That's a question a team of astronomers is trying to answer, and if they find any, what tales those moons might tell.
Distant moons could yield insights into the formation and evolution of planetary systems unlike our own. In addition, they could also host environments hospitable for life, even if the planets they orbit don't.
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Finding so-called exomoons is a daunting task. But as Kepler searches for Earth-size planets orbiting at Earth-like distances around sun-like stars, the Hunt for Exomoons with Kepler (HEK) aims to use similar techniques to find a new trove of celestial objects.
Even within our solar system moons may offer the potential for life. Jupiter's moon Europa and Saturn's moon Enceladus, for instance, have piqued interest because evidence indicates they have liquid water – a key ingredient for life – beneath their ice surfaces. Another Saturnian moon, Titan, does not appear habitable but is fascinating in its own right, with rain, lakes, and rivers of liquid hydrocarbons.
Yet these moons fall far outside the sun's habitable zone – the place where, if a planet has a sufficient atmosphere, the amount of starlight it gets is "just right" to allow water to exist as liquid, ice, and gas at or near the surface.
During its search for planets inside their stars’ habitable zones, Kepler could also find evidence of moons worthy of further study.
The HEK team is looking for moons with more heft than any in our solar system – at least 10 percent of Earth's mass. By contrast, the largest moon in the solar system, Jupiter's satellite Ganymede, has about 2.5 percent of Earth's mass.
The project, which began in earnest about four months ago, already has yielded objects the team wants to examine in more detail, according to David Kipping, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
These objects "were really easy to spot; they just stuck out like sore thumbs," Dr. Kipping says. "We have about four or five top, top candidates which we are looking at very closely."
How you look for an exomoon
But where planet-hunters have experience in identifying a planet's signature, he cautions, the same can't be said for moon hunters. It may be a year or so before the team will know whether or not it has found its first moon.
Kepler finds planets by staring at roughly 150,000 stars simultaneously for subtle changes in brightness when a planet eclipses its sun.
Kipping's team is hunting for moons in much the same way. In principle, a large moon can betray its presence as it eclipses part of the star, or even as it transits its planet as the planet crosses in front of the star.