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Between Size A and SGG: Super-Earths, Hyceans, and Gas Dwarves

This article originally appeared in the November/December 2021 issue.

* One T-mass is the mass of Terra (Earth), approximately 5.971024kg. It is also sometimes written as ME.Sol System (where Humaniti originated, no matter what the Vilani Identity types claim) is a G2V yellow dwarf star with eight proper planets – four rockballs, the largest Trav UPP Size 8, two Large Gas Giants (318 and 91 T-masses*), and two Small Gas Giants (15 and 18 T-masses), with nothing in size between that Size 8 single T-mass rockball and the 15 T-mass gas giant.

Planets of such intermediate size were postulated when the Solar Nebula model of planetary formation became dominant at Tech Level 6bis (Early Cold War). Confirmation of their existence didn’t occur until TL 9 (Y2K), when astronomical instruments had become sensitive enough to “see” the literally-microscopic signatures of exoplanets many parsecs away, at which point said intermediate-sized planets were found not only to exist, but to be common – once again, Sol System was abnormal.

Originally these intermediate-sized transition planets were all lumped together under the generic title “Super-Earths”, i.e., probable rockballs “larger than Earth but smaller than Uranus”. (For the purposes of this page, “Generic Super-Earth” will refer to planets of 2 to 10 T-masses.)

This generic name was later split into at least three distinct types: True Super-Earths, Gas Dwarves, and Hycean Planets. True Super-Earths are additionally divided into Super-Earths, Super-Venuses, and Super-Mercurys, though “Super-Earth” remains as a generic name for all.

True Super-Earth

A True Super-Earth is a rockball larger then Earth, from UWP Sizes 9 to 12+, though often Size 9 worlds are not included and Super-Earth starts at Size 10 (over 2 T-masses). Because of the Helium Capture Threshold (see below), the maximum size of a True Super-Earth is around UWP Size 12-15 (which translates to between 3 and 4 T-masses).

As a rockball’s Size (diameter) increases, so does its Atmospheric Density and Hydrographic percentage, to the point that Super-Earths tend towards Dense-atmosphere waterworlds. At least in the outer Habitable Zone (HZ) or near Outer Zone; with denser atmosphere comes increased greenhouse effect. For this reason, Super-Earths are normally found farther out than T-normal habitables; too much solar flux and you have a Super-Venus instead.

As a rule-of-thumb, a dense-atmosphere Super-Earth will be found up to a full Orbital Zone outward than an equivalent T-normal.


The smaller Super-Earths may actually be more conducive to life than a T-normal, especially if orbiting a K-class orange dwarf. The higher hydrographic percentage and hotter core guarantee active plate tectonics, and the denser atmosphere’s greenhouse effect tends to prevent (or at least reduce) extreme temperature variations.

The greenhouse effect also allows the planet to have habitable surface temperatures into the Outer Zone (out to approximately the solar-flux equivalent of Mars), increasing the effective size of the Habitable Zone.

A K-class orange dwarf sun stays longer on the main sequence before growing bright and hot enough to trigger runaway global warming, allowing more time for life to originate and evolve. (That and there are usually two or three orange dwarves for every yellow dwarf like Sol.)

Example: Telerine (habitable Super-Earth)

Telerine (Paryan III, AA99744-B, Ag) in the Dole Moving Group (“Jump Destination: Telerine”, Freelance Traveller #74, Mar/Apr 2016) is a typical habitable Super-Earth.

From afar, Telerine appears as peach-tinted clouds both forming into gas giant-style cloud bands along the equator and swirling over a wine-red oceanic surface dotted with islands, all blurred by the thick hazy atmosphere.

A Size 10 world of a K1 sun, Telerine has the characteristics of a Super-Earth: High gravity (1.2 G), Dense Atmosphere (2000 millibars at sea level), high Hydrographics (over 90%), fast-spinning (18-hour day), and seismically/volcanically active. It misses out on Superhabitable only by its higher-than-optimal Hydrographic percentage. All these characteristics have side effects:

The high gravity requires 2-G Maneuver Drives to land and take off from the surface, and the extra strain increases the chance of health problems (primarily cardiac).

The Dense atmosphere doubles the force of winds and creates a greenhouse effect that evens out temperatures across the globe. The nitrogen partial pressure comes close to edge of nitrogen narcosis at sea level; at higher altitudes too-low oxygen limits breathability, replacing too-high nitrogen at sea level; storms and weather can be extreme. Water boils at 120C instead of 100C. Between the thick atmosphere and the K-class solar spectrum, sunburn is almost unknown.

The high Hydrographic percentage leaves little or no land to interrupt the “fetch” of storms and waves. Seas can go 50 meters in storms, and 300-kph hypercanes with tsunami-like storm surges menace the coastlines.

The short day in combination with the Dense atmosphere affects weather; 90% of the clouds hug the equatorial belt of the world, and can form several layers of cloud decks.

Seismic/volcanic activity means most/all of the landmasses (the largest of which are the size of Greenland or Madagascar) are recently volcanically active, further limiting the habitable/arable land area. The main natural disasters are mega-storms, volcanic eruptions, earthquakes, and their resulting (mega-)tsunamis.

Almost all native life is oceanic, with very little land life.

Example: Quantol (maximum-sized borderline-habitable Super-Earth)

Quantol (Pentathos IV, CCD8753-7, Ag, NI) in the Scattered Worlds (which may eventually show up in Jump Destinations) is right at the Helium Capture Threshold. Despite this, it has enough habitable surface to maintain a small colonial population.

A Size 12 planet located outside the HZ of its orange dwarf sun with the approximate solar flux of Mars, Quantol is the outermost, largest, least populated, and lowest-technology of the three terraformed worlds in its compact five-planet system.

From afar, Quantol is mostly obscured by the haze of its very dense atmosphere; the lower the altitude, the more opaque the atmosphere. Oceans and lowlands are invisible, with continental highlands “ghosting” through the blue-white haze like the surface of Wasphome (“Jump Destinations: Geolan & Wasphome”, Freelance Traveller #79, Jan/Feb 2017); only the highest mountains and plateaus – the only human-habitable regions – are fully-visible under an overlay of cloud formations.

Higher up the Super-Earth spectrum, Quantol’s characteristics are more extreme than Telerine’s: High gravity (greater than 1.4 G), Dense/High Atmosphere only breathable at high altitudes, high Hydrographics percentage (though still low for its size), and more seismic/volcanic activity than Telerine.

Like Telerine, the high gravity requires 2-G Maneuver Drives to land or take off; the gravity (20% over Telerine’s) has affected the physiology and health of its long-term human inhabitants: Quantolians tend to be slightly-shorter than average, stocky “peasant” build, heavily-muscled, heavily-boned, and prone to heart problems.

Quantol’s atmosphere is on the verge of a Hycean world, including 7% helium in a low-oxygen, mostly-nitrogen mix. Sea-level pressure and density is over ten times standard (over 10,000 millibars), to the point where the nitrogen is toxic and even the helium is problematic. Only in the extreme highlands and mountains peeking through the eternal low-altitude haze is the pressure and density low enough to be breathable (though tainted by low oxygen), seriously limiting the habitable land area. This and the gravity are the main reasons for the world’s low population.

Quantol’s Hydrographic percentage (80%) is very low for its size; other worlds this size tend to “ocean planets”, waterworlds whose oceans are so deep they practically form the crust and upper mantle (like Asilra in Virtchok System, profiled below). This is a result of no gas giants insystem; without GG gravity wells to throw ice asteroids inward, most of the system’s water remains as ice outside of the frost line. As is, Quantol’s oceans are well over 100C from low-altitude greenhouse effect; all that keeps them liquid is sea-level atmospheric pressure (again, much like Asilra).

(Note: For planets in systems without gas giants or multiple systems where the other component(s) disrupt the frost line, roll 1D – 7 + Size for Hydrographics percentage instead of 2D – 7 + Size.)

A world this size should have enough angular momentum for a 12 to 14-hour day. However, Quantol has been tidally-braked by its moon Tetran (D322562-7, NI, Poor) to a 25-hour day.

A world this size with the additional tidal stress of a large moon will be seismically and volcanically-active, and Quantol is no exception. Though since the only habitable areas are the highest of the highlands, tsunamis are not a problem.– only earthquakes and explosive subduction-zone volcanoes from high plate tectonics. The low-altitude haze is often darkened by volcanic plumes.


A Super-Venus happens when a Super-Earth is too close to its sun and gets a runaway greenhouse effect during formation. The Hydrosphere boils away and reacts with outgassing to form a Cytherean world – Superdense acid-vapor atmospheres, ocean-bottom air pressures, and surface temperatures of over 400C.

Super-Earths tend to have Dense atmospheres to begin with, so it doesn’t take much to push one over into a Super-Venus. This is why True Super-Earths orbit at the outer edge of their star’s HZ, if not a little way into the Outer Zone itself. The larger the Super-Earth and denser the original atmosphere, the farther out it has to be to avoid going Cytherean. Individual worlds may vary in Size, Atmosphere, and Hydrographics, but generally any Super-Earth in the Inner Zone (and some in the HZ depending on size and position in the HZ) will actually be a Super-Venus.

As the name implies, a Super-Venus is juat that – a Venus writ large, with even more extreme surface conditions and atmosphere than a typical Cytherean. Don’t even think of visiting one.

Super-Mercury (including lavaworlds)

A Super-Mercury happens when the atmosphere is “thin” enough to where the planet’s surface becomes even barely visible. This happens in Close “Torch Orbits” where the solar flux/solar wind/solar flares are able to wear away the Dense to Super-Dense Exotic atmosphere.

Or the planet may not have accreted that much atmosphere or hydrographics to begin with. Or both.

If any other planets of significant mass are in adjacent orbits, a Super-Mercury will resemble a Super-Io, with massive volcanism and seismic activity beneath an atmosphere now thickened and opaque from volcanic plumes – actually intermediate between a true Super-Mercury and Super-Venus; the distinction is that the atmosphere is too shallow and thin to be a true Super-Venus.

A world this close will normally be tidal-locked, an eyeball world whose bright face iris/pupil will be a seething mass of active volcanoes directly under the sun where the crust is weakened by sun-heat. Or it may be in a resonance lock (like Mercury) where the “days” are longer than the “years”; in this case the volcanic activity forms an “equatorial belt” of volcanoes whose center of activity slowly migrates to stay beneath the sun.

In both cases, while such a “lava world” is spared the ocean-bottom pressures of a super-dense atmosphere, said atmosphere will still be Exotic-to-Corrosive from volcanic gases. The surface may or may not be visible beneath the haze of the atmosphere.

Helium Capture Threshold

Helium capture occurs when the world’s surface gravity becomes great enough to prevent helium from escaping. This usually happens around 1.5 G (UPP size 12, 3-4 T-masses) for a world with similar solar flux to Earth; the closer to the sun, the higher the surface gravity needed; the farther out, the lower.

(Note: In Habitable Planets for Man (1964), Stephen H Dole sets the Helium-capture threshold at 1.5 G surface gravity; this corresponds to a UWP Size 10 planet, as Dole factored in increasing density with increasing mass. Classic Traveller’s table of “Standard Worlds” (Book 2 p.37, The Traveller Book p.79) does not take this density increase into account; there, 1.5 G corresponds to a Size 12 planet.)

Helium is the second most common element in the universe (including protostellar molecular clouds), much more plentiful than heavier elements. When the gravity becomes enough to prevent helium escape, the helium in the atmosphere builds up, increasing the atmosphere’s depth and surface pressure still further and increasing the planet’s mass until the gravity is high enough for actual hydrogen capture. This is how gas giants accrete; a protoplanet that achieves helium capture and then hydrogen capture is headed for gas gianthood fast unless something stops the process – like the sun igniting and the new solar wind ending planetary accretion by pumping the gases out of the system in Herbig-Haro Object jets like a miniature quasar.

Depending on the size of the original rockball core, this results in two types of transitional planet: a “Hycean” world or an actual Gas Dwarf.

Hycean Worlds

A Hycean world is a large rockball/waterworld with a dense atmosphere rich in hydrogen and helium, but where the hydrogen and helium do not dominate the mass. They are the true transition between Super-Earth and Gas Dwarf/SGG, whose accretion stopped before they reached full GG-hood while leaving enough “solid” surface. Ultra-large waterworlds with ultra-dense, ultra-deep, hydrogen-rich atmospheres, like a more extreme version of Asilra in Virtchok System (“Jump Destination: Korvo”, Freelance Traveller #75, May/Jun 2016).

Hyceans can be life-bearing – extremophile microbes only (Life Score 2 or 3, max), with most of those locked up in the deeps and only low amounts of specialized aero-bacteria (single-celled or colony aeroplankton) reaching altitudes where a ship could survive. But then, nine out of ten life-bearing worlds never get beyond bacteria.

These extremophiles’ metabolisms would operate on a reducing instead of oxidizing biochemistry, “breathing” the hydrogen that is a major atmospheric component.

Example: Asilra (dense/exotic atmosphere waterworld Super-Earth)

A Telerine-sized waterworld in the Dole Moving Group, Asilra (Virtchok III, XAAA000-0) is too small to be a true Hycean (just below the helium-capture threshold for its orbit), but illustrates all the characteristics of one on a smaller, less-extreme scale:

Gas Dwarf
(because “Very Small Gas Giant” sounds stupid)

A Gas Dwarf is like a Hycean, but where the hydrogen and helium start to dominate; the rocky core is proportionally smaller than a Hycean, to where the planet resembles a Small Gas Giant instead of a Super-Venus. Though Gas Dwarves tend to be more massive than Hyceans (farther along the transition), there is a lot of overlap; the larger Hyceans are more massive than the smaller Gas Dwarves.

Characteristics of a Gas Dwarf as opposed to a Hycean:

Gas Dwarves are more likely in the Outer Zone, especially near or outward of the frost line, where the light gases are more plentiful, the ambient temperature is colder, and the critical mass for helium capture is smaller.

They can be skimmed for fuel but this is a desperation move, as the skim would require 2-G to 3-G Maneuver Drives and the skimmed fuel would be heavily Contaminated with Helium and heavier gases, requiring multiple refining passes to decontaminate the fuel. (Rule of thumb is Contaminated fuel refines into Unrefined Fuel at best.)

There is no clear boundary between a Gas Dwarf and an SGG, but the Scout Service uses a frankly-arbitrary border of 10 to 12 T-masses. Anything over this and the planet is definitely on the downward density curve of an SGG.