Here is a description of a drake-like equation
to estimate their numbers in our galaxy. Now let us look at these Mars or bigger planets around dark, mostly Super Gas Giants (since some might form around smaller Saturn sized systems, especially in older, inner regions of the galaxy where the ratio of heavy elements is much larger) that have heavy warm liquid oceans, large amount of hydrothermal tidal heating, and if we believe J. Marvin Herndon, a huge nuclear fission core that has kept the tectonic plates and a strong magnetic field going for billions of years into the present. These planets would be very watery and will have a thin coating of ice on their oceans- and can be considered to be mega versions of Europa, and I theorize on the basis of the potentially large number of brown dwarfs and Super Gas Giants that might be out there, might outnumber terrestrial planets bathed by warm solar radiation. The forms of life that might evolve in such a harsh, changing but also well-protected environment is unimaginable- especially since many such systems would be much older than our Sun. The strong magnetic field of these Super Earth Europas would also protect them from any radiation belt of the mother gas giant (which would actually be quite weak and even absent because these planets are far from any major star). It is important to note that once our Sun dies, Europa and yet undiscovered worlds around gas-giants in or beyond Kuiper belt might continue to remain warm and hospitable just below the surface because of a strong tidal effect supported by their large parent and the accompanying siblings.
Open waters in the dark sky?
Some with tidal effects strong enough might even have localized liquid water surface environments: imagine a moon close to the mass of Earth around a solitary gas giant, say 10 times the mass of Jupiter orbiting it at the same distance as Io is from Jupiter. A large amount of intense radiation from the mother gas giant from the first few million years of their existence can blast off much of the highly volatile gases and material give such a planet a density substantially higher than that of Io or Europa leaving behind a much more Earth like system, with continents, a huge warm ocean, open waters in many regions, and lots of lightening, snow-storms and rain driven by hundreds of active volcanoes distributed on the surface, and the parent somewhat warm gas-giant covering a large part of the sky on one side of the planet. The global ocean would be much larger than that of Earth kept liquid because of tidal heating and a huge amount of tectonic activity, and strong sources of both radiative and heat energy near hundreds of ever erupting volcanoes. A thick atmosphere- perhaps mostly of nitrogen, methane and other gases spewed by the volcanoes. Such large volcanoes would even create small continents. Such a dynamic habitable system full of fresh nutrients would be very suitable for evolution of complex, fast-adapting and perhaps even intelligent life- compared to the much slower changing environment in the oceans of Earth- one argument for why land animals on Earth got smarter faster. On Earth we know of at least one hydrothermal species that uses the volcanic radiation for photosynthesis! It would be such a surreal environment; on many such worlds where the right combination of mass, density and parent gas giant creates an atmospheric pressure not too high; in some warmer volcanic regions a human could scuba dive naked or walk in shirt sleeves on a planet with no Sun! A planet far from being dark with glowing streams of lava dominating the scenery and lighting in many places, massive lightning storms visible both from the atmosphere of the dark gas giant and the local storms brewed by the volcanoes. If such a world does exist in large numbers, the possibilities are just mind-boggling, especially since such worlds can remain habitable for a long time. Even if 1 percent of Super-Europas are so hot, that would count to potentially tens of millions to a billion Worlds. Since the numbers are so unknown, and these worlds can exist in our backyard, it becomes imperative as we develop better infrared and imaging capabilities to try to find such large, dark gas giants in our neighborhood. Another important point is that SETI researchers should perhaps not be surprised if they cannot find a visible source for an apparently alien signal. With Jupiter still radiating (40% more?) a lot more energy than it receives from Sun, an object the size of Jupiter or bigger would still be relatively visible in the infrared, especially if it happens to be within a light-year from Earth. Once we detect such very nearby potential habitats, they should be potential SETI search targets and the first "stars" that robotic probes might be able to reach. But more than that, this idea highlights one important range of habitable worlds that might be as big if not bigger than habitable worlds around visible stars.
