Image by flflflflfl from Pixabay

One of the essential skills that has to be in every GM’s toolkit is the ability to interleave research into subjects that they, and their players, know nothing about into the stories that they craft for their campaigns.

Way back in September 2014, I produced an abbreviated list of subjects that a GM arguably had to at least seem to be expert in, as part of The Expert In Everything?.

In 2016, I followed up with Lightning Research: Maximum Answers in Minimum Time, in which I describe the research techniques that I employ in order to be able to pull off that sleight-of-hand.

It’s not something that comes up every weekend, and quite often it’s fairly superficial, but recently I’ve had a couple of more intense such sessions.

I can’t really talk about one of them, because the adventure in question is still unfinished, and is months away from actually being played (in the Adventurer’s Club campaign), and I can’t talk about the second, because it would give too much away about the Adventurer’s Club adventure that’s only just started, but the third example is from the Doctor Who campaign, which lives rather closer to the margins. The relevant part of that adventure was played yesterday (as I write this), and written almost completely in the 48 hours prior, so it’s the most fresh example of the three, to boot.

What’s more, the research in question is inherently interesting (at least in my opinion), and that made choosing this topic an easy decision.

But on top of all that, a chance discovery (on my part) of a couple of tangentially-relevant stories provides an extra bonus that cemented the choice.

Here’s what I’ve mapped out for this article, a table of contents if you will.

1. A Golden Period
2. Planetary Types
   1. Classification Structure
   2. Terrestrial Planets
      • Carbon Planet
      • Coreless Planet
      • Desert Planet
      • Dwarf Planet
      • Ice Planet
      • Iron Planet
      • Lava Planet
      • Ocean World
      • Super-Earth
      • Mega-Earth
      • Sub-Earth
      • Ultra-short period planets
   3. Gaseous Planets
      • Controversy: When Is A Brown Dwarf Not A Brown Dwarf?
      • Gas Giant
      • Hot Jupiter
      • Super-Jupiter
      • Super-Puff
      • Eccentric Jupiter
      • Puffy Planets
      • Helium Planet
      • Gas dwarf (aka Mini-Jupiter)
      • Ice Giants
      • Mini-Ice Giant (aka Mini-Neptune)
      • Hot Neptune
      • Super-Neptune
      • Ultra-Hot Neptune
   4. Selected Other Types Of Planet
   • Blanet
   • Cthonian Planet
   • Circumbinary Planet
   • Circumtriple Planet
   • Disrupted Planet
   • Double Planet
   • Ecumenopolis
   • Mesoplanet
   • Eyeball Planet
   • Pulsar Planet
   • Sub-brown Dwarf
   • Sub-Neptune
   • Toroidal Planet
   • Ultra-short Period Planet (USP)
   • Superhabitable Planet
3. Unusual Exoplanets List
4. Nearby Habitable Systems
5. List Of Named Exoplanets
6. The State Of Cosmological Texts
7. Controversy: Universal Expansion

 
That’s as far as I got in this article, so in part 2, there will be:

8. Dr Who: Venturi Station
   1. The Needs Of Story
   2. Dr Who: Defining Venturi
   3. Further Research
   4. Plot outline
   5. A Cast Of Characters: Integrating Introductions
   6. Just-In-Time Infodumps vs. Background Teasers
   7. Spacing The Pacing
   8. Adventure Content
9. Fantasy: Many Planes of Cosmological Grit

So this is, on the face of it, going to be an absolutely huge article. But by being brief, I have hopes that it can be completed and published on schedule.

Misplaced hopes, as it turns out – though I got a huge amount done, it wasn’t complete, as you can see from the above.

A Golden Period

We are living in a golden age in terms of advancing our cosmological knowledge. Never before have we learned so much about the universe we inhabit in such a short period of time.

The first exoplanet was detected in 1988, but that one (Gamma Cephei Ab) wasn’t confirmed until 2002. Just four years after that first detection, in 1992, exoplanets were confirmed around PSR B 1257+12 (which is a mouthful! This is a millisecond pulsar located in the constellation Virgo, believe it or not! And just three years after that, the first confirmed exoplanet around a normal star was found, 51 Pegasi b, currently named Dimidium.

As of 1 May, 2022 (just two weeks ago), there have been 5017 confirmed exoplanets discovered in 3,694 planetary systems, 822 of which have more than one planet. On top of that, there are 6587 detections from three mission still awaiting confirmation.

That’s absolutely massive – potentially, 11,600 exoplanets in 30 years, or an average of about 387 a year. More than one a day, on average!! Even if we assume that none of those unconfirmed contenders turn out to be real (something that I doubt), that’s still more than 167 a year, or about one every 2 days, 4 hours.

Arguably, though, it’s not the fact of their existence that is the headline news these days, it’s what has been determined about their natures, something we have been getting better at, and that will take a quantum leap forward once the Webb Space Telescope becomes fully operational, any day now.

Any science-fiction property – be it a game, campaign, adventure, novel, short story, TV series, or a movie – that predates 1992 and even hints at something to do with space is at a near-certainty of being out out-of-date, and that risk remains significant even if it was first played/published/broadcast just yesterday.

This presents both a huge demand on GMs as well as a huge opportunity. To make your game or game product cutting edge, all it needs to be is up to date with respect to the general classification and frequency of occurrence of exoplanet types, or to contain some good reasons for having differing answers. That’s the opportunity. The responsibility comes with avoiding the loss of credibility that comes from not being up-to-date.

Talk about a risk-vs-reward ratio…

So, this article will look to lay some foundations for a GM to at least pretend to know more than he does on the subject.

Much of it will be mercilessly cribbed from Wikipedia, which is an excellent resource for these known-fact information compilations, and which is kept relentlessly up-to-date by enthusiastic planet-watchers and astronomy buffs.

Image by Chil Vera from Pixabay

Planetary Types

Classification Structure

Planets are, these days, generally divided into three main types, sometimes dubbed “Rocky”, “Gaseous”, and “Unusual”. Astronomers generally use “Terrestrial” for the first and “Other” for the last, but – for the most part – these definitions are too broad for usage these days.

Terrestrial Planets

These are planets composed mostly of silicates, rocks, and metals. The four innermost planets of our solar system (Mercury, Venus, Earth, and Mars) all qualify.

There are several sub-types. Because this information wasn’t actually needed for my Dr Who adventure, I did very little research on them, in fact I’ve had to supplement my research for this article.

• Carbon Planet – a (theoretical) type of planet that contains more carbon than oxygen. Some possible examples have been found but their nature has not been confirmed.
• Coreless Planet – A coreless planet is a theoretical type that has no metallic core (and hence, no magnetic field). They are likely to be found in cooler regions (cosmically speaking) and further from the star.
• Desert Planet – a theoretical type of planet with a surface similar to Earth’s hot deserts. Studies suggest that Desert Planets have a broader habitable zone than ocean planets.
• Dwarf Planet – Probably the most famous non-Earthlike category, thanks to the controversy over the reclassification of Pluto. a Dwarf Planet is clearly a world (defined as possessing sufficient gravity to force it into a spherical shape) in solar orbit. The controversy stemmed from a later exclusion of Dwarf Planets from the list of Planets – while, ironically, retaining the term in the category title. It’s currently estimated that there are from 5 to 120 such bodies in our solar system, including Pluto, Ceres, Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia). And the observant will note that there are already more than 5 names on that list!
• Ice Planet – These planets have a surface of frozen volatiles such as water, ammonia, and methane. They may possess liquid oceans beneath an ice layer. In our solar system, the only known examples are planet-sized moons like Ganymede and Titan, but it is estimated that they will be common worlds in the Milky Way.
• Iron Planet – If a Coreless Planet is one without an earth-like metallic core, Iron Planets are the opposite, planets that have little or nothing but planetary cores. Mercury is the largest example in our solar system, but it is expected that there will be larger versions commonly in existence because Iron is one of the most abundant substances in the universe.
• Lava Planet – Terrestrial planets with a surface mostly or entirely covered by molten lava. There are several developmental paths that could lead to such planets – newborn planets, planets who have recently undergone a collision event, or a planet that orbits very close to the surface of its star. There have been possible Lava Planets found in at least three solar systems, but there are no examples in our solar system.
• Ocean World – planets containing a substantial amount of water either as a surface hydrosphere or subsurface ocean. There are no prizes for guessing that the Earth is the most famous example. The term is sometimes used to refer to hypothetical planetary bodies with oceans of liquid other than water – ammonia (Titan) or Hydrocarbons (Titan again!). Theoretical studies by NASA have recently (2020) found that exoplanets with oceans are going to be far more common than was previously expected. Since water is a high-probability foundation for the development of life, this suggests that life may also be more common than was thought likely even after the confirmed discovery of exoplanets.
• Super-Earth – A super-earth is a terrestrial ocean exoplanet with a mass of 5-10 earths. The term, which is not intended to carry implications regarding surface conditions, was coined to fill the gap between earth-like Ocean Worlds and Mega-earths. The term “gas dwarf” is sometimes used to describe planets at the upper end of the size scale, though Mini-Neptune is more commonly employed. There is some controversy over whether or not such planets should be considered Terrestrial or Gas Giants; they bridge the gap between those classifications.
• Mega-Earth – The term is not yet routinely accepted; it was used to describe Kepler-10c when that world was first discovered to be a Neptune-mass planet with a density considerably greater than that of Earth. Further study showed Kepler-10c to be a typical volatile-rich planet weighing just under 1/2 of the initial mass estimate, so the term no longer applied to that exoplanet, but it is believed that such worlds remain theoretically possible.
• Sub-Earth – These are terrestrial planets that are considerably smaller than Earth, and include Mercury and Mars, even though the former is also considered to be an Iron Planet. One of the earliest exoplanets confirmed is a sub-Earth around the millisecond Pulsar PSR B1257+12. The smallest example found to date, despite these being the most difficult exoplanets to detect, is WD 1145+017 b which has a radius just 15% of Earths, making it somewhat smaller than Pluto.
• Ultra-short period planets – These are planets with orbital periods (years) of less than one earth day, and only seem to occur around stars of less than about 1.25 solar masses. While all the known examples are Hot Jupiters (see below), it’s theoretically possible for a ‘rocky’ world to posses this characteristic as well. Of course, such would be at temperatures sufficient to melt or even boil almost every substance known, so the appellation “rocky” might be something of a misnomer in such cases!

As you can see, the definitions and classifications of terrestrial exoplanets are still evolving and while there are some areas of consensus, there are also some areas of disagreement; at this time, you could not say that there was a rigorous classification system. Right now, we are still discovering just what is possible within this category. This is one of the bleeding edges of Astronomy.

Gaseous Planets

Things are a little more settled with the Gaseous Planets. These are largely oriented into “Families” based on the commonly known exemplars within our solar system – Jupiter, Saturn, and Neptune.

I’ve been a little more fulsome (and eschewed the bullet-point format for this part of the discussion because this was the focus of my research for the Dr Who adventure.

But, before we get there, there was an unexpected area of controversy that shows that this sub-field is also still evolving, and that I found fascinating, which deserves presentation in a sidebar.

Controversy: When Is A Brown Dwarf Not A Brown Dwarf?

A few years ago, it was all so clear-cut. A Brown Dwarf was a star that was hot enough to fuse Deuterium, at least for a while, but not ordinary Hydrogen, which happened nicely and neatly at a mass of 13 Jupiters. Therefore, anything larger than that was a Brown Dwarf, and anything smaller was a Gas Giant, a Hot Jupiter.

And then we found a few exoplanets whose density was too low to permit such fusion but which were well over the “13Mj” (Jupiter Mass) limit, and a few Brown Dwarfs were found that clearly did or had supported fusion that were only 10Mj in size.

And then a few stellar bodies / rogue planets were found that were Brown Dwarfs in every respect but underwent no fusion, presumably because chance had given them insufficient Deuterium, just to completely demolish this rosy little picture.

The dividing line / criteria for distinguishing one class of extrasolar object from the other is now the subject of “hot” debate. The official dividing line is still the mass of 13 Jupiters, but this is more often ignored than it is followed. Extra-solar planets as large as 60 Mj are now on the record – and it is worth noting that the upper limit of Brown Dwarf sizes are 60-90 Mj.

Officially, an extrasolar body that orbits a star and does not show Deuterium Fusion is considered a planet, even above the 13Mj ‘limit”. Infrared and X-Ray observations are thus considered definitive, and this is likely where the consensus will land, in my opinion.

Currently, there are two different methods of differentiation, one based on formation and the other on the physics of the interior; these can yield contradictory results in which one method classifies an object as a star (brown dwarf) while the other does not.

But I find it both amusing and fascinating that an entire planetary definition has come and gone without my noticing!



Image
provided by flflflflfl from Pixabay

Gas Giant

Gas Giants are planets composed mainly of Hydrogen and Helium. Jupiter is the definitive example within the solar system. Saturn is often considered a second example, but some prefer to classify it into a separate class of giant planet due to a larger Helium content.

The visible ‘surface’ of Gas Giants consists of an outer layer of compressed hydrogen and helium surrounding a layer of liquid metallic hydrogen, with a rocky core at the interior. Within the layer of compressed gasses are visible clouds mainly composed of water and ammonia.

“Metallic” hydrogen is a state of matter in which hydrogen becomes electrically conductive due to extreme pressure. A Jovian core is at such a high temperature and pressure (20,000°K) that its physical and chemical properties are not yet fully understood.

Theoretically, gas giants can be divided into five separate sub-sub-classes according to their atmospheric attributes, which produce distinctive appearances. These features are (I) Ammonia Clouds, (II) Water Clouds, (III) Cloudless, (IV) Alkali-metal clouds, and (V) Silicate clouds.

Interestingly, cold Hydrogen-rich planets more massive than Jupiter but less than about 1.6 Mj will be larger in volume than Jupiter, but above this limit, gravity actually causes the planet to shrink back toward the size of Jupiter. Even Brown Dwarfs are very close to the typical Jupiter in size.

Heat from the interior carried upward by local storms is a major driver of weather on gas giants, predominantly thunderstorms, and much if not all of the heat escaping the interior follows this mechanism, which is thought to be very similar to the mechanism that creates storms on Earth. The heat flows develop into small eddies and vortices within the clouds, causing them to form ‘curls’.

The Great Red Spot is a high-pressure anticyclone system in which winds swirl at between 430 and 680 kilometers an hour. It has been observed to swallow smaller storms whole. Exposure to UV radiation creates brown organic compounds, which are then sucked into the upper atmosphere; it is hypothesized that these get stuck in Jupiter’s Great Red Spot, creating it’s red color. It is expected that such phenomena will be frequent occurrences in extrasolar planets of this size.

Condensation of helium creates a Helium Rain on gas giants, but the different masses of Jupiter and Saturn cause them to have different climatic mechanisms associated with this phenomenon. Some astronomers distinguish the two planetary types according to the Helium behavior; In ‘typical size’ gas giants, this distinction is largely ignored, but it remains relevant when considering much larger examples. The surface temperature of Saturn is about 900°C.

Hot Jupiter

When a Jupiter-class planet is very close to its primary, it has a very short orbital period (‘a year’) and becomes super-heated. We’re talking about “years” of less than 10 days, for the most part (one has been found with a year of 111 Earth days, and one has been found with year of just 1.3 earth days!)..

These exoplanets are the easiest to detect using the radial-velocity method because they induce relatively large ‘wobbles’ in the parent star’s motion.

The term ‘Hot Jupiter’ is an informal designation that is almost universally employed.

Although there is great diversity amongst the Hot Jupiters that have been discovered to date, there is a long list of common attributes.

They must have a mass of somewhere between 0.36 and 11.6 Jupiter masses, for example.

They have very circular orbits with low eccentricities; there are competing theories as to why.

Some collide with (and are absorbed into) their parent stars, otherwise they would be even more common.

Many have unusually low densities. They tend to be much larger than their mass warrants, and it’s not clear exactly why; several explanations have been posited but none proven to be the dominant mechanism.

The closeness to the Primary causes most to be tidally locked, with one side always facing the parent star. This is thought to create extreme and exotic atmospheric conditions. The day-night temperature differential is estimated to be about 500°C.

They are more commonly found around F- and G- type stars and are less common around K-type stars. Hot Jupiters around red dwarfs are extremely rare. In general, their prevalence decreases exponentially with absolute stellar magnitude.

More than half of the Hot Jupiters studied have orbits that are misaligned with the axis of rotation of their parent stars, and a significant fraction have completely retrograde orbits. There are many proposed explanations for this.

Even when taking surface heating from the proximate star into account, many Hot Jupiters have a larger planetary radius than expected; this may be caused by interactions between the atmospheric winds and the planets’ magnetosphere creating an electric current through the planet that heats it up. The hotter the planet, the greater the ionization of the atmosphere, which in turn leads to a greater magnitude of magnetosphere interaction and hence a larger current being generated, leading to greater heating and expansion of the planetary atmosphere. This theory matches evidence relating to observed correlations between inflated radii and planetary temperatures.

Super-Jupiter

A Super-Jupiter is an exoplanet that is considerably more massive than the planet Jupiter. By 2011, there were 180 confirmed Super-Jupiters, some hot and some cold. Up to about 80 Jupiter masses, their size remains very similar to that of our local Jupiter. Beyond that, they become so massive that fusion can initiate, turning what might have been a planet into a brown dwarf.

That means that their surface gravity and density are proportional to their mass. CoRoT-3b has a mass of around 22 Mj and is predicted to have an average density greater than that of Osmium, the densest natural element under standard conditions. It’s surface gravity will be over 50 times that of Earth.

Super-Puff

Super-puffs have a mass comparable to that of the Earth (up to a few times Me) but a radius larger than that of Neptune. This gives them a very low density. They are cooler and less massive than low-density Hot Jupiters.

One hypothesis is that they have continuous outflows of dust to the top of their atmosphere, so that the true surface is much smaller than the apparent surface. Gliese 3470 b is considered a possible example of this mechanism.

Another possibility is that some super-puffs are smaller planets with large ring systems which are being mistaken for planetary surfaces. HP 41378 f is considered an example. See also “Puffy Planet,” below.

Ultra-hot Jupiter

Ultra-hot Jupiters are Hot Jupiters with a daytime temperature in excess of 2200°K. At such temperatures, most molecules dissociate into their constituent atoms and stream away from the hot side to the night side (tidally locked, remember) where they recombine into molecules again. Logically, there must be a reciprocal flow from cold side to hot, completing a closed cycle, but what this could be remains to be determined.

The most extreme example is TOI-1431b, which was found to have an orbital period of just 2-and-a-half days; it’s day-side temperature is 2427°C, hotter than 40% of the stars in the Milky Way, and even it’s night-side temperature is 2300°C.

Eccentric Jupiter

An Eccentric Jupiter is a gassy exoplanet that orbits its star in an eccentric orbit, which carries it both close to its primary and some distance away. HD 96167 has a comet-like orbit that carries it out from roughly the equivalent of the orbit of Mercury to the equivalent of the center of the Asteroid belt (in terms of orbit size).

Together with the discovery of Hot Jupiters, Eccentric Jupiters required a complete reexamination of theories of solar system formation as the existing theories did not adequately explain the phenomena.



Image by Reimund Bertrams from Pixabay

Puffy Planets

Puffy Planets are less-extreme Super-puffs; they have a large radius and low density. They are sometimes called “Hot Saturns” due to their density being similar to that of the ringed planet.

They orbit close to their parent star, and the intense heat from solar radiation absorption plus internally-generated heat inflates the atmosphere.

Six have been confirmed so far, and it is suspected that some Hot Jupiters will be reclassified as Puffy Planets upon further examination.

Most puffy planets will be at or below Jupiter mass because anything greater would generate enough gravity to counter Puffing, and keeping them at roughly the same size as Jupiter.

Hot Jupiters wit masses less than that of Jupiter and temperatures in excess of 1800°K are so inflated and puffed out that they are on unstable evolutionary paths that will eventually cause their atmospheres to evaporate and become lost to the planet.

Helium Planet

A Helium Planet is one with a helium-dominated atmosphere, which contrasts with “ordinary” gas giants like Jupiter and Saturn. We don’t have an example of a Helium Planet in our solar system, but there have been some attempts to shift the boundary of “helium-dominated” to cause Saturn to fall into this classification.

Two formation mechanisms have been posited that would lead to Helium Planets – remnants of White Dwarf stars and Hydrogen evaporation from standard Jupiter-like gas giants.

Helium stars would have a white or gray color.

It is expected that a distinguishing feature of the chemistry of Helium Planets that would separate them from regular Jupiter and Jupiter-family exoplanets would be evidence of carbon monoxide and dioxide in the atmosphere, resulting from a loss of the hydrogen that would normally bond with the carbon to form methane.

Gliese 436 b is a possible Helium planet and does exhibit this chemical signature.

Gas dwarf (aka Mini-Jupiter)

Wikipedia lists a planetary type by this name but doesn’t have, and appears never to have had, a page on the subject, or a mention anywhere else of the planetary type. That means that I can only theorize, with at least a 50-50 chance of being totally wrong.

So, speculating (but not wildly), the existence of Mini-Neptunes (see below) could suggest the existence of similar Gas Dwarf planets that have a hydrogen-dominated atmosphere. These would need to be sufficiently distant from their parent star that they would not outgas sufficient Hydrogen to change the nature of the planet to a dwarf Helium Planet; their small size would suggest that they would have trouble holding on to an atmosphere in any event, so it’s entirely possible that if the hydrogen were to go, so would the helium, leaving only the heavier compounds of a Mini-Neptune (and a much smaller planet).

This also means that they would be sufficiently distant that significant Puffing would not take place, resulting in a cool, sub-Jovian planet in both mass and size.

That’s all semi-educated guesswork. It’s just as likely that someone created the category by mistake – but at least it sounds logical.

Ice Giants

The original meaning of the term Gas Giant included Uranus and Neptune, but these days the significant differences in their chemistries have placed them in a separate class, Ice Giants. These are giant planets comprised mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur.

In astrophysics and planetary science, the term “ices” refers to chemical compounds that are volatile, i.e. have freezing points above about 100°K.

Ice Giants lack a significant solid surface, they are primarily composed of liquids and gasses. The principles of their formation is problematic because the building blocks that would have gone into Gas Giants such as Jupiter would have been close to solar escape velocities, and more likely to have been thrown out of the solar system or into cometary orbits than to have accreted into the planets that we see today.

Either there were a lot more of these than solar system formation theory suggests, or the formative principle is different; proposals based around pebble accretion or gravitational disk instabilities have been proposed that might plug this theoretical gap. Many Ice-giant candidates have been observed amongst the exoplanets discovered, suggesting that they are relatively common in the Milky Way, and therefore that the mechanism must be fairly ubiquitous.

The gaseous outer layers of Ice Giants have several similarities to those of Gas Giants. These include long-lived equatorial winds of high speed, polar vortices, large-scale circulation patterns, and complex chemical processes driven by ultraviolet radiation from above and mixing of layers in the lower atmosphere. Their chemical compositions promote different processes to those of the Jupiter family.

Because they receive far less sunlight than the Jovian family, internal heating becomes far more relevant to the atmospheric weather of Ice Giants. There are still no complete models explaining how the observed atmospheres of ice giants function. This is a hot subject of research in planetary physics as exoplanetary properties are not yet understood, even in general theory.

The largest visible feature on Neptune is the Great Dark Spot, which forms and then dissipates every few years. It is similar in size to the Great Red Spot, which has persisted for centuries.

Of all giant planets of the solar system, Neptune emits the most internal heat per unit of absorbed sunlight (an approximate ratio of 2.6). Saturn has the next highest (ratio of 1.8); Uranus emits the least, at a ratio of about 0.26, which is so extremely different that it is attention-getting. The suspicion is that the extreme axial tilt of the planet, 98°, is relevant to this observation. That axial tilt also creates seasonal patterns unique within the solar system.

The internal heat of Uranus is very low; it is the coldest planet in the solar system, with a temperature in the upper atmosphere of -224°C. The deepest parts of the mantle are so hot and under so much pressure that methane decomposes into elemental carbon. One potential result is that the mantle will experience a rain of liquid diamonds. Higher up, sunlight causes methane to form compounds like Acetylene and Diacetylene, and this can potentially support organic chemistry of great complexity in the regions that bound the diamond nucleation zone and the upper atmosphere, potentially supportive of life.

Because of their large sizes and low thermal conductivities, planetary interior pressures range up to several hundred GPa (Giga-Pascals) and temperatures of several thousand Kelvins.

The compressibility of water in ice-giant models could be off by as much as 1/3, according to an announcement in Match 2012. The consequences of this are still reverberating through the relevant planetary science studies.

The magnetic fields of Uranus and Neptune are both unusually displaced and tilted. Their strengths are intermediate between Gas Giants and those of terrestrial planets, 50 and 25 times those of the Earth, respectively. Despite these greater field strengths, the equatorial field strengths are only 75 and 45 percent of that of the Earth, indicating that the sources are relatively deeper than the field strength itself indicates, compared to Earth. The magnetic fields are believed to originate in an ionized convecting fluid-ice mantle deep within the planets.

Mini-Ice Giant (aka Mini-Neptune)

A gas dwarf is a gas planet with a rocky core that has accumulated a thick envelope of volatiles but which is less massive than Neptune but which has a radius of 1.7 and 3.9 earth radii. They are divided into a three-tier classification regime based on the metallicity of short-period exoplanets (in astronomy and astrophysics, a “Metal” is any substance that is neither Hydrogen nor Helium.

Studies of such planets are loosely-based on what we have learned about Uranus and Neptune. Without a thick atmosphere, they would be classified as ocean planets instead. The 1.7 Earth-radius lower limit is a little fuzzy; the dividing line is considered to be somewhere between 1.6 and 2.0 Earth radii. Any planets with a larger radius that have been observed had significant levels of Hydrogen and/or Helium, as did any planet more massive than approximately 6 earth-masses. Beyond this observation, they appear to have a diversity of compositions that are poorly explained by simple models using a single mass-radius relationship.

It is this diversity of composition that explains the “fuzziness” mentioned in association with the lower limit; depending on the composition, the dividing line can be as low as 1 Earth-radius to as high as 20 earth-radii.

Neptune-like planets are considerably rarer than sub-Neptunes despite being only slightly bigger. This “radius cliff” separates sub-Neptunes from Neptunes, with a dividing line of 3 earth-radii. The divide is thought to be a function of planetary formation; the atmospheres of planets smaller in radius than the limit struggle to achieve the pressures required to force Hydrogen into the magma ocean, which limits radius growth. Only once the magma ocean becomes Hydrogen saturated can radius growth continue, and that can only occur with planets larger than the critical boundary.

Hot Neptune

As might be expected from the description / definition of a Hot Jupiter, a Hot Neptune is a Neptunian giant planet that orbits close to it’s star, typically in an orbit of less than 1 AU (closer to it’s star than the orbit of the Earth around our star). The first Hot Neptune to be found was Gliese 436 b, in 2007; it is located about 33 light years from our solar system.

Recent observations have revealed a significantly larger population of Hot Neptunes than was previously expected. In part, this can be explained by these planets being easier to detect, for the same reasons as Hot Jupiters, but this is not enough to account for the discrepancy. One notable example is Kepler-56 b, which has a mass somewhat larger than Neptune’s and orbits its star at a radius of just 0.1 AU, closer than Mercury orbits the Sun.

Super-Neptune

A super-Neptune is a giant planet of Neptunian character which is more massive than the planet Neptune. These are generally described as being approximately 5-7 times the size of Earth and with masses of 20-80 Earths. Exceeding this limit generally requires the planet to have sufficient Hydrogen and/or Helium to qualify it as a gas giant, even under the revised nomenclature. Planets falling within this mass range might also be referred to as a Sub-Saturn, indicating that the additional mass contains more Hydrogen-Helium than is normal for a planet within the Neptunian class. However, “sub-Saturn” is not an official designation, while “Super-Neptune” is.

There have been relatively few planets of this kind to be discovered. The mass gap between Neptune-like and Jupiter-like planets is thought to occur because of “Runaway accretion” occurring for planets of larger than 20 earth-masses; once the threshold is crossed, it becomes so much easier for them to acquire additional mass that they rapidly grow into planets the size of Jupiter or larger, much of it Hydrogen / Helium, which pushes the planet out of this category and into the Super-Jupiter category.

Ultra-Hot Neptune

So far, only one Ultra-Hot Neptune has been confirmed; LTT 9779 b has an orbital period (year) of 19 hours and an atmospheric temperature in excess of 1700°C. It is so close to its star that its atmosphere should have evaporated into space, so its existence requires an unusual explanation.

There is a possible second example awaiting confirmation orbiting Vega. It is believed to be slightly more massive than Neptune, orbiting Vega every 2.43 days, and – due to the highly energetic star – has a temperature of 2500°C, which will make it the second-hottest planet on record if it is confirmed.



Image by Chil Vera from Pixabay

Selected Other Types Of Planet

There are a number of other planetary types whose names caught my attention. If they subsequently proved interesting enough, I’ve kept them on the list below; if not, they’ve been redacted.

• Blanet – A blanet is a hypothetical exoplanet class that orbit black holes. They are basically planets like any other, i.e. they have enough mass to be rounded by their own gravity but not enough to start fusion reactions. In 2019, a team of astronomers and exoplanetologists showed that there is a safe zone around a supermassive black hole that could harbor potentially thousands of blanets in stable orbits. What is more, the accretion disks that form around black holes would force matter into this safe zone and so foster planet-building processes, so this is considered to be a self-fulfilling prophecy, a natural trait of such Black Holes. The term is a portmanteau of “BLAck hole” and “plaNET”.
• Cthonian Planet – Cthonian planets are a hypothetical class of celestial objects resulting from the stripping away of an gas giant’s atmosphere and outer layers through hydrodynamic escape. Such stripping is most likely to occur as a result of stellar proximity. The remaining rocky or metallic-core planet(oid) would resemble a terrestrial planet in many respects but significantly differ in others. If the core material of a gas giant (or even a brown dwarf) has an appropriate composition, it can stay compressed for billions of years despite the loss of the atmosphere that contained enough mass to perform the compression. HD 209458 b is an example of a gas giant in the process of becoming a Cthonian Planet, a process that will take many billions of years. Similarly, Gliese 436b has already lost 10% of its atmosphere. CoRoT-7b is the first exoplanet that might be fully transformed into a Cthonian Planet, but other researchers dispute this classification. TOI 849 b, a planet more massive than Neptune and located very close to its host star was found in the Neptunian Desert (a region of space virtually devoid of planets), may also be Cthonian.
• Circumbinary Planet – A planet that orbits two stars instead of one; the stars are usually in a binary star system. At one point, these were considered impossible, as the presence of the binary star would both disrupt planetary formation and ‘suck up’ all the building material needed for such formation; but several examples have been discovered, disproving this belief. The discovery has shed fresh light on some aspects of planetary systems such as orbital dynamics and axial tilt precession. It has also been noted that there is a general absence of such planets around shorter-period binary systems, suggesting that the old theory might not be completely inaccurate after all.
• Circumtriple Planet – A celestial mass that orbits three stars in a trinary system at the same time. Star System GW Ori contains a huge accretion disk of dust and gas located about 1300 light-years from earth; astronomers have observed a gap in the cloud and hypothesized that a planet has swept that region clear. There are other gravitational oddities about the star system that could be explained by the presence of a planetary body of Jupiter size. The body itself has not been observed. If it exists, it will be an extremely rare phenomenon in the universe, potentially the rarest type of planet in the known universe, and quite likely the only example within the Milky Way.
• Disrupted Planet – A planetary body disrupted or destroyed by nearby or passing astronomical body or object; the study of the process is known as Necroplanetology. For a long time, it was thought that the Asteroid Belt of our solar system was a Disrupted planet (some science-fiction suggested that it had contained an advanced civilization which had destroyed itself and its planet, which would – ironically – mean that it no longer met the criteria for this designation). This theoretical origin of the Asteroid Belt may have inspired the origin story of Superman as a refugee from the destroyed planet of Krypton. It is no longer thought that the Asteroid Belt was ever a solid singular body; it is rather thought that the gravitational attraction of Jupiter disrupted the planetary formation process.
• Double Planet – The typical ratio of masses between a planet and a satellite is around 10,000 to 1. In extremely rare cases, a satellite may be massive enough that both it and the planet it orbits with both revolve around a point external to both planetary bodies. The Earth-Luna double is one such example; both actually orbit about a point of balanced mutual attraction. Pluto and Charon were proposed to be an even better example, but this proposal failed when Pluto was controversially struck off the list of planets. This resulted in the definition being amended to state that one of the planetary bodies in question must be a bonafide planet before a system could qualify as a double-planet. Later revelations about the origins of the moon have only made the probability of double-planets forming even more rare than it was already thought to be; in conjunction with arguments about the significance of Luna to the development and evolution of life on Earth, and of sentience, some have suggested the absence of a moon of significant mass relative to an earth-like planet may constitute a choke-point in the formation of life-bearing worlds.
• Ecumenopolis – The name given to describe a (hypothetical) planet-wide city. Term coined in 1967, but the concept originated in Asimov’s Foundation series, and was initially proposed by American religious leader Thomas Lake Harris.(1823-1906). Lake also depicted interstellar empires and “ancient astronauts” in his writings, making him a lost grandfather of Science Fiction.
• Mesoplanet – Speaking of Asimov, this term is another of his contributions to science. Mesoplanets are planetary mass objects smaller than Mercury but larger than Ceres. Asimov observed that there was a considerable gap between the smallest object considered an undoubted major planet (Mercury) and the largest object undoubtedly considered a minor object (Ceres) and proposed the term to describe objects fitting into that size gap. At the time, only one planetary object, Pluto, fell into the category, and rather than declare it arbitrarily one or the other, he suggested placing it into this new subcategory (“Meso” means “middle” in Greek). Other objects have since been discovered that would be included are Eris, Haumea, Makemake, Gonggong, Quaoar, probably Sedna, and possibly Orcus. Astronomers generally describe these as “Dwarf Planets” these days; other, smaller, bodies have been proposed by astronomers disagree about their potential dwarf planet status.
• Eyeball Planet – A hypothetical model describing tidally-locked planets induces spatial features which come to resemble an eyeball. The concept is that the planet will be hottest at the perpetual ‘noon’ and cool somewhat at positions removed from this point, producing ring-shaped zones in which some materials liquefy and others do not. The theoretical model thus posits a series of concentric rings as surface features of some such planets. It further calculates two viable types, a ‘hot’ eyeball and a ‘cool’ eyeball, depending on he chemistry of the liquid and whether or not it forms on the near-solar side or the far side. Kepler-1652b is potentially an eyeball planet, and the TRAPPIST-1 system may contain several such planets.
• Pulsar Planet – Pulsar planets are discovered through the influence of the planet’s gravity on pulsar, inducing a wobble that impacts on the precision of the timing of the pulses produced by the Neutron Star. The discovery was unexpected; such stellar objects have previously gone supernova and it was thought that any planetary bodies orbiting such stars would have been destroyed in the explosion. In 1991, it was announced that a planet had been detected around PSR 1829-10, but this was later retracted – just before the first real pulsar planets were announced. Two astronomers announced the discovery of a multi-planet system around PSR 1257+12, and these became the first two extrasolar planets confirmed to exist. There was some initial doubt about the discovery because of the prior retraction and because of questions about how pulsars could have planets; however, the planets proved to be real and cosmology had to adapt to incorporate their existence. Two additional smaller planets were later added to the system using the same technique, but one has since been retracted. The oldest known planet is currently a Circumbinary Pulsar Planet, 12.6 billion years old. It is believed to have been a planet orbiting the pulsar’s companion star before becoming a Circumbinary planet. In 2006, a Magnetar, 13000 light years from earth, was found to have a circumstellar disk, thought to have formed from metal-rich debris (NB. the astronomical definition of ‘metal’) left over from the supernova about 100,000 years ago. The disk appears to be quite similar to those around sun-like stars, suggesting that planetary formation may be possible even around Pulsars, and that Pulsar Planets may be far more common than previously suspected. It may be that these have unique properties in common due to their origins, which would elevate the term “Pulsar Planet” into the main types of planetary bodies.
• Sub-brown Dwarf – Just when you thought the Planet-Brown Dwarf controversy was complicated enough… These are astronomical objects formed in the same manner as stars and brown dwarfs (through the collapse of a dust cloud) but of planetary mass, and therefore below the limiting mass of the fusion of deuterium. Some astronomers call these free-floating planets, some call them planetary-mass brown dwarfs or Y spectral class brown dwarfs, and some label them rogue planets.
• Sub-Neptune – Again, a definition that seemed settled until now – the term has also been applied to planets with a smaller radius than Neptune but a larger mass, or to a planet with smaller mass or larger radius, like a super-puff. Both meanings can also be used in the same publication. Consistency seems lacking but consensus will eventually resolve this confusing situation… it is to be hoped.
• Toroidal Planet – A hypothetical exoplanet with a torroidal or doughnut shape. While there is no rigorous theoretical understanding as to how one could form in nature, the shape itself is potentially quasi-stable. It is considered extremely improbable that any naturally-occurring Toroidal Planet will ever be discovered, and should one be found, it will immediately become classified as one of, if not the, rarest object in the universe. But so many exoplanets are now thought to exist that if there is a formation mechanism that has been overlooked so far, no matter how improbable, it remains possible that such a planet exists – somewhere.
• Ultra-short Period Planet (USP) – Exoplanets with orbital periods (years) of less than one earth-day are designated USPs. Few exceed two earth radii in size. About one in 200 sun-like (G-type) stars has an USP, and most of them have an earth-like composition (70% rock 30% iron). K2-229b has a higher density, suggesting a more massive iron core, while WASP-47e and 55 Cnc e have a lower density and a corresponding composition of more pure rock, or a rocky-iron body surrounded by a layer of water or other volatile substance. The main difference between these planets and Hot Jupiters is that USPs almost always have longer-period planetary companions, while Hot Jupiters are rarely found with other planets within a factor of 2-3 in orbital period.
• Superhabitable Planet – The Drake Formula, which I wrote about in A Game Of Drakes and Detectives: Where’s ET?, often seems to assume that because life was known to have evolved on Earth, Earth and its solar system must reflect the optimum chances for the development of life. In 2014, Rene Heller and John Armstrong introduced the concept a Superhabitable planet, a hypothetical exoplanet or exomoon that may be even better-suited to the emergence and evolution of life. Critical of the existing conceptual models as unjustifiably anthropocentric and geocentric, they proposed to establish a profile for exoplanets based on planetary and stellar features; analyzing the measurable properties of a planet which offered the greatest potential likelihood of life, they identified eight characteristics and concluded that existing search methodologies had ignored the most likely targets for success. 24 planets matching the Superhabitable profile have been identified but – so far – only two have been confirmed: Kepler-69c and Kepler-1126b. One of the unconfirmed, KOI 5715.01, is regarded as potentially the best match to the profile.


Image by WikiImages from Pixabay

Unusual Exoplanets List

Wikipedia also maintains a couple of lists that are worth keeping an eye on, and occasionally exploring. This first of these is a list of exoplanet extremes.

This list is full of one jaw-drop after another.

Nearby Habitable Systems

Next up, we have a list of the Habitable Systems closest to Earth – (but refer to the discussion of Superhabitable Planet above for relevant discussion).

List Of Named Exoplanets

It would be nice if this list explained why these particular exoplanets were considered worthy of being named, given that so many have now been found that these have clearly been singled out. But this List Of Named Exoplanets is what it is.

The State Of Cosmological Texts

There have been so many changes in the last couple of years that if you have an astronomy or cosmology text that is more than 5 years old, it’s almost certainly out-of-date, and even hot-off-the-presses publications run the risk of being significantly outdated in some areas. That doesn’t make older reference works useless – well, not completely – but it does mean that they might well mislead more than educate. At best, they are a starting point. I’d offer up a recommendation or two, but they would almost certainly date very poorly.

If this subject is relevant to your games, the advice is therefore to seek out the most recent book written for your academic level in this subject. This is going to be an evolving landscape – cutting-edge right now might be a year out of date, and might get supplanted as “most recent” six weeks from now – good luck if you happen to be reading this in late June, not so much, if not.

That advice also presumes that all such works are going to be equal. We all know better than that, don’t we?

You might use reviews to assess this factor, but there are complications. The newer a book is, the less likely it is that it will have a lot of reviews – that’s number one. And you can’t assume that all reviews are created equal, that’s number two. Unfortunately, that’s where you are going to have to use your own best judgment. But I have a suggestion for you to consider. And it comes from, of all places, learning the art of musical composition.

You see, I couldn’t decide between the For Dummies book on the subject and the Complete Idiot’s guide. So, feeling extravagant at the time, I bought both. And what I learned was that the combination was better than either book on its own. What one explained in a way that left me confused, the other explained clearly. Often, one would provide the foundation concepts and the other would expand on that material.

I would, therefore, select a university which has an Astronomy or Cosmology course and which lists study materials online and select the first-year textbook their course demands; and then look for an alternative pitched just a little down from that, in hopes that it would fill in any foundational blanks in the more advanced book. (Actually, because I already know a little on the subject, I might go for a second-year text and a supplementary first-year text, but you get the general idea).

Image by Alex Myers from Pixabay

Controversy: Universal Expansion

How fast is the universe expanding?

Well, my answer has always been a slightly vexed “no-one knows”, because the further away we look, the further back in time our information is. So we can only ever know how fast it was expanding, and one datum is not enough to extrapolate a current value.

But I thought that this one known datum was at least fairly solidly known – at least until I read this answer on Quora:
Krister Sundelin’s answer to “Why is there a crisis in cosmology?”

In a nutshell, we now have several different methods of calculating the Hubble Constant (the rate of expansion of the universe), which have been refined and made more reliable and accurate over the years – and the three methods don’t agree.

It’s possible that the solution to this problem lies in the logical fallacy I espoused above, but it’s equally possible that it doesn’t – I would need to do a lot more research to try and answer that question.

It’s worth noting that the answer given is months old at this point, and that brings in the pace of cosmological discovery that I mentioned earlier. I made an attempt to do that further research, or at least to see if the discrepancy had been resolved since; what I learned was that it has, if anything, deepened. There is now evidence that the rate of expansion is not constant, but is a variable that has changed over time – and that the rate of expansion is accelerating in a non-linear way.

If my complaint was the sole factor at play, it might explain a constant change, but not a non-linear one. What’s more, while a consensus has been reached that this is (or was) taking place, no adequate explanation has been found as to why.

If you’re as intrigued as I was, aside from the link given above, I would suggest reading
Ian Kimber’s answer to “What is the Hubble Constant controversy, and how would it change the way we understand the cosmos?”

and

Anders Rehnberg’s answer to “Why is there not a unanimous way to calculate the Hubble Constant?”

Finally, it may be worthwhile looking at the most directly relevant Wikipedia page, on Hubble’s Law

Okay, so here we are, about 2/3 of the way through the article (which is already more than ten times the usual length) and I’m right out of time. So I’ll pick this up with a shorter “Part 2” article next week, which might also give me the time to explain why this can all be relevant to D&D….