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Eying exomoons in the search for E.T.

When I was young, the only planets we knew about were the ones in our own solar system.

Astronomers presumed that many of the other stars in the night sky had planets too, but this was sheer speculation. We could never know for sure, the thinking went, because such planets were ridiculously small and faint. To ever see or study them seemed a complete impossibility. "Extrasolar planets," or "exoplanets," were a , but not of professional astrophysics.

It's hard to believe that there was once such a simple time. The first was in 1991, identified by the tiny wobbles experienced by the parent star as its exoplanet swung around it. Since then, the . There are now around , with almost 4,000 other known candidates. There are exoplanets , and others . Their orbits around their parent stars range from to . And the ones we know about are just a tiny fraction of the we now believe are spread throughout our Milky Way galaxy.

But while the golden age of exoplanets has barely begun, an exciting additional chapter is also taking shape: the hunt for exomoons.

Beyond Earth-like planets to exomoons

An exomoon is a moon orbiting a planet, which in turn is orbiting another star. You may not have ever heard of exomoons before now. But if you're a fan of films such as "," "" or "," this should be familiar territory: in all three cases, most of the action takes place on an exomoon.

But what about real life? How many exomoons do we know of? At the moment, zero.

But to find the real-life analogs of and .

You might think searching for tiny rocks orbiting distant planets around faint stars hundreds or thousands of light years away is the ultimate example of an obscure academic pursuit. But exomoons are poised to become a big deal.

The whole reason exoplanets are exciting is that they're a path to answering one of the grandest questions of all: "" As we find more and more exoplanets, we eagerly ask , and whether . However, so far we've yet to find an exact match to Earth, nor can we yet really know for sure whether any exoplanet, Earth-like or otherwise, hosts life.

Enter exomoons in the search for life

There are several reasons why exomoons, these little distant worlds, may be the key to finding life elsewhere in the universe.

First, there's the stark reality that life on Earth may not have happened at all .

The Earth's axis is tilted by 23.5 degrees relative to its motion around the sun. This tilt , and because this tilt is relatively small, seasons on Earth are mild: most places never get impossibly hot or unbearably cold. One thing that has been crucial for life is that this tilt has stayed the same for very long periods: for millions of years, the angle of tilt has .

What has kept the Earth so steady? The .

In contrast, Mars only has , which have negligible gravity. Without a stabilizing influence, Mars has gradually tumbled back and forth, its over millions of years. Extreme changes in climate have resulted. Any Martian life that ever existed would have found the need to continually adapt very challenging.

Without our moon, the Earth, too, would likely have been , rather than the relative certainty of the seasons that stretches back deep into the fossil record.

The gravity of the . Billions of years ago, the ebb and flow of the oceans produced an alternating cycle of high and low salt content on ancient rocky shores. This recurring cycle needed to generate the first DNA-like molecules.

Exomoons might have Earth-like environments

Overall, as we continue to hunt for another Earth somewhere out there, it seems likely that a twin of Earth, but without a moon accompanying it, would not look familiar. Finding exomoons is a key part of finding somewhere like here.

Meanwhile, we shouldn't be discouraged by the fact that most exoplanets found so far are , with hostile environments unlikely to support life as we know it. What we don't know yet, crucially, is whether these exoplanets have moons. This prospect is exciting, because exomoons are expected to be smaller rocky or icy bodies, .

This is hardly speculation: Titan (a moon of Saturn) has even denser than Earth's, while are thought to exist on (another moon of Saturn) and on and (both moons of Jupiter). Thus, if there is any other life out there somewhere, it may well not be found on a distant planet, but .

The . While exomoons are too faint to see directly, astronomers are deploying in their searches. Those moons are assuredly out there by the billions – and soon we will find them. It won't be too much longer before these tiny worlds help us answer huge questions.