Enceladus, Europa, Ganymede, Titan, Triton, Pluto, Eris...they may all have, or have had, large oceans of liquid water trapped beneath a frozen crust. That poses some interesting questions.
I've written before on these pages (and elsewhere) about the wealth of evidence for internal bodies of liquid water in our solar system. Since the Pioneer, and then Voyager and Galileo missions, the icy shrouded moons of the giant planets have prompted the notion of layered interiors that could include oceans.
Relatively simple physical modeling of the balance between self-gravity and multi-phase material pressure inside these objects suggests that internal ocean zones can happen. And there is a wealth of pieces of evidence to support this, which - taken altogether - point to an all but unassailable truth.
These facts range from the extraordinary surface topography of places like Europa, to the 101 active geysers of Enceladus (and perhaps geysers on Europa too), the salty particles making Saturn's E-ring, and the silicate nanoparticles that could have originated in hydrothermal environments in Enceladus, as well as the sulfates scattered across Europa's surface, the slithering of Titan's outer crust, and the in-situ and remote measurements of induced magnetic fields in Europa and Ganymede that suggest a conductive internal medium.
I think at this point only the most contrived of explanations could avoid concluding that most of the liquid water in our solar system resides beyond the Earth - conceivably 15 to 16 times or more the volume of all terrestrial surface water.
The specific configurations of many of these internal oceans includes a direct or indirect interface with 'hot rocks' - the silicate-rich inner cores of larger bodies. Both Enceladus and Europa have evidence for precisely this, which bolsters the notion of habitats capable of seeding and evolving life as we know it - from chemoautotrophic organisms living off mineral REDOX opportunities to who-knows-what.
For near-term science these discoveries represent a golden opportunity, and confirm that proposed missions like NASA's Europa Clipper or ESA's JUpiter ICy moons Explorer (JUICE) are excellent ideas that should go ahead. But it's also increasingly clear that there should be much more ambitious exploration, a proper sampling mission to Enceladus, a proper lander mission to Europa and so on.
One of the hurdles for the broader community to get on board may be, in my opinion, that we've been so indoctrinated with the notion of Earth as a cosmic 'oasis' that it's hard to see beyond this. Our planet is indeed a jewel, but it's a jewel who's worth is somewhat in the eye of the beholder. If we came from, say, Mars, we'd probably think that the thin atmosphere and exquisitely caustic dryness of martian soil were lovely, beautiful things (well, I guess they are, but not immediately for most of us).
It really is possible that we have the habitability of our solar system backwards. While it's true that the internal oceans of the outer solar system may mostly be places of low energy flux, with slow-living organisms more akin to the deep terrestrial biosphere, it's also possible that hydrothermal environments are extensive and capable of supporting more vigorous ecosystems. And what the dark oceans lack for in energy they may make up for in sheer bulk.
It would be truly ironic and, dare I say, Copernican, to discover that even in our home system the phenomenon of life is not actually centered on the Earth.