Solar System are located. Appropriately named “gas giants” because of their composition, these planets dwarf the rocky (terrestrial) planets of the inner Solar System many times over.
Just take a look at Saturn, the gas giant that takes its name from the Roman god of agriculture, and the second largest planet in the Solar System (behind Jupiter). In addition to its beautiful ring system and its large system of moons, this planet is renowned for its incredible size. Just how big is this planet? Well that depends on what your frame of reference is.
First let’s consider how large Saturn is from one end to the other – i.e. it’s diameter. The equatorial diameter of Saturn is 120,536 km ± 8 km (74,898 ± 5 mi) – or the equivalent of almost 9.5 Earths. However, as with all planets, their is a difference between the equatorial vs. the polar diameter. This difference is due to the flattening the planet experiences at the poles, which is caused by the planet’s rapid rotation.
The poles are about 5,904 km closer to the center of Saturn than points on the equator. As a result, Saturn’s polar radius is about 108,728 ± 20 km (67,560 ± 12 mi) – or the equivalent of 8.5 Earths. That’s a pretty big difference, and you can actually see that Saturn looks a little squashed in pictures. Just for comparison, the equatorial diameter of Saturn is 9.4 times bigger than Earth, and it’s about 84% the diameter of Jupiter.
Volume and Surface Area:
In terms of volume and surface area, the numbers get even more impressive! For starters, the surface area of Saturn is 42.7 billion km² (16.5 billion sq miles), which works out to about 83.7 times the surface area of Earth. That’s smaller than Jupiter though, being only 68.7% of Jupiter’s surface area. Still, that is pretty astounding when put into perspective.
On the other hand, Saturn has a volume of 827.13 trillion km³ (198.44 trillion cubic miles), which effectively means you could fit Earth inside of it 763 times over and still have room enough for about twenty Moons! Again, Jupiter has it beat since Saturn has only 57.8% the volume of Jupiter. It’s big, but Jupiter is just that much bigger.
Mass and Density:
What about mass? Of course Saturn is much, much, MUCH more massive than Earth. In fact, it’s mass has been estimated to be a whopping 568,360,000 trillion trillion kg (1,253,000,000 trillion trillion lbs) – which works out to 95 times the mass of Earth. Granted, that only works out to about 30% the mass of Jupiter, but that’s still a staggering amount of matter.
Looking at the numbers, you may notice that this seems like a bit of a discrepancy. If you could actually fit 763 Earth-sized planets inside Saturn with room to spare, how is it that it is only 95 times Earth’s mass? The answer to that has to do with density. Since Saturn is a gas giant, its matter is distributed less densely than a rocky planet’s.
Whereas Earth has a density of 5.514 g/cm³ (or 0.1992 lb per cubic inch), Saturn’s density is only 0.687 g/cm3 (0.0248 lb/cu in). Like all gas giants, Saturn’s is made up largely of gases that exist under varying degrees of pressure. Whereas the density increases considerably the deeper one goes into Saturn’s interior, the overall density is less than that of water – 1 g/cm³ (0.0361273 lb/cu in).
Yes, Saturn is quite the behemoth. And yet, ongoing investigations into extra-solar planets are revealing that even it and its big brother Jupiter can be beaten in the size department. In fact, thanks to the Kepler mission and other exoplanet surveys, astronomers have found a plethora of “Super-Jupiters” in the cosmos, which refers to planets that are up to 80 times the mass of Jupiter.
I guess the takeaway from this is that there’s always a bigger planet out there. So watch your step and remember not to throw your weight (or mass or volume) around!
We have written many articles about Saturn for Universe Today. Here’s Ten Interesting Facts About Saturn, The Orbit of Saturn, how Long is a Year on Saturn?, What are Saturn’s Rings Made Of?, How Many Moons Does Saturn Have?, What’s the Weather Like on Saturn?, and What is the Atmosphere Like on Saturn?
We have also recorded an entire episode of Astronomy Cast all about Saturn. Listen here, Episode 59: Saturn.
Since it’s discovery in the mid-19th century, Neptune has consistently been a planet of mystery. As the farthest planet from our Sun, it has only been visited by a single robotic mission. And there are still many unanswered questions about what kind of mechanics power its interior. Nevertheless, what we have learned about the planet in the course of the past few decades is considerable.
For example, thanks to the Voyager 2 probe and multiple surveys using Earth-based instruments, scientists have managed to gain a pretty good understanding of Neptune’s structure and composition. In addition to knowing what makes up its atmosphere, planetary models have also predicted what the interior of the planet looks like. So just what is Neptune made of?
Structure and Composition:
Neptune, like the rest of the gas giant planets in the Solar System, can be broken up into various layers. The composition of Neptune changes depending on which of these layers you’re looking at. The outermost layer of Neptune is the atmosphere, forming about 5-10% of the planet’s mass, and extending up to 20% of the way down to its core.
Beneath the atmosphere is the planet’s large mantle. This is a superheated liquid region where temperatures can reach as high as 2,000 to 5,000 K (1727 – 4727 °C; 3140 – 8540 °F). The mantle is equivalent to 10 – 15 Earth masses and is rich in water, ammonia and methane. This mixture is referred to as icy even though it is a hot, dense fluid, and is sometimes called a “water-ammonia ocean”.
Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere. Unlike Uranus, Neptune’s composition has a higher volume of ocean, whereas Uranus has a smaller mantle. Like the other gas/ice giants, Neptune is believed to have a solid core, the composition of which is still subject to guesswork. However, the theory that it is rocky and metal-rich is consistent with current theories of planet formation.
In accordance with these theories, the core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.
Due to its smaller size and higher concentrations of volatiles relative to Jupiter and Saturn, Neptune (much like Uranus) is often referred to as an “ice giant” – a subclass of a giant planet. Also like Uranus, Neptune’s internal structure is differentiated between a rocky core consisting of silicates and metals; a mantle consisting of water, ammonia and methane ices; and an atmosphere consisting of hydrogen, helium and methane gas.
Neptune’s atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core, where it reaches pressures of about 10 GPa – or about 100,000 times that of Earth’s atmosphere. At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium, with a trace amount of methane.
As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s is darker and more vivid. Because Neptune’s atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune’s more intense coloring.
Neptune’s atmosphere is subdivided into two main regions: the lower troposphere (where temperature decreases with altitude), and the stratosphere (where temperature increases with altitude). The boundary between the two, the tropopause, lies at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere at a pressure lower than 10-5 to 10-4 microbars (1 to 10 Pa), which gradually transitions to the exosphere.
Neptune’s spectra suggest that its lower stratosphere is hazy due to condensation of products caused by the interaction of ultraviolet radiation and methane (i.e. photolysis), which produces compounds such as ethane and ethyne. The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide, which are responsible for Neptune’s stratosphere being warmer than that of Uranus.
For reasons that remain obscure, the planet’s thermosphere experiences unusually high temperatures of about 750 K (476.85 °C/890 °F). The planet is too far from the Sun for this heat to be generated by ultraviolet radiation, which means another heating mechanism is involved – which could be the atmosphere’s interaction with ion’s in the planet’s magnetic field, or gravity waves from the planet’s interior that dissipate in the atmosphere.
Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet’s magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours.
This differential rotation is the most pronounced of any planet in the Solar System, and results in strong latitudinal wind shear and violent storms. The three most impressive were all spotted in 1989 by the Voyager 2 space probe, and then named based on their appearances.
The first to be spotted was a massive anticyclonic storm measuring 13,000 x 6,600 km and resembling the Great Red Spot of Jupiter. Known as the Great Dark Spot, this storm was not spotted five later (Nov. 2nd, 1994) when the Hubble Space Telescope looked for it. Instead, a new storm that was very similar in appearance was found in the planet’s northern hemisphere, suggesting that these storms have a shorter life span than Jupiter’s.
The Scooter is another storm, a white cloud group located farther south than the Great Dark Spot. This nickname first arose during the months leading up to the Voyager 2 encounter in 1989, when the cloud group was observed moving at speeds faster than the Great Dark Spot.
The Small Dark Spot, a southern cyclonic storm, was the second-most-intense storm observed during the 1989 encounter. It was initially completely dark; but as Voyager 2 approached the planet, a bright core developed and could be seen in most of the highest-resolution images.
The Voyager 2 probe is the only spacecraft to have ever visited Neptune. The spacecraft’s closest approach to the planet occurred on August 25th, 1989, which took place at a distance of 4,800 km (3,000 miles) above Neptune’s north pole. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton – similar to what had been done for Voyager 1‘s encounter with Saturn and its moon Titan.
The spacecraft performed a near-encounter with the moon Nereid before it came to within 4,400 km of Neptune’s atmosphere on August 25th, then passed close to the planet’s largest moon Triton later the same day. The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the center and tilted in a manner similar to the field around Uranus.
Neptune’s rotation period was determined using measurements of radio emissions and Voyager 2 also showed that Neptune had a surprisingly active weather system. Six new moons were discovered during the flyby, and the planet was shown to have more than one ring.
While no missions to Neptune are currently being planned, some hypothetical missions have been suggested. For instance, a possible Flagship Mission has been envisioned by NASA to take place sometime during the late 2020s or early 2030s. Other proposals include a possible Cassini-Huygens-style “Neptune Orbiter with Probes”, which was suggested back in 2003.
Another, more recent proposal by NASA was for Argo – a flyby spacecraft that would be launched in 2019, which would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would be on Neptune and its largest moon Triton, which would be investigated around 2029.
Given its distance from Earth, it is no secret why the Trans-Neptunian region remains mysterious to us. In the coming decades, several proposed missions are expected to travel there and explore its rich population of icy bodies and the giant planet for which it is named. From these studies, we are likely to learn a great deal about Neptune and the history of the Solar System.
We have written many interesting articles about Neptune on Universe Today. Here’s Who Discovered Neptune?, What is the Surface of Neptune Like?, What is the Surface Temperature of Neptune?, How Many Moons Does Neptune Have?, What’s the Atmosphere of Neptune Like?, What Color is Neptune?, The Orbit of Neptune: How Long is a Year on Neptune?
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.
“Hitler’s acid” is a colloquial name used to refer to Orthocarbonic acid – a name which was inspired from the fact that the molecule’s appearance resembles a swastika. As chemical compounds go, it is quite exotic, and chemists are still not sure how to create it under laboratory conditions.
But it just so happens that this acid could exist in the interiors of planets like Uranus and Neptune. According to a recent study from a team of Russian chemists, the conditions inside Uranus and Neptune could be ideal for creating exotic molecular and polymeric compounds, and keeping them under stable conditions.
The study was produced by researchers from the Moscow Institute of Physics and Technology (MIPT) and the Skolkovo Institute of Science and Technology (Skoltech). Titled “Novel Stable Compounds in the C-H-O Ternary System at High Pressure”, the paper describes how the high pressure environments inside planets could create compounds that exist nowhere else in the Solar System.
Professor Artem Oganov – a professor at Skoltech and the head of MIPT’s Computational Materials Discovery Lab – is the study’s lead author. Years back, he and a team of researchers developed the worlds most powerful algorithm for predicting the formation of crystal structures and chemical compounds under extreme conditions.
Known as the Universal Structure Predictor: Evolutionary Xtallography (UPSEX), scientists have since used this algorithm to predict the existence of substances that are considered impossible in classical chemistry, but which could exist where pressures and temperatures are high enough – i.e. the interior of a planet.
With the help of Gabriele Saleh, a postdoc member of MIPT and the co-author of the paper, the two decided to use the algorithm to study how the carbon-hydrogen-oxygen system would behave under high pressure. These elements are plentiful in our Solar System, and are the basis of organic chemistry.
Until now, it has not been clear how these elements behave when subjected to extremes of temperature and pressure. What they found was that under these types of extreme conditions, which are the norm inside gas giants, these elements form some truly exotic compounds.
As Prof. Oganov explained in a MIPT press release:
“The smaller gas giants – Uranus and Neptune – consist largely of carbon, hydrogen and oxygen. We have found that at a pressure of several million atmospheres unexpected compounds should form in their interiors. The cores of these planets may largely consist of these exotic materials.”
Under normal pressure – i.e. what we experience here on Earth (100 kPa) – any carbon, hydrogen or oxygen compounds (with the exception of methane, water and CO²) are unstable. But at pressures in the range 1 to 400 GPa (10,000 to 4 million times Earth normal), they become stable enough to form several new substances.
These include carbonic acid, orthocarbonic acid (Hitler’s acid) and other rare compounds. This was a very unusual find, considering that these chemicals are unstable under normal pressure conditions. In carbonic acid’s case, it can only remain stable when kept at very low temperatures in a vacuum.
At pressures of 314 GPa, they determined that carbonic acid (H²CO³) would react with water to form orthocarbonic acid (H4CO4). This acid is also extremely unstable, and so far, scientists have not yet been able to produce it in a laboratory environment.
This research is of considerable importance when it comes to modelling the interior of planets like Uranus and Neptune. Like all gas giants, the structure and composition of their interiors have remained the subject of speculation due to their inaccessible nature. But it could also have implications in the search for life beyond Earth.
According to Oganov and Saleh, the interiors of many moons that orbit gas giants (like Europa, Ganymede and Enceladus) also experience these types of pressure conditions. Knowing that these kinds of exotic compounds could exist in their interiors is likely to change what scientist’s think is going on under their icy surfaces.
“It was previously thought that the oceans in these satellites are in direct contact with the rocky core and a chemical reaction took place between them,” said Oganov. “Our study shows that the core should be ‘wrapped’ in a layer of crystallized carbonic acid, which means that a reaction between the core and the ocean would be impossible.”
For some time, scientists have understood that at high temperatures and pressures, the properties of matter change pretty drastically. And while here on Earth, atmospheric pressure and temperatures are quite stable (just the way we like them!), the situation in the outer Solar System is much different.
By modelling what can occur under these conditions, and knowing what chemical buildings blocks are involved, we could be able to determine with a fair degree of confidence what the interior’s of inaccessible bodies are like. This will give us something to work with when the day comes (hopefully soon) that we can investigate them directly.
Who knows? In the coming years, a mission to Europa may find that the core-mantle boundary is not a habitable environment after all. Rather than a watery environment kept warm by hydrothermal activity, it might instead by a thick layer of chemical soup.
Then again, we may find that the interaction of these chemicals with geothermal energy could produce organic life that is even more exotic!
It’s impossible to do an article about Uranus without opening up the back door to a spit storm of potty humour. I get it, there’s something just hilarious about talking about your, mine and everyone’s anus. And even if you use the more sanitized and sterile term urine-us, it’s still pretty dirty, in an unwashed New York stairwell kind of way. You’re in us? No.
This is a no-win solution. It’s a Kobayashi Maru scenario here. We’re all doomed.
Can we call a truce? I dare you commentators, to keep the YouTube comments as pure and clean as driven snow, so we can focus on the super interesting science. Think of the children.
Let’s set the stage, I’m going to let planetary astronomer Kevin Grazier give you the proper pronunciation to clear our minds and let us move forward with grace and civility.
Strictly speaking, it’s pronounced Youranous, is the pronunciation.
As you probably know, Uranus… I mean Ouranus. No, I can’t do it, my brainwashing is too far along. Save yourselves!. Anyway, Uranus is the 7th planet from the Sun, and the 3rd largest planet in the Solar System. Jupiter and Saturn get all the spacecraft and Hubble space telescopes, but Uranus is an incredibly worthwhile target to visit.
It’s almost exactly 4 times larger than Earth and has its own set of strange dusty rings – perhaps left over from a shattered moon. It has at least 27 moons, that we know of, and many more interesting features that would fascinate astronomers, if we had a spacecraft there, which we don’t. Which is ridiculous. We’ve only made one close flyby of Uranus by Voyager II back in 1986.
We’ve seen Pluto up close, but there are no plans to visit Uranus? Madness.
Anyway, perhaps one of the strangest aspects of Uranus is its tilt. The planet is flipped over on its side, like a Weeble, that wouldn’t unwobble.
Actually, all the planets in the Solar System have some level of axial tilt. The Earth is tilted 23.5 degrees away from the Sun’s equator. Mars is 25 degrees, and even Mercury is 2.1 degrees tilted. These tilts are everywhere.
But Uranus is 97.8 degrees. That’s just 0.2 degrees shy of a 90s boy band.
You might be wondering, why have it be more than 90 degrees. High school geometry tells me that 97.8 degrees is the same as 82.2 degrees. And that’s true. But astronomers define the angle as greater than 90 degrees when you take its direction of rotation into account. When you describe it as turning in the same direction as the rest of the planets in the Solar System, then you have to measure it this way.
What could have done that to Uranus, how could it have happened?
The fact that Uranus is flipped over on its side tells us that the calm clockwork motion of the Solar System hasn’t always been this way. Shortly after the formation of the Sun and planets, our neighborhood was a violent place.
The early planets smashed into each other, pushed one another into new orbits. Some planets could have been spun out of the Solar System entirely, while others might have been driven into the Sun. Our own Moon was likely formed when a Mars-sized object crashed into the Earth. Other moons might have been captured from three body interactions between worlds. It was mayhem.
The Solar System that you see today contains the survivors. Everything that wasn’t delivered a death blow.
And something really tried to deliver a death blow to Uranus, very early after it formed. We know this because the moons of Uranus orbit at the same tilt as the planet’s axis. This means that something smashed into Uranus while it was still surrounded by the disk of gas and dust that its moons formed from.
When the massive collision happened, the planet flipped over, wrenching this disk with it. The moons formed within this new configuration.
Astronomers think it was more complicated than that, however. If it was a single, massive collision, models suggest the planet would just flip over entirely, and end up rotating backwards from the other planets in the Solar System.
It’s more likely that another collision or even a series of collisions put the brakes on Uranus’ end over end roll, putting it into its current configuration. It boggles the mind to think about what must have happened.
Having such a huge axial tilt makes a big different to Uranus. As it travels around the Sun in its 84-year orbit, the planet still has its poles pointed at fixed locations in space. This means that it spends 42 years with its northern hemisphere roughly pointed towards the Sun, and 42 years with its southern hemisphere in sunlight.
If you could stand on the north pole of Uranus, the Sun would be directly overhead in the middle of summer, and then it would make bigger and bigger circles until it dipped below the horizon a few decades later. Then you wouldn’t see it for a few decades until it finally reappeared again. It would be very very strange.
Of course, it’s a gas planet, so you can’t stand on it. If you could stand on it, we’d all be marveling at your ability to stand on planets.
Here we are in our calm, ordered Solar System, everything’s business as usual. But if you look around, you realize it’s pretty amazing that our planet is even here. Poor sideways Uranus is a testament to our good luck.
Our Solar System is a fascinating place. Between its eight planets and many dwarf planets, there are some serious differences in terms of orbit, composition, and temperature. Whereas conditions within the inner Solar System, where planets are terrestrial in nature, can get pretty hot, planets that orbit beyond the Frost Line – where it is cold enough that volatiles (i.e. water, ammonia, methane, CO and CO²) condense into solids – can get mighty cold!
It is in this environment that we find Neptune, the Solar System’s most distance (and hence most cold) planet. While this gas/ice giant has no “surface” to speak of, Earth-based research and flybys have been conducted that have managed to obtain accurate measurements of the temperature in the planet’s upper atmosphere. All told, the planet experiences temperatures that range from approximately 55 K (-218 °C; -360 °F) to 72 K (-200 °C; -328 °F), making it the coldest planet in the Solar System.
Of all the planets in the Solar System, Neptune orbits the Sun at the greatest average distance. With a very minor eccentricity (0.0086), it orbits the Sun at an semi-major axis of approximately 30.11 AU (4,504,450,000,000 km), ranging from 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion.
Neptune takes 16 hours 6 minutes and 36 seconds (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. In addition, the planets axial tilt also leads to variations in the length of its day, as well as variations in temperature between the northern and southern hemispheres (see below).
Due to their composition, determining a surface temperature on gas or ice giants (compared to terrestrial planets or moons) is technically impossible. As a result, astronomers have relied on measurements obtained at altitudes where the atmospheric pressure is equal to 1 bar (or 100 kilo Pascals), the equivalent of air pressure here on Earth at sea level.
It is here on Neptune, just below the upper level clouds, that pressures reach between 1 and 5 bars (100 – 500 kPa). It is also at this level that temperatures reach their recorded high of 72 K (-201.15 °C; -330 °F). At this temperature, conditions are suitable for methane to condense, and clouds of ammonia and hydrogen sulfide are thought to form (which is what gives Neptune its characteristically dark cyan coloring).
But as with all gas and ice giants, temperatures vary on Neptune due to depth and pressure. In short, the deeper one goes into Neptune, the hotter it becomes. At its core, Neptune reaches temperatures of up to 7273 K (7000 °C; 12632 °F), which is comparable to the surface of the Sun. The huge temperature differences between Neptune’s center and its surface create huge wind storms, which can reach as high as 2,100 km/hour, making them the fastest in the Solar System.
Temperature Anomalies and Variations:
Whereas Neptune averages the coldest temperatures in the Solar System, a strange anomaly is the planet’s south pole. Here, it is 10 degrees K warmer than the rest of planet. This “hot spot” occurs because Neptune’s south pole is currently exposed to the Sun. As Neptune continues its journey around the Sun, the position of the poles will reverse. Then the northern pole will become the warmer one, and the south pole will cool down.
Neptune’s more varied weather when compared to Uranus is due in part to its higher internal heating, which is particularly perplexing for scientists. Despite the fact that Neptune is located over 50% further from the Sun than Uranus, and receives only 40% its amount of sunlight, the two planets’ surface temperatures are roughly equal.
Deeper inside the layers of gas, the temperature rises steadily. This is consistent with Uranus, but oddly enough, the discrepancy is larger. Uranus only radiates 1.1 times as much energy as it receives from the Sun, whereas Neptune radiates about 2.61 times as much. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. The mechanism for this remains unknown.
And while temperatures on Pluto have been recorded as reaching lower – down to 33 K (-240 °C; -400 °F) – Pluto’s status as a dwarf planet mean that it is no longer in the same class as the others. As such, Neptune remains the coldest planet of the eight.
We have written many articles about Neptune here at Universe Today. Here’s The Gas (and Ice) Giant Neptune, What is the Surface of Neptune Like?, 10 Interesting Facts About Neptune, and The Rings of Neptune.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.