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What struck me when reading about the "super-Earth" type of exoplanet is that many people seem to assume these planets are either like terrestrials in our Solar system, or mini-Neptunes/Gas Dwarfs. Reading more about this struck me as I realized an "in-between" exoplanet type probably exists and many of the superterrestrials discovered to this date. These two papers https://arxiv.org/abs/1606.08088 https://arxiv.org/abs/1311.0329 are rather enlightening on this matter. For example of what struck me: Also, on Wiki This basically suggests that there is a class of planet that: - has a solid or molten lava surface - at the same time, has an atmosphere that is H/He rich, and while FAR lighter than Neptune (for this reason I would say calling them sub-Neptunes is not really accurate as the pressure and temperature, as high as they are, are orders of magnitude lower than those in the water mantle of Neptune), still much denser, hot and crushing than Venus (pressure on the surface of Venus is 9 MPa, the pressure on a hypothetical 5 ME/2 Earth radius planet is 2 GPa or 222x as much as on Venus, one of the theoretised pressures for Kepler 11b is 1 GPa). Now, these planets have some unimaginable surface conditions, but unlike Neptune, you can still say there is a surface there, and to compare, the pressure at the top of the water mantle of Neptune is 200 GPa or 100-200x as much as at the surface of these planets (the temperature at the top of Neptune's mantle is 3000 K). In case of Kepler 11 b, this atmosphere is likely to be steam/supercritical water (a sort of an inbetween phase between liquid and gas). So, basically, there are probably many planets that are not really terrestrials as we know them from our Solar system, but not ice dwarfs like Neptune let alone gas giants like Jupiter. Of course, if we ever get there, exploring them would be the ultimate challenge of building landers, but the chemistry and processes (as they might feature processes we know from Neptune along with terrestrial geological phenomena like volcanoes) might be very fascinating. 55 Cancri is also sending some rather interesting signs that it is probably something we have not seen yet: http://www.space.com/32416-super-earth-55-cancri-e-super-hot-weather.html http://news.nationalgeographic.com/2015/05/150506-volcano-planet-space-cancri-astronomy/ Yet, despite the temperature differences suggesting a thin atmosphere, one was indeed discovered, and it is a hydrogen/helium one with a mix of... probably hydrogen cyanide: https://www.spacetelescope.org/news/heic1603/ So here is some conflicting evidence. On one hand, the large temperature variations and possible evidence of volcanic ash blocking emissions suggests a relatively thin atmosphere. On the other hand, spectral evidence suggests a hydrogen/helium atmosphere. On one hand, the planet is 8.63x as massive as Earth, so it could have gathered a H2/He envelope. On the other hand, it is on an extreme torch orbit, worse than Kepler 10b and Corot-7b, which have practically no atmosphere, according to transit data. Yet this one has, yet no H2O was detected (while for a Neptune like planet it is the major component), drastic temperature variations, possible volcanism... yet it apparently retained some light gases. The radius is 2x of Earth, mass 8.63x of Earth, so it is much denser that the 5 ME/2 Earth radius hypothetical "borderline" planet. That would suggest it has a hydrogen atmosphere, but one with a lower surface pressure than 2 GPa. Basically, what I am saying is that some exoplanets might have hydrogen/helium atmospheres that have high pressures, but have not retained the extensive hydrogen envelopes like Neptune or Jupiter. I think planets like this might be frequent in torch orbits, as the gravity holds SOME of the light gases, but not everything. The result might be an unholy Venus/Neptune hybrid with features common to both ice giants and terrestrials. But this does not end, apparently, some planets managed to grow to Neptune like masses while being so dense that they are clearly fully terrestrial and free of any H/He envelope: https://www.eurekalert.org/pub_releases/2014-06/hcfa-afa053014.php The planet likely has a superheated ocean of water, but no helium or hydrogen: https://en.wikipedia.org/wiki/Kepler-10c However, there are also apparently planets as light as Earth with an extensive light gas envelope: http://www.nature.com/news/earth-mass-exoplanet-is-no-earth-twin-1.14477 https://en.wikipedia.org/wiki/Gas_dwarf So, apparently, planets can be Neptune sized and terrestrial, Earth sized and with a huge gas envelope, or anything in-between. This is also why I don't really like when people throw terms like "super-Mars", "super-Earth", "super-Venus", or "super-Mercury". I think there is a very big factor that determines what a planet is like and that is - how exactly did it form and in what conditions. A planet is more that its orbital parametres. Sorry if this got too long. Just had an urge to air my thoughts and stimulate a discussion. I personally feel that the obsession of astronomy about finding "Earth-like" planets limits our horizons and knowledge. I am fascinated by bizzare planets, even those that probably have no life (through the soup like atmospheres of "borderline" planets might have surprises waiting for us...), and think apart from the joy of knowledge and aesthetics (when we eventually manage to photograph them) they might eventually offer a lot to humanity. EDIT - In addition, planets like the ones with a big, but not quite ice giant atmosphere (like the "sub-Neptunes" mentioned in the paper) might offer life bearing conditions if they are rogue planets: https://en.wikipedia.org/wiki/Rogue_planet#Retention_of_heat_in_interstellar_space