Mounting discoveries of planets around other stars are fueling anticipation among most astronomers that our solar system is a reasonable model for the kinds of objects that probably exist around many stars. If they are right, then the galaxy could be loaded with billions of planets -- and a far greater number of moons.

Some of these satellite worlds could have the ingredients for life and the ability to support it, said experts interviewed by SPACE.com.

These presumed extrasolar moons are hidden in the scientific shadows of their planetary companions. None have been found, so none can be studied. Few top researchers theorize about them. Scientific literature on the topic is thin.able -->

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	An extrasolar world might have multiple moons, as depicted in this illustration by John Whatmough. Scientists say there are probably a lot of the moons in the galaxy, and some might support life. Find more of Whatmough's art at www.extrasolar.net.
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The reasons are simple: there are sexier targets to hunt for, such as potential Earth-like planets around other stars. Also, a handful of moons right here in our solar system carry their own intrigue, perhaps even the possibility of life, and are relatively easy to study. Finally, current methods do not promise easy detection of a moon around a planet around another star.

But they must be there.

"Moons form so commonly in our solar system that it would be ludicrous to think that this is unique," said Peter Ward, co-author of the book "Rare Earth" and professor of geological sciences at the University of Washington.

And now that astronomers have found about 100 planets orbiting other stars, the case for the existence of extrasolar moons is all but a foregone conclusion, awaiting that last, all-important bit of evidence: a discovery.

By one method of speculation, it is possible to put a very rough number on how many moons might be out there. Assuming there are 300 billion stars in the Milky Way Galaxy, and figuring about 10 percent are roughly Sun-like and might harbor planets, and that solar system formation is somewhat similar to what happened here, one recent study estimated there could be about 30 billion Jupiters and an equal number of Earths.

Not even taking into account other possible planets around those stars, and considering only Jupiter's four large moons and the one around Earth, simple math dictates the possible presence of 150 billion moons, which could be a very conservative estimate.

A pretty meaningless number, to be sure, but one that illustrates the possibilities.

Alan Boss, a top planet-formation theorist from the Carnegie Institution of Washington, said it is "highly likely" that some of the exoplanets already discovered have satellites.

Nearly every class of large object astronomers can study -- from asteroids to planets to stars -- is known to have something orbiting around it. Boss said it seems to be a universal conclusion that satellite-making is a high-probability byproduct of round-object-making.

Moons, in fact, are about the only big round things we know of that don't seem to have moons.

Ward, the author, agrees that it's reasonable to assume there could be life on extrasolar moons. Complex life, however, is another story. The gravity of a giant planet would tend to reel in a solar system's debris, and moons have a habit of getting in the way.

"The biggest challenge might be impact rate," Ward said. "Jupiter's moons are blasted by asteroids and comets. Jupiter brings them in, and boom."

Other theorists have suggested that microbial life might struggle to gain any sort of complexity in such an environment. Many astrobiologists, meanwhile, suspect that simple life could survive some level of bombardment. Tiny simple creatures might even exist on one or more moons in our solar system. The leading candidate is Europa, a satellite of Jupiter.

No one expects fish swimming in the liquid sea under Europa's frozen crust, but even the discovery of microbes there would instantly double the number of known worlds where life exists. Prospects for life elsewhere would suddenly become nearly as certain as the Sun's rising each day.

A key factor in determining whether a moon might support complex life is whether the moon can survive long enough to allow evolution to run its course. On Earth, it took billions of years for life to advance beyond pond scum.

Jason Barnes is one of the few people running calculations on the prospects for extrasolar moon longevity. The University of Arizona researcher recently looked into whether giant exoplanets could indeed harbor moons in long-term, stable orbits.

Barnes did not consider how or whether satellites are created, but he speculates: "If extrasolar planets formed in the same way as planets in our solar system -- and it's not yet clear if they did -- then it's logical to assume they could form satellites."

Barnes and his colleague, David O'Brien, studied the orbital conditions are necessary to allow a satellite to endure. The work was based on what's known about the orbital mechanics of our own solar system.

In their study, published in the Aug. 20 issue of the Astrophysical Journal, the researchers show that a large planet orbiting too close to a star can't support moons over billions of years. The moons "crash into the planet," Barnes said.

Jupiter does support moons, of course, and they've been there a long time. But there is very little of the life-giving solar radiation in that neck of the planetary woods. So how close to a star can a giant planet be and still harbor long-lived moons?