Argthtjtr

This creature was void-born in the farthest depths of time in this Universe. Drawing its energy from electromagnetic radiation, and sealed against the vacuum of space, this drifting creature establishes itself in stable orbits, Lagrangian points, and in the wakes of comets. With the patience of a microbe, it can be ejected from solar systems and wait millions of years to re-establish itself. So, by fortune, it has spread, over billions of years, across this galaxy and beyond.

How does space plankton work?

It must:

• Be able to survive and reproduce in the vacuum of space.


• Generate energy somehow using electromagnetic radiation. It would be cool if multiple strains could utilize different frequencies.


• Go dormant as necessary to survive millennia or more in slow, sublight travel across galaxies. Upon reaching a suitable new habitat, it must be able to reproduce and grow once more.

Anatomically Correct Series

Their bodies are made of concentric shells of photon absorbing materials, arranged in a quantum cascade fashion:

• The outer layer absorb photons of energy $E_1$, re-emitting a photon of energy $E_1 - delta E = E_2$
  


• The following layer absorb photons of energy $E_2$ and re-emits at lower energy $E_3$
  

This cascade proceeds ad libitum.

Such principle can be applied starting from any point in the electromagnetic spectrum, and energy can be kept within metastable molecules to be released on demand.

The organism feeds on capturing travelling ions encountered during the travel. When the size of the organism goes above a certain threshold it simply splits in two smaller beings which depart one from the other. This works as reproduction for them.

Absorbed photons and ions provide momentum to them, keeping their eternal space travel.

Panspermia perhaps

Perhaps we can look at how a particular species like this could evolve.

If it was, for instance, a similar origin to a plant species, converting light and gas into cell structures, it may be conceivable for alien plant-like species to develop seeds in much the same way as fungal spores, light enough to be easily ejected into space.

If such spores were in the trillions, floating out from its origin system, it might be possible that in millions of years it could float to adjacent star systems, or into gas clouds.

In this instance, in further millions of years, it needs to:

• Maintain its DNA (or similar) structure impervious to radiation, or evolved to reproduce naturally far away from extreme sources of radiation to prevent its demise.


• be able to find, or bump, into others of its kind to mate, or simply reproduce asexually.


• gain mass in order to reproduce and spread.

One way to gain mass is simply to wait millions of years, where through the gradual force of gravity hydrogen and other light elements may 'stick' to it, and slowly become the genesis of a new cell. Once sufficient mass is gained, it could again 'explode' and send more spores/seeds out to other star systems, for the next generation.

Each generation could be hundreds of thousands, if not millions, of years - a mere blink of the eye cosmologically.