So far we have considered life on planets similar to Earth: terrestrial planets of sufficient size, orbiting within a zone of moderate temperatures, with liquid water on their surface. But can there be life in other environments as well? The Apollo astronauts had to go through quarantine measures upon their return from the Moon because no-one was willing to take the risk of importing something alien, infectious from there. (However, not a single trace of life was found on the Moon -- not very surprising.) Mars can be considered a close call towards an Earthlike world; it is considered possible that primitive life exists there. The Viking probes couldn't find anything; but it is yet not certain that there is no life on Mars.

Other places in our solar system have been considered to be possibly life-bearing as well. No-one really expects life on Mercury or Venus (though even this need not be entirely impossible!), or, on the other hand, on the moons of Saturn (except Titan -- see below), or in the Uranus, Neptune or Pluto systems. But it might be possible that there is life on Jupiter: while the outer layers of its vast atmosphere are colder than -100 degrees C, its inner parts are hot -- there must be an area of moderate temperatures in between. Jupiter's atmosphere contains methane, ammonia, water, hydrogen sulfide and other molecular compounds, and huge thunderstorms and lightnings have been observed. Thus, Urey-Miller processes might be possible, and some scientists actually assume that Jupiter's clouds are coloured by complex organic compounds. This, however, is the first step towards biological evolution, and perhaps, actually life exists on Jupiter, floating in its atmosphere. These life forms, however, would be incredibly alien, different from anything ever to be found on an Earth-like planet.

The next place sometimes mentioned as a possibly life-bearing world is the second of Jupiter's Galilean moons: Europa. At first glance, Europa doesn't seem very convincing -- it is entirely encrusted with ice and has no atmosphere. However, its ice crust is extremely smooth -- just as if it had been molten several million years ago. It is assumed that this is actually the case: Europa, on its orbit between Io and Ganymede, suffers intense tidal friction; this could have molten its crust and might still keep the lower layer of it in liquid state, i.e. there might be an underground ocean beneath its solid surface! However, if there is life there, it is life in utter darkness, which rules out sunlight as energy source. It therefore would have to live off something else, perhaps volcanic gases. It is unknown whether this is feasible, which means that Europa might be just as dead as the other moons of Jupiter. (Nobody expects life on Io with its sulfur volcanoes, especially in light of the deadly radiation this moon is exposed to, or on the frozen worlds of Ganymede and Callisto, which show no hints of an underground ocean.)

Another place often suspected to be capable of bearing life is Saturn's largest moon, Titan. It has a dense, opaque atmosphere containing organic material. However, one thing is certainly missing there: liquid water. Titan is simply too cold to allow for this. It is, however, assumed that there is something liquid on Titan -- a mixture of light hydrocarbons (like methane and ethane) and several more complex substances. The possibility of life on Titan depends on whether such a medium can play the role of water or not. This issue is discussed below.