One of the most exciting discoveries of the 1990s was the detection of massive worlds orbiting stars beyond the Sun. Astronomers were surprised to find that, unlike in our own solar system, some of these giant planets orbit within the "habitable zones" of their suns — that is, in the regions where liquid water could exist and, in theory, life could thrive.

It seems highly unlikely that life, much less intelligent life, could arise on (or rather, in) these giant, gaseous planets. But what about their moons? Could they offer roughly Earth-like conditions where a technological civilization might evolve?

In our own solar system, the larger a gas giant is the greater the total mass of its satellites. So perhaps extrasolar giants more massive than Jupiter have moons as large as Mars or even Earth. How livable might these worlds be?

One factor determining a moon's suitability for life is the stability of its orbit, which can be disrupted by the close proximity of its sun. Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1 astronomical unit from a Sun-like star. The major moons of our own solar system's gas giants all have orbital periods well within this range, between 1.7 and 16 days.

Moreover, the total angular momentum of a gas giant's system of moons seems to be roughly proportional to the planet's mass. If a similar scaling law applies to more massive giants and brown dwarfs outside our solar system, the orbital periods of their moons will still fall within the upper limit for stability. At the lower end of the range, the orbits will still be well outside the Roche limit where a moon would be sheared apart by tidal forces. So, giant planets and brown dwarfs in a star's habitable zone indeed seem likely to have large moons in stable orbits.

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