Are there any other elements which could form as broad a variety of compounds as carbon? Pulp-quality space fantasies often envision organisms based on silicon instead of carbon. The authors of these works usually have silicon equivalents to organic compounds in mind (every carbon atom replaced by a silicon atom). This, however, is impossible. Silicon equivalents of some simple compounds are known to exist, but they are highly unstable substances which can only be sustained under laboratory conditions. Most organic compounds, however, including the more complex ones, don't have silicon equivalents. But there might be another kind of silicon-based life. Silicates show a wide variety of forms (though not as wide as organic compounds). Molecules consisting of chained silicate (SiO₄), phosphate (PO₄), sulphate (SO₄) and aluminate (AlO₄) units might exist under suitable conditions. It is, however, entirely unknown whether such conditions do occur in nature, and whether life could evolve from such molecules. But if it exists, it might thrive in places where no organic life could exist, such as Venus.

But even weirder things might exist. Physicists assume that atomic nuclei have a shell structure similar to the electron hull. It is speculated that under the immense pressure to be found at the surface of neutron stars, these shells might interlock just as electron shells do and form `nuclear molecules'. No-one knows what properties such nuclear molecules have and whether these are able to form complex, self-organizing and self-replicating structures. There might even be life forms which are not composed of matter as we know it at all, but of supersymmetric or bosonic matter, or just standing wave patterns of creative energy.

And finally, there might be life forms around that have evolved from more `normal' life forms into something utterly bizarre. Intelligent computers, networks of superconducting nanotechnical nodules or anything else. The advanced descendants of star-faring races might be perfectly adapted to living in open space such that they consider planets to be dangerous gravity traps and avoid getting near them whenever they can; an Earth-like world might be as hostile to them as the oceans or the reducing atmosphere of a juvenile terrestrial planet are to us. They probably won't care about sun-like stars at all (the only interesting thing about sun-like stars is the possibility of Earth-like planets, anyway, so if you don't care about Earth-like planets, why care about sun-like stars?), but cluster around those stars which put out really big amounts of energy -- supergiants, recurrent novae and their like, which are all places where our SETI researchers who focus on Earth-like life never look.