Synthetic life

Nanomachine swarms

Advanced metal or crystal based life forms. Built as mechanical cell-based machines by an organic alien race.

Think of a tiny 3D printer that, if you give it raw materials, can create another tiny 3D printer.

Grey goo

If a species invented a nanomachine that could self-replicate from widely available materials, and that nanomachine escaped into the environment and without any failsafe measures, it could multiply so rapidly that its progeny could consume an entire planet.

This type of scenario is called 'grey goo'.

Eric Drexler, Engines of Creation

Imagine such a replicator floating in a bottle of chemicals, making copies of itself…the first replicator assembles a copy in one thousand seconds, the two replicators then build two more in the next thousand seconds, the four build another four, and the eight build another eight. At the end of ten hours, there are not thirty-six new replicators, but over 68 billion. In less than a day, they would weigh a ton; in less than two days, they would outweigh the Earth; in another four hours, they would exceed the mass of the Sun and all the planets combined.

Eric Drexler, Engines of Creation

Early assembler-based replicators could beat the most advanced modern organisms. 'Plants' with 'leaves' no more efficient than today's solar cells could out-compete real plants, crowding the biosphere with an inedible foliage. Tough, omnivorous 'bacteria' could out-compete real bacteria: they could spread like blowing pollen, replicate swiftly, and reduce the biosphere to dust in a matter of days. Dangerous replicators could easily be too tough, small, and rapidly spreading to stop — at least if we made no preparation. We have trouble enough controlling viruses and fruit flies.

You can imagine a visiting spaceship venturing too close to a planet encased in grey goo. They may unknowingly encounter a particle or two in the upper atmosphere and end up with a stowaway on the outside of the ship, which may spread the goo to another planet.