H-bit

H-bits are hydrogen-based aliens hypothesized by scientists Gerald Feinberg and Robert Shapiro. They are made of solid hydrogen and live in liquid hydrogen, and therefore require extreme cold. Their metabolism is based not on chemistry but on the conversion of hydrogen between its isomers, o-hydrogen and p-hydrogen.

Anatomy & Physiology
An H-bit consists of a tiny mass of solid hydrogen with a small amount of trapped helium for flotation. The H-bit has an exterior of p-hydrogen, an internal layer of o-hydrogen, and an inner cavity containing liquid or gaseous helium for flotation. An H-bit could have an extendible "arm" like an amoeba's pseudopod.

Since an H-bit's metabolism is very simple, relying on only a few different molecules and a few different processes, an H-bit can be much smaller and simpler than any Earthly life form. An H-bit could be as small as 10 nanometers and consist of as few as a thousand molecules, far smaller and simpler than even a virus.

This describes a single-celled H-bit. Conceivably, an H-bit could be a multicellular organism, analogous to a plant or animal.

Genetics
H-bits do not have chemical genomes. Instead, their genome consists of a three-dimensional lattice of solid hydrogen, with o-hydrogens and p-hydrogens always in certain positions. This records information in much the same way that bases in nucleic acids do.

Metabolism
H-bits derive energy from the conversion of o-hydrogen to p-hydrogen. They can obtain this by bringing their internal o-hydrogen to their surfaces to interact with o-hydrogen in the ocean, converting both molecules to p-hydrogen. H-bits can also absorb solar radiation to produce o-hydrogen, which will then decay to p-hydrogen and provide energy. Impurities in the H-bit's body can collide with p-hydrogen to produce o-hydrogen, and H-bits can also use these impurities to control the rate of decay from o-hydrogen to p-hydrogen.

H-bits on the surface of their homeworld can use solar radiation for energy, rather like photosynthesizing plants. H-bits in areas too dark for this can feed on surplus o-hydrogen in their environment.

Homeworld
Feinberg and Shapiro hypothesized the planet Cryobus as a possible home for H-bits. Cryobus is much farther from its dim star than Neptune is from the Sun. Thus, it is incredibly cold, below 20 Kelvin. At this temperature, nearly every element exists in a solid state. Only hydrogen and helium are not frozen. Therefore, Cryobus' atmosphere consists of hydrogen and helium over a sea of liquid hydrogen. Some impurities and bits of solid hydrogen exist in this frigid sea, but no other variety exists. The H-bits live in the liquid hydrogen sea, though they could conceivably evolve to live on land if any exists.