Though the search for alien life has focused on planets, it's possible that living creatures dwell on moons in our solar system and others.

The first real search for extraterrestrial life was a reconnaissance of the moon. In the early 1600s, Johannes Kepler trained a low-grade telescope on that pockmarked rock, and figured that the roundish craters he saw were the domes of subterranean cities, teeming with lunar inhabitants.

Kepler was badly mistaken, of course, though that wasn't obvious for another century when better telescopes suggested that the dry, airless moon was likely to be deader than Latin. But today there's a renewed interest in satellite worlds — moons — as homes to life.

Phil Sutton, an astrophysicist at the University of Lincoln in northeast England, recently pointed out that moons around gas giant planets — planets similar to Jupiter and Saturn — could be bristling with biology.

Why should anyone think that?

In the past few decades, we've learned that moons don't have to rely on starshine to stay warm. If they're in orbit around a king-size planet — even one that's at a great distance from their sun — they could still have reservoirs of liquid water and thus the potential for life as we know it. That's because the gravitational pull holding these moons in check isn't necessarily constant but varies slightly depending on their shape, their orbit and whether they have sibling moons nearby.

As a consequence, such moons are subject to periodic stretching and squeezing. The distortions are small — typically on the order of a few meters — but this never-ending moon massage produces internal warming, just like kneading bread dough causes it to heat up slightly.

The surprising bottom line is that even moons that are captive to planets in the frigid outer realms of a solar system might be sufficiently heated to have liquid-water oceans below their surface, rather than a layer of rock-hard ice. Picture deep, pitch-black seas, barely above freezing and laced with salts. While that's less than ideal for your personal lifestyle, microbes — with their lower standards — might consider these subsurface oceans perfectly satisfactory.

So when scientists think about where they might find biology in our own neighborhood, as opposed to other star systems, they're particularly enthused by three moons of Jupiter — Europa, Callisto and Ganymede — and two of Saturn — Enceladus and Titan. Liquids are thought to ebb and flow in, or on, all of these small orbs. And in the more than four billion years since they were whelped, it's hardly unthinkable that some might have spawned life. So, if you also count Mars, that makes a half-dozen nearby extraterrestrial worlds where life may be moving and shaking.

Sutton's point is this: Moons seem to be as numerous as gnats (in our own solar system there are nearly 40 times as many known moons as planets). Presumably, they're also plentiful in other solar systems. And any that are relatively big and in orbit around large planets will have a source of heat no matter how far they are from their host star. So we should consider these moons in our searches for life, and not just restrict our attention to star systems that might have an Earthlike planet.

That's easier said than done, of course. Just finding a so-called exomoon (a natural satellite orbiting a planet in a far-off solar system) is a crushing technical challenge. So far only one candidate has been reported, a Neptune-size satellite believed to orbit Kepler 1625b, a planet that's a daunting 8,000 light-years from Earth. Even if we're right about this exomoon, how might we ever learn if it has life — especially if that biology exists in an underground aquifer? That's a problem left for the student, and most likely a student a generation or two hence.

Extraterrestrial microbes may not fully float your boat, of course. Anyone who's seen the movie "Avatar" might find the idea of a Pandora-like moon more tantalizing. This fictional world was home to a species of blue-skinned aliens who traded in the exotic element unobtanium (whatever that is) to build up their foreign reserves. This sort of extraterrestrial would likely not arise in the buried oceans of a satellite like Europa, but Pandora was much bigger than the moons in our own solar system. Under the right circumstances, a large moon could have surface oceans and an atmosphere, although maybe no unobtanium.

There are roughly a trillion planets in our galaxy. The number of moons is probably close to 100 trillion. That's a lot of real estate, and it's probably not all lifeless. Indeed, as Sutton points out, it's entirely possible that the zip code of most cosmic life is on a moon, not a planet.

This article was originally published at nbcnews.com: https://www.nbcnews.com/mach/science/why-alien-moons-may-be-great-place-find-extraterrestrial-life-ncna1017391