The search for life on Mars is almost always expected to be microbial, either belonging to the bacterial domain, or the prokaryotic Archaea. Entire research programs and endeavors are designed with this in mind; instrumentation aims to find biosignatures of these little critters when they land on these neighboring worlds of ours.

As spotted by Gizmodo, a new review published in the journal Astrobiology emphasizes a longstanding concern of plenty of astrobiologists. Namely, why are we assuming that the life we’ll find is bacterial, when it could as easily turn out to be viral?

Viruses sit awkwardly in a gray area between the living and the non-living. They can do some things that organisms can do, but they can’t do others.

That ambiguity doesn’t mean they aren’t important though. Far from it: as the authors of the new paper point out, “viruses are the most abundant biological entities on modern Earth.” They’re incredibly resilient, everywhere, and they’ve influenced life and biogeochemistry for hundreds of millions of years.

Their ubiquity suggests that they could very well be found on alien worlds, and yet, as the team points out, even NASA isn’t really paying much attention to this possibility. The 2015 NASA Astrobiology Strategy – a somewhat crowdsourced document that outlines the agency’s thoughts on how best to search for life in the darkness – only mentions viruses six times.

The team behind this paper – a trio of scientists from the University of California, Berkeley, the Tokyo Institute of Technology, and Portland State University – not only request that agencies like NASA do a better job at incorporating viruses into their thinking. They also point out that, before we start looking for alien viruses, we need to decide what a virus actually is.

As aforementioned, it’s complex. We can’t just be talking about virions – the inert form seen under a microscope – which they compare to a “spore” that only replicates within cells. A virus refers to the entire replication-based life cycle, and “considering the virion to be a virus is analogous to considering an acorn an oak tree.”

The team suggests, fairly sensibly, that whether or not a virus is alive is a moot point. If a virion was discovered in an extraterrestrial sample, few would argue that this isn’t demonstrative of “life”.

Either way, finding such evidence will likely be a considerable challenge.

“Finding a virus in space is totally different from finding bacteria,” Dr Ben Libberton, a microbiologist at the MAX IV Laboratory in Lund, Sweden, who was not involved in the paper, told IFLScience.

“Bacteria replicate independently, but viruses we know about have to replicate in eukaryotic cells. If we find a virus, it would probably belong to a complex ecosystem.”

Libberton also mentioned bacteriophages, viruses that specifically infect bacteria. Perhaps, then, we’ll get a double whammy, and discover both at the same time.

It’s possible that we could detect viruses based on the nucleic acids or the shells (capsids) they leave behind, which we could then compare with Earth’s own. “But if life is significantly different from that on earth then it will be very difficult to detect it,” he added.

The paper is rich in detail, but one point stands out: viruses are astronomically abundant, especially in our oceans. With that in mind, perhaps when we finally drill beneath the icy crusts of Europa and Enceladus, a complex viral ecosystem will be revealed.

“More than a century has passed since the discovery of the first viruses,” the authors conclude in their study. “Entering the second century of virology, we can finally start focusing beyond our own planet.”