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a) The gas giants: Jupiter & Saturn
Chapter 4 was dedicated to the Sun and Chapter 5 to the terrestrial planets, the Moon and the asteroid belt. Those planets and their satellites are also called “telluric” in reference to their Earth-like composition or “rocky”, both of those referring to the metallic core and silicon compounds. In contrast to this, beyond the frost lines, compounds called “volatiles” because they vaporize easily such as methane or carbon dioxide do condensate because the temperatures are cool enough and this helps increase the rate of accretion of planetesimals. The resulting planets have composition reflecting those of stars and most of the universe, which is dominated by hydrogen and helium, elements that are called “gases” in astrophysics as opposed to the metallic elements covering all the rest of the periodic table, from lithium onward.
Thus, when the term “gas giant” is used, it does not refer to the gaseous state of matter but to its hydrogen and helium-rich makeup. In the case of Jupiter, the outer layer is molecular hydrogen (H2) with clouds of ammonia crystal subjected to intense weather patterns, including the famous red spot, followed by a large volume of mostly fluid metallic hydrogen and the core is thought to be a mix of rock and ice including many of the denser elements such as carbon and oxygen.
I appreciate the reference to metallic hydrogen just after stating that hydrogen falls into the “gas” category rather than the “metallic” one can raise eyebrows but this is a different concept, essentially a state matter with similarities to plasma that hydrogen can experience under extremely high pressure and hot temperatures where the electrons are stripped away from the nucleus and the resulting protons behave like a fluid with metallic properties, namely they conduct heat and, with rotation and convection coming into play, they generate a very powerful magnetosphere.
The current state of the solar system is not a reflection of how it started and orbits have been perturbed and protoplanets ejected, colliding or falling into the Sun. So we can only guess as to why Jupiter is so big though it is clear that being the first planet to form out of a protoplanetary disk helps and Jupiter was second after the Sun.
Writing about Jupiter and the Sun, the radius of the planet is nearly 70,000kms or just over 10% of that of the Sun and so the volume ratio is only about a thousandth (0.1%) but with a density not fundamentally different from our star’s and on average 5.2 AU separating them, this is enough to have a barycentre for the Jupiter-Sun system lying just outside of the Sun, at 1.06 solar radii. The order of magnitude in volume is the exact opposite when it comes to comparing Jupiter and Earth since the gas giant has a radius 11.2 times larger than our planet; however because it is less dense the giant is merely 318 times more massive. In fact, Jupiter accounts for about 70% of the entire mass of the solar system excluding the Sun.
Unsurprisingly then, its orbit only deviates by 0.32° from the invariable plane. There is little else of note about the giant’s orbit: it is mildly eccentric (0.05) and hardly tilted (3.13°). On the other hand, its rotation is something to behold as it spins on itself in just under 10h, translating into an equatorial velocity of 12.6km/s and a 6.5% equatorial bulge. Shifting our attention away from the planet itself as an orbiting mass and to the bodies orbiting it, its natural satellites, there is potentially plenty to write about. Indeed, we know of 95 of them, though most are only a few kms in diameter. Among these, four are of particular interest. By order of size it would be Ganymede, Callisto, Io and Europa, and together they are called the Galilean moons because they were discovered by Galileo Galilei at the end of 1609 with his newly invented telescope.
- Io is the innermost of the four and is extremely volcanically active on account of the tidal forces exerted by its parent planet and the other Galilean moons. With an iron core and silicate rock, in many ways its composition resembles that of a telluric planet.
- Whereas Io’s diameter is 5% larger than the Moon’s, Europa is 10% smaller and its orbital period is exactly twice that of Io at 3.55 days. It also has an iron core and silicon rock but its crust is frozen water estimated to be 100km thick. It is hypothesized the bottom part could be in liquid form on account of internal heating from the moon’s core and so Europa is a prime candidate for extra-terrestrial life in the Solar System. Were it to be the case, it would very likely be non-complex, congregating around hydrothermal vents, the way it is thought to have started on Earth. Also, it lies within Jupiter’s magnetosphere so it is shielded from deleterious cosmic rays and solar winds though the strength of the field also creates issues of its own.
- Ganymede orbits Jupiter in 7.16 days, with a resonance ratio of 4 with Io and 2 with Europa. Its diameter is 50% larger than the Moon’s and it takes the medal of the largest natural satellite in the Solar System. In fact, at 5,268 kms it is larger than Mercury’s 4,880 kms though it is only 36% as dense and therefore not as massive. Besides an iron core and silicate rock, it features an internally-generated magnetic field, a thin oxygen atmosphere and quite likely a large internal saltwater ocean. The JUICE probe (Jupiter icy moon explorer mission) will tell us more when it arrives in December 2031 (this is the plan at the time of writing so this date is subject to delays).
- Callisto is the furthest of the four Galilean moons from Jupiter and is only marginally smaller than Mercury. This increased distance and the fact that is not in resonance with the other three result in much less tidal heating and less exposure to radiations from Jupiter’s magnetic field. Its composition is mostly rocks and volatiles in ice form, with possibly a saltwater ocean beneath the crust. At the surface there is some water ice and a thin atmosphere with carbon dioxide and perhaps even oxygen. There is less chance the sub-surface water could harbour life as there is no interface with rocky material and no tidal heating.
Much further out, on average 9.58 AU from the Sun, which is almost twice as far as Jupiter, orbits the second of the gas giants in our Solar System: Saturn. Its orbit is somewhat eccentric (0.056) so there is a full astronomical unit of difference between the planet’s aphelion and perihelion, and the sidereal orbital period is a long 29.4 Earth years or over 24,000 Saturn days lasting 10.5 hours. Since the planet has a mean radius of 58,232 kms (9.14 Earth radii) this translates into an equatorial velocity of nearly 10km/s which creates an equatorial bulge of 10%, so even more pronounced than Jupiter’s, and wind speeds in excess of 1,800km/h.
Similarly to the aforementioned giant, Saturn’s interior is composed of a dense core surrounded by metallic hydrogen but it is not as dense, in fact its mean density is one eighth that of Earth at 0.687 g/cm3 (1 is the density of liquid water at 1 atm pressure). Consequently, even though its radius is only 17% smaller than Jupiter’s, the ratio for volume drops to 58% and to 30% for mass. Still, when combined, the two gas giants account for 92% of the planetary mass in the Solar System. So there is them, and the rest.
Of course, when one thinks of Saturn, the first feature that comes to mind is not mass or orbit, it is the thin rings of mostly water ice extending outward from 6,600kms up to 120,000kms. Thin here means really thin, about 20m on average, yet we can see them clearly with amateur optical equipment because they consist of highly reflective particles, most of which are less than 10m in diameter. As I write this near the end of March 2025 however, they have just disappeared from view for a few days as they align perfectly with our line of sight. Indeed, Saturn has an axial tilt of 26.73° so the angle they form when seen from Earth changes over time and every half Saturn-year we cross the ring plane. The main hypotheses around the origin of these rings propose they are the remnants of an earlier natural satellite that suffered a collision or was ripped apart by tidal forces exerted by its parent planet.
This still leaves quite a few moons for Saturn, 274 on the current roaster to be precise. The best known and most interesting of these are Titan and Enceladus. The latter because although very small with a mean diameter of just over 500km, it has plenty of liquid water interfacing with rock surfaces and some tidal heating, a potential recipe for primordial life forms. In comparison, Titan is the largest moon of Saturn and second only after Ganymede in the Solar System and not by much: their mean radii are 2,574km and 2,634km respectively. What makes this moon so interesting is a combination of factors, including its dense atmosphere and surface features often described as Earth-like since they include the equivalent of seas, sand dunes and rivers. This means both weather patterns and liquid at the surface, though considering the temperature of around -180°C this would not be water of course. Instead it is liquid methane (CH4) and ethane (C2H6) while the atmosphere is dominated by nitrogen (95-98% depending on the altitude) and methane. Put together, this means there is a proper methane cycle on Titan, in many ways analogous to the water cycle present on Earth we covered in S3 Section 2.d. As for the interior, there is a rocky core, high pressure ice and an icy crust with a layer of ammonia-rich liquid water in between at a temperature estimated to be -97°C. Another plausible candidate for extra-terrestrial life within the Solar System.
b) The ice giants: Uranus and Neptune
Way further out from the two gas giants lay the two ice giants, the last two planets in the Solar System. The terminology of the former is correct in that they are mainly composed of hydrogen and helium, which is not the case of the ice giants, but those are not, or at least no longer, primarily made of volatile ice and instead they hide oceans made of ammonia and water in a state of supercritical fluid (I am inserting the link to the Wikipedia entry for SCF in the last section of this chapter). The other main elements present besides nitrogen and oxygen are carbon and sulphur while for the atmosphere it is mainly hydrogen and helium.
Uranus, the first of the two ice giants, has an orbit which sees the planet being on average 19.2 AU distant from the Sun, exactly double the number for Saturn, and its orbital period is 84 Earth-years or 42,718 Uranian solar days. The planet has 28 natural satellites though none of them is worth writing home about from there since the largest one has a diameter of less than 1,600kms, and it also surrounded by rings consisting of small dark particles. So far, so unspectacular considering what we have seen regarding other planets. However Uranus does have a couple of very eccentric features…
The first is a head spinning axial tilt of 82.23° retrograde or 97.77° prograde according to the right-hand rule – for which I am providing the Wikipedia link at the end of the chapter if you are curious. To imagine this, 90° corresponds to the equator so think of the planet as a ball rolling on a surface inclined by 7.8% rather than a basketball spinning on your finger. It isn’t tidally locked with the Sun however, so this means four seasons of 21 years if we are to adopt an Earthly analogy, except that during most of the winter for the polar regions, they remain entirely in darkness, for that many years. This is one of the reasons why Uranus is the coldest planet in the Solar System with a lowest point recorded in its tropopause (a boundary within the atmosphere we discussed for Earth in S3 Section 3.a). The proposed explanation for the unconventional tilt is it being the consequence of a major impact eons ago and, in the process, a major portion of its core temperature would have been lost to space.
The second very unusual aspect of the planet is the orientation and the location of its magnetic dipole. The dipole is completely off-centre, by a full third of the radius, compared to the planet’s centre of mass. Furthermore, it is not aligned with the axis of planetary rotation – and not by a slim margin: a 59° tilt. This creates an asymmetric field where the magnetosphere extends much further out on one side and is comparatively weak on the other side. Are the rotation tilt and unconventional magnetic field coordinates and orientation a coincidence? In isolation it would be tempting to answer no, yet Neptune the other ice giant has a less unusual tilt of 28°32 – so not far off that of Earth – and it also showcases a magnetic field which is 47° off from the rotational axis and a dipole situated 0.55 radius from the planet’s centre of mass. So it could be that the dynamo effect arises from convection in the fluid oceans located in the interior rather than from the core. You may want to (re)read S1 Section 8.e to better understand the nature of magnetic fields.
The radius of Neptune is 24,622km so it is comparable with, though just 3% shorter than, the radius of Uranus. However, its mean density is actually significantly higher at 1.638 g/cm3 versus 1.27 g/cm3 so it is actually more massive. Hence, Neptune is the 4th largest planet in the Solar System and ranks third in terms of mass while the ranking is the opposite for Uranus. The combination of volume and density also translates into a strong surface gravity of 1.14g, only bested by Jupiter in the solar system.
Neptune has 16 moons with Triton the only large body in the lot; it has a diameter about 21% that of Earth and is primarily made up of water ice, nitrogen ice and a rocky-metallic core. Besides the magnetic field, the convection happening in the inner oceans and ensuing heat exchange mechanisms might also be responsible for the puzzlingly extreme winds of up to 2,200km/h experienced in the planet’s atmosphere. This remains an area of studies and hypotheses. As for orbit, it extends another 10 AU beyond the path of Uranus, on average 30.1 AU from the Sun with hardly any eccentricity and an orbital period of a whopping 164.8 Earth-years or 89,666 Neptunian solar days – devising a simple calendar sounds like quite a challenge there
c) The Kuiper Belt
Further out, beyond Neptune’s orbit at 30 AU and up to 50 AU is another type of belt, not rock-metal asteroids this time but rock and volatile ices mostly composed of ammonia, water and methane, those same compounds found aplenty in the ice giants. This region of space called the Kuiper Belt is much wider than the asteroid belt and an estimated two orders of magnitude more massive in aggregate at about 2% the mass of Earth. Most of those objects are small, nonetheless there are a few exceptions, the best-known being Pluto, the one-time planet that was demoted to the status of dwarf planet.
This dwarf planet has five known natural satellites, including Charon with a mean radius of 606km, which is more than half the 1,188kms of Pluto. Why was Pluto reclassified then? Some use the word demoted but I don’t think Pluto really minds. In 2006, the International Astronomical Union formally defined, for the first time, the criteria for a body in the Solar System to make the cut as a planet. These are 1) be in orbit around the Sun, 2) be massive enough to have a nearly rounded shape accommodating hydrostatic equilibrium, and 3) having cleared the neighbourhood around its orbit. Those bodies not meeting the third criteria would be deemed dwarf planets and Pluto clearly fails that test since it accounts for only 7% of the total mass of the objects in its orbit.
The more interesting aspect of Pluto is its orbit taking it as far as 49.3 AU from the Sun and as close as 29.66 AU, meaning it does cross into Neptune’s orbit though it never goes anywhere near the giant, with a full orbit taking a long 248 Earth-years. The interesting feature is not so much its 0.25 orbital eccentricity, rather it is the orbital resonance it shares with Neptune: Pluto goes around its elliptical orbit twice for every three trips Neptune completes. Orbital resonance is essentially the existence of a ratio of integers in orbiting periods and is a key concept in the formation of stellar systems and generally the orbiting of any number of bodies in a system. Depending on circumstances, the extra tug it provides to the less massive of the bodies can have a stabilizing or destabilizing effect, the latter ultimately resulting either in a collision or more probably a gravity assist-type of change in velocity propelling the smaller bodies towards a new inner or outer orbit in the best case, or into the Sun or into outer space in the worst case. Typically, the effect will be a stabilizing one when the objects are never too close from one another, as is the case in the Pluto-Neptune resonance, and in other cases it will be destabilizing. For instance, in the asteroid belt, there are huge gaps with hardly any bodies in the bands matching the resonance of 3:1, 5:2 and 2:1 with Jupiter, even 7:3 sees a noticeable drop in density of orbiting asteroids. An extreme example of resonance with ratios 4:2:1 called Laplace resonance can be seen between the Galilean moons of Jupiter; Io goes 4 times around Jupiter while Europa goes twice and Ganymede once.
The Neptune-centric resonance greatly impacts the Kuiper Belt with many objects, in addition to Pluto, in the 3:2 band and only a few in the 2:1 band. Over several millions of years, this would have had the potential to move the giants inward or outward to the orbits they currently occupy and so to understand the genesis of the Kuiper Belt these historical shifts need to be modelled. There are two main strands to the prevalent hypothesis; in the first, the belt consists of planetesimals that never reached the critical mass to coalesce, probably because the density of matter so far out in the protoplanetary disk was insufficient, and in the second, most of the original content of the belt has been displaced, either inward or in the majority of cases, outward. I thought it would be useful to visualize the distance and densities involved so I have included a diagram showing the Sun at the centre, the Giant Planets symbolized by their initials (J, S, U, N), the Greeks and Trojans at the Lagrange points on either side of Jupiter, and in blue and red the Kuiper Belt. The diagram is drawn to scale, in astronomical units, based on positions in January 2015.
Figure 4: Objects in the Kuiper Belt
Credit: WilyD (CC BY-SA 3.0)
The Kuiper Belt is home to several minor planets, those bodies classified neither as planets nor as comets. Among them are several Trans-Neptunian objects, being objects lying beyond the orbit of Neptune. Some are commonly accepted to make the cut to dwarf planets such as Haumea, Makemake, Quaoar, and Orcus. However, there are also Trans-Neptunian objects much further out, in particular in the formation called the Scattered Disk where Eris, the most massive of the dwarf planets, and Sedna, another dwarf planet, are located.
The Scattered Disk is a lot more diffuse because the orbits of the objects within can be both elliptical, approaching the Sun as close as 30-35 AU and venturing out as far as 100 AU, and heavily inclined compared to the invariable plane. Similarly to objects in the Kuiper Belt, those in this area of the Solar System are mostly composed of volatile ices and thought to have been scattered into their current orbit due to interferences from Neptune, hence the name.
d) The Oort Cloud and comets
Most of the short-period comets are thought to originate from either the Scattered Disk or the Kuiper Belt as a result of the above-mentioned Neptunian gravitational interference. To use the IAU wording, a comet is “a body made of rock and ice, typically a few kilometres in diameter, which orbits the Sun.” As a comet approaches the Sun, it warms and some of its ice sublimates (the transition from solid to gas) and is then blown by the solar wind into a long tail of gas directed away from the Sun. In fact, there are two tails, one is made of dust and the other gas, mostly water. The most famous of these is Halley’s comet, it makes an apparition – that is the technical term – every 72 to 80 years, with the last one dating from 1986 and the next perihelion scheduled for end July 2061.
Short-period comets refer to those having orbital periods under 200 years with the remaining being classified as long-period. These are thought to originate mostly from the Oort Cloud and many of the other Trans-Neptunian objects could have followed the same path. The Oort Cloud has never been observed because it is simply too far away considering how faint the objects located there are, nevertheless it is an important aspect of the current theory surrounding the formation of the Solar System. It consists of two sub-regions: the torus-shaped (think of a ring donut as opposed to a flat flying ring) inner cloud (also called Hills cloud) aligned with the solar ecliptic starting at around 2,000 AU and extending anywhere between 50,000 and 200,000 AU depending on models, and a spherical cloud, the outer Oort Cloud, encapsulating all the planets and even the Kuiper Belt and the Scattered Disk. This is way beyond the heliopause at around 120 AU, in interstellar space.
Again, we are looking at original planetesimals from the protoplanetary disk that have not coalesced and drifted due to planetary interactions as well as quite possibly the influence of other stars, not just the Sun. Indeed, the centre of gravity of the Milky Way exerts tidal forces and other stellar systems in orbit within the Milky Way can de-orbit some of those objects. This boundary where galactic gravitational pull balances out with that of our star is where the Oort Cloud is thought to terminate.
e) Trivia – Space weather
As you may recall, the heliopause marks the extent of the heliosphere, this part of space primarily dominated by the Sun; not merely its gravitational pull since as we have just seen other planets play an important role in that respect, but also several of the physical phenomena emanating from it. These and the effects they have on the rest of the planets, Earth in particular, and various human technologies, comes under the name of “space weather”.
The main drivers of space weather are solar flares and solar winds, and the disturbances created by or dangers stemming from those vectors are either magnetic in nature or the result of the high energy particles contained in solar rays. The latter can play havoc with electronics and organisms not shielded from them by the ozone layer and the ionosphere. The former can interfere with Earth’s magnetosphere and transfer energy into it giving rise to geomagnetic storms causing not just pretty auroras but surges in electrical currents that can bring down large parts of our modern telecom and electricity distribution infrastructure. Navigation systems also tend to be affected because disturbances to the ionosphere impact molecular density and signal propagation, resulting in erroneous outputs.
As for the effects of space weather on intra and interstellar travel, we’ll revert to it in S3 Section 10.d, in the chapter dedicated to cosmological exploration.
f) Further reading (S3C6)
Suggested reads:
- Wikipedia on Supercritical fluid: https://en.wikipedia.org/wiki/Supercritical_fluid
- Wikipedia on Right-hand rule: https://en.wikipedia.org/wiki/Right-hand_rule
- Wikipedia on Orbital resonance: https://en.wikipedia.org/wiki/Orbital_resonance
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