Titan that is the largest moon of Saturn is the only moon in our solar system that has a dense atmosphere. Titan is also the only object in our solar system except for earth that has liquids on the surface. Titan has just like earth a landscape with rivers and lakes it has a variating climate with wind and rain. The river and lakes are liquid methane and the atmosphere consists of nitrogen. The average surface temperature in -179 C or -290 F. Not even the most resistant living organisms that are known on Earth would ever survive on Titan and many scientists find it very unlikely that life could exist on Titan. However, it would not be theoretically impossible for life on Titan. Perhaps organisms that been developed on Titan differs from the one developed on Earth. Perhaps those lifeforms like freezing liquid methane just like we like water. Huygens probe that landed successfully on Titan in 2005 was not equipped to provide evidence for biosignatures. More probes to investigate Saturn’s moons has been proposed in the future, though.
In our solar system, the inner planets are terrestrial and the outer planets are gas giants and according to the theory of solar system formation, massive Jovians can only form in the cold outer regions of the system.
But still, it is very common that exosolar systems have so-called hot Jovians. A hot jovian is a gas giant that is orbiting close to its sun. The first one orbiting a sun-like star was discovered In 1995 by Michel Mayor and Didier Queloz around the star 51 Pegasi. One explanation could be that young large Jovians formed in the outer regions of their solar system gains enough gravitational force that it reduces the planet's orbital energy causing the planet to migrate inward. 827 hot Jovians and 139 warm Jovians have been discovered so far. If a jovian is in the habitable zone of its star it could have moons. Just like Titan the moon could have atmosphere and oceans and those oceans could be water instead of methane and the temperature could be similar to earth. Perhaps an alien civilization exists on a moon instead of a planet. The science fiction movie Avatar from 2009 takes place on the moon Pandora orbiting the gas giant Polyphemus in the Alpha Centauri A system. Alpha Centauri A does look like a pleasant home for exoplanets the star emits far less radiation than our sun, but no one is found so far.
We have not discovered any Exomoons yet because as those are very hard to detect and confirm using current techniques. In July 2017 Hubble Space Telescope found signs of a Neptune-sized exomoon around the planet Kepler-1625b. Theoretical it could be possible with James Webb Space Telescope in the future to detect images of exomoons. Some scientists estimate that there are as many habitable exomoons as habitable exoplanets. There are several candidates with possible habitable exomoons in the PHL exoplanet catalog. It is possible both in the Android app and in this website catalog to filter for candidates with possible habitable exomoons.
Enceladus is the sixth biggest moon of Saturn. The moon that was discovered by William Herschel in 1789 is named after the giant Enceladus of Greek mythology. It won't live up to its name when it comes to size. With a diameter of 500 km, the moon would fit within the north sea. But it will live up to its name when it comes to being one of the strongest candidates for finding non-terrestrial microbial life in our Solar system. Studies have shown half of Earth's biomass lives long under the surface. As microbial life is not just a surface phenomenon the chances of finding this kind of organism outside the so-called goldilocks zone of a star increases. The strongest candidates are moons around gas giants that under their thick icy surface has oceans heated with volcanic activity. One of those candidates is Jupiter's moon Europa that I wrote about in this . And the other is Enceladus.
Image credit: NASA/JPL-Caltech
On January 16th, 2005 Nasa's probe Cassini's was on a flyby pass Saturn when it captured a very unusual event on Enceladus. There was a bright light coming from the south pole of the moon shining out into space. There were enormous geysers plumbing out vapor into space, more than 8000 times larger than geyser on Earth. Nasa immediately decided that Cassini should have a new mission. To study Enceladus. The probe was reprogrammed from earth to fly into these plumbs and find out was the vapor was made of. It took three years of reprogramming until Cassini was able to do that and the result showed that the vapor was liquid water. Some of the water will fall back on Enceladus as snow but the most will make of the material in Saturn's E ring. The result also showed that the water contains salt which is a very important ingredient for life. The chemicals in the water were very similar to earth and it also showed that something inside Enceladus was heating the water. On June 27, 2018, scientists reported the detection of complex organics compounds from the Cassini data. The Cassini mission ended in September 2017 with the probe crashing into Saturn's atmosphere.
It is very hard getting a probe to investigate Enceladus within a reasonable budget. Several future missions to Enceladus that have been proposed has been canceled. Here is are a list of candidates that are under study and could be our next mission to the icy moon: