Super-Earths and the Three Planet Types

When I was a boy the existence of planets outside of our solar system had yet to be confirmed. There were some observations of Barnard’s star that seemed to suggest there was a planet there, but that was about it. Now we have confirmed the existence of more than 1,800 extra-solar planets, or “exoplanets”. Sometimes our knowledge advances in bounds.
The following may be of interest to those planning an adventure in space. Often in science-fiction we are told a world is “high gravity” or “low gravity” with no thought as to how this may effect the planet’s atmosphere or the form of the world itself. As will become clear, a “3 g inhabitant” won’t be a strong, heavy-set humanoid.
Recently I was watching a program about the solar system and it mentioned a theory that the solar system was atypical of planetary systems. The belief was that Jupiter had hogged most of the “planet-building” materials, resulting in four, relatively small, rocky, inner worlds. The program claimed that most exoplanet systems had larger rocky worlds, or “super-earths”. Wikipedia gives the definition “super-earth” as a planet up to ten times the mass of Earth. These could be rocky, oceanic, gas dwarves or “mini-neptunes”. Most super-earths do not seem to be particularly Earth-like! While we have discovered exoplanets smaller than Earth, it is worth bearing in mind that larger planets are probably more likely to be discovered, and this potential sampling error may slew the results as to what the “typical size” of a planets is.
We know life has evolved on at least one small rocky world. Logically, there are two options:
  • The first is that life can only evolve on smaller rocky planets. If super-earths and gas giants are the usual planet-types, then life is rare.
  • The second option is that life can evolve on super-earths, and possibly gas worlds (Mother nature doubtless still has many surprises for us! It would be foolish to say that life on gas worlds is definitely is not possible). Such lifeforms would have evolved in higher gravity, greater atmospheric pressures and possibly atmospheres containing more helium and hydrogen. Other factors being equal, most life in the universe would be very different to what we are used to.
As I researched these ideas, I came across an interesting paper [1]. The authors had taken the radii and mass of solar and known exoplanets and correlated them. If you know both a planet’s mass and radius you can estimate its density and determine if it is mainly of rock or gas. The results indicated that planets divided into three categories, which the authors termed “Jovian”, “Neptunian” and “Terran”. The latter term in particular should be treated with reservation, since it includes solid planets of anything smaller than twice the Earth’s mass, and atmospheres may range from none to that of Venus. 

The Neptunian planets ranged from more than twice the Earth’s mass to about 130 times; or up to 0.41 times (±0.07) of the mass of Jupiter. Saturn is therefore a Neptunian world, be it one of the larger examples. 

Jovian planets ranged from 0.41 times the mass of Jupiter and range up into Brown Dwarfs. The upper limit of the Jovian planets is where gas giants achieve enough mass to start burning their hydrogen and become stellar bodies, or stars. The difference between Jovian and Neptunian planets is that the Jovian are so massive they begin to “self-compress”. You will note that on the chart below that increases in the mass of Jovian worlds produces very little increase in size. As the paper states: “A defining feature of the Jovian worlds is that the MR power-index is close to zero (0.04 ± 0.02), with radius being nearly degenerate with respect to mass.” [1]
One of the things this scheme reveals is that most “super-earths” are in fact smaller Neptunian planets. The upper limit for rocky planets seems to be twice the mass of Earth (2 ± 0.7 M⊕), or about 1.75 times its radius (R⊕). Above this the gravity of the body retains so much atmosphere that a Neptunian world is formed.
Rocky super-earths do exist, but they are not that much bigger than Earth, which is at half the mass that a rocky planet could be. Viewed under the above classification scheme our solar system is no longer atypical. Its rocky worlds are near the centre of normal range and it has three Neptunian worlds.
Another investigation [2] contributes to our understanding of the above:
‘The team discovered that no matter what material a planet is made of, the mass/diameter relationship follows a similar pattern.
All materials compress in a similar way because of the structure of solids,” explains Seager. “If you squeeze a rock, nothing much happens until you reach some critical pressure, then it crushes. Planets behave the same way, but they react at different pressures depending on the composition. This is a big step forward in our fundamental understanding of planets.”
This explains how objects of rock, ice or metals can seem to exhibit similar mass to radius characteristics.
Someone asked me how I thought planets should be classified. The terms “Jovian”, “Neptunian” and “Terran” are possibly a little too specific. For the first two I see no problem with “J-class” and “N-class”. We will doubtless see some sub-divisions. We have observed “hot-Jupiter” and “hot-Neptune” exoplanets that are closer to their suns than the solar examples. Both classes cover a considerable range of mass and size. Planets will doubtlessly be classed by their size relative to the planets their class is named after. Our own solar system contains both a sub-Neptunian and a super-Neptunian planet.
The naming of the third group is more problematic. Calling a celestial body “Terran”, “terrestrial” or “Earth-like” implies features other than just being a ball of rock or ice. Terms such as “T-class”, “Telluric”, “E-class” or even M-class do not solve this. In the above passages I have used the term “rocky”, but for some of these bodies ice or metallic composition may seem more apparent. Given Neptunian and Jovian worlds are both described as “gas”, “solid” or “dense” may be an acceptable term for the third class. Unfortunately “S-class” (and T-class) is already used for a class of star and also for asteroid classification. Since these objects are denser than gas worlds I will use the term “Rho-class”. These are my personal terms for the three classes of planet and do not have any formal scientific standing, useful as they may be. Feel free to adopt them.
Taxonomy of the Rho-class is likely to be more intricate, since these are the worlds mankind will be most interested in. You will note that I have started calling the members of the Rho-class “worlds” or “bodies” since many of its members are not strictly speaking “planets”. The study above [1] included moons and dwarf planets in this class and covered objects of less than a thousandth the mass of Earth. It is unlikely we will be satisfied with simply classing the members of this class as “hot” or “cold”, “larger” or “smaller”. Detail on whether these temperatures are comfortable, tolerable or extreme will be desirable. The absence, presence, density and composition of an atmosphere will be relevant. The availability, abundance and state of any water will also be significant. In James White’s “Sector General” series of books lifeforms were given a four-letter classification that provided information on their nature. It is possible that a multi-letter system might be developed for celestial objects in the third class. Something similar is already used for stellar classification.
The current definition of “super-earth” is not particularly useful, given most planets with this designation are more accurately described as sub-Neptunian. A good case can be made for reserving this term exclusively for “solid” (Rho-class) planets that are larger and/or more massive than the Earth.
Would we possibly find life on the solid super-earths? The Wikipedia article notes that:
According to one hypothesis, super-Earths of about two Earth masses may be conducive to life. The higher surface gravity would lead to a thicker atmosphere, increased surface erosion and hence a flatter topography. The end result could be an “archipelago planet” of shallow oceans dotted with island chains ideally suited for biodiversity. A more massive planet of two Earth masses would also retain more heat within its interior from its initial formation much longer, sustaining plate tectonics (which is vital for regulating the carbon cycle and hence the climate) for longer. The thicker atmosphere and stronger magnetic field would also shield life on the surface against harmful cosmic rays.”
There is a school of thought [5] that our Earth is not the optimal configuration for the evolution of life and larger rocky super-earths may be closer to the ideal.
Life may be more likely on larger “Earths” than worlds our own size!
Theorized characteristics of such “superhabitable” worlds are twice the Earth’s mass and 1.2 to 1.3 times its radius, preferably orbiting a K-dwarf star that is smaller but longer lived than our sun, giving a world more time to develop a robust biosphere. K-dwarves also subject their planets to less potentially harmful UV light. Current evidence suggests that super-Earths around small stars are substantially more abundant throughout our galaxy than Earth-sun analogues

What would the life on a rocky super-earth be like? An “archipelago planet” suggests the majority of life will be aquatic or amphibious. Islands, and the shallow waters around them, tend to favour biodiversity. Not only will a rocky super-earth have more than 50% greater surface area than Earth, but more of it will be available for supporting rich ecosystems. Shallow oceans will probably mean more abundant life on the sea bottom. Shallow seas are defined as having an average depth of 200m, so sunlight usually reaches the bottom, which favors underwater flora. Smaller land masses mean less likelihood of desert areas that can support relatively few lifeforms. A denser atmosphere may prevent the formation of ice sheets to result in a lower thermal difference between different regions of the planet.
My initial thought was that the greater gravity, atmospheric pressure and geography may make colonizing the land less attractive, or favour smaller terrestrial creatures? Just how much greater will the gravity of such a world be?
The world of Kepler 442b is slightly above the suggested superhabitable values at 1.34 R⊕ and 2.34 M⊕. If this world is of similar composition to Earth its surface gravity is estimated to be 30% higher the Earth. The formulae given in GURPS Space 4e.[p.85] yield similar results: A world of 1.95 M⊕ and the same density as Earth would be 1.25 R⊕ and surface gravity 25% higher than Earth. A very dense (1.4) planet of the same mass would be 12% larger than Earth and have around 60% higher gravity. Worlds within the suggested parameters for super-habitable are likely to have 20-30% higher gravity.
This webpage [3] gives us an idea what this might mean:
In any given mass category high-gee animals should have shorter, stockier bones than those evolving in low-gee environments. To provide proper support, bone cross-section must increase directly with weight. Weight is the product of mass and gravity, so bone diameter must be proportional to the square root of gravity.
Let’s apply this to man. The typical human femur, the most perfectly cylindrical and largest single bone in our bodies, measures 3.5 centimeters in diameter. Using the above square-root relation, we find that the thigh-bone should increase to 4.9 cm on a two-gee world or shrink to 1.6cm on a 0.2-gee planet for identical support of a 70 kg human body mass. Experiments have confirmed that animals reared in high gravity grow thicker bones, stronger hearts, and lose fat, but alien creatures will not appear wildly over- or under-built as compared with Earth life of equal mass.”
Using the example given, 1.3 g gives our “man” a femur 4cm in diameter rather than the 3.5cm of 1 g.
The higher gravity of a super-habitable world may not be that significant an effect on bioforms, at least not to the extent of preventing the conquest of the land. Creatures will be stockier and possibly smaller for a given mass. There are suggestions that a superhabitable world’s denser atmosphere with more oxygen would instead favour the evolution of larger plants and creatures.
The above webpage also notes that: The main factor fixing avian size is atmospheric pressure, not gravity as some erroneously believe. On high-pressure worlds, alien bird creatures can have surprisingly small wings and large masses.”
A superhabitable world may have flying lifeforms, some of considerable size. Flying creatures would be important in connecting island ecosystems. Many ground-dwelling creatures may be the flightless descendants of flying ancestors.
The gravity of Earth is not sufficient to retain hydrogen and helium. An interesting feature of rocky super-earth worlds is that the gravity is high enough to retain helium, but not hydrogen. Would this have an effect on the ecosystem? Presumably most of the helium would be at high altitudes. Might some lifeform be able to utilize helium as a resource? One can envision a sort of “sky-algae” that floats in the high winds.
The gravity of a Rho-class super-earth will be tolerable to baseline humans. Other than the extra weight they are carrying, travelers may find such worlds rather pleasant.
On thing the above findings implies is that terrestrial-type lifeforms are more likely to originate from K-dwarf:super-Rho systems. Humans may find themselves  physically the wimps of the Galactic federation, most other sapients they encounter originating from 1.2-1.3 g worlds.
Planets of more than 2 M⊕ will be N or J-class worlds. If they have life it will be of a form that never walked on its planet’s surface. The rocky core of an N-class world will be beneath thousands of kilometres of atmosphere and at tremendous pressures and temperatures. J-class worlds may not have rocky cores, current theories being that their core would be liquid metallic hydrogen. If there are lifeforms on J and N-class worlds they will probably live in the upper atmosphere, or possibly the lower pressure region of the liquid layers. The usual suggestion is a floating creature. There may be actively flying lifeforms that like a shark or a swift never stop moving, or hitch a ride on larger floating creatures. Also possible are lifeforms with a large surface area, like dandelion fluff, that allow themselves to be kept aloft by the winds.
Would such worlds produce space-faring races if a thick atmosphere seldom lets them see the stars?
[5] Better Than Earth, René Heller, Scientific American 312, January, 2015