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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Keeping an Atmosphere
For a moon to be friendly to advanced life, it certainly needs an atmosphere. To have enough gravity to hold onto an atmosphere, it obviously must be larger than Earth's own airless Moon, which has a mass 0.012 times that of Earth. But how much larger?
Darren Williams, James Kasting, and Richard Wade at Penn State University have examined this problem in detail. They found that a number of processes allow a world's atmosphere to escape.
The first is well known. Some of the gas atoms at the top of an atmosphere will get kicked by random thermal collisions to faster than the planet's escape velocity and fly away. For an atmosphere to last long, this process must be kept very slow. Either the temperature at the top of the atmosphere must be low, or the world must be massive enough to have a high escape velocity.
For a body with a Mars-like density and an Earth-like atmospheric temperature structure, calculations show that the mass must be at least 7 percent of Earth's to retain most of its atmosphere for 4.6 billion years (Earth's current age).
The escape of some atmosphere is not necessarily fatal. On Earth, carbon dioxide can be replenished from the vast stores locked up in carbonate deposits and available to be weathered out. However, the loss of some other biologically important gases, such as nitrogen, is irreversible.
A major loss mechanism for nitrogen is called dissociative recombination. This process starts when a positively charged nitrogen molecule at the top of the atmosphere combines with an electron to produce a pair of free nitrogen atoms. The energy released by this reaction gives the atoms enough of a kick to escape. Estimates based on Mars's nitrogen-loss rate indicate that dissociative recombination becomes negligible for a world with more than 12 percent of Earth's mass at a distance of 1 a.u. from a Sun-like star.
A potentially greater threat to a moon's atmosphere is sputtering. This process occurs when energetic charged particles hit the atmosphere and kick molecules into space. The gas giants in our solar system, and presumably others as well, have magnetospheres with radiation belts potent enough to completely erode the atmosphere of an orbiting Earth-like moon in only a few hundred million years.
One way to blunt this type of atmospheric loss is shielding by a moon's own strong magnetic field. Measurements by NASA's Galileo spacecraft hint that large moons might have magnetospheres of their own with the required strength. Galileo has detected a strong Earth-like magnetic field around Jupiter's moon Ganymede, which has a mass only 2.5 percent of Earth's.
Researchers once believed that small bodies like Jupiter's major moons could not possess such strong fields at all. But these moons orbit deep inside Jupiter's own powerful magnetosphere. According to models developed by Graeme Sarson (University of Exeter) and his colleagues, a strong ambient field helps initiate the circulation needed to produce a vigorous dynamo effect in the core of an even slightly active moon, leading to a strong field for the moon itself. Taken together, these observations and models hint that planet-size moons can maintain protective magnetic fields that they would not have in isolation.
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Maintaining Geologic Activity
For a moon to have an active dynamo producing a magnetic field, it must have a source of internal heat. But even more internal heat is required to drive another process that may be essential: the geologic activity needed to keep the carbonate-silicate weathering cycle going, which regulates the global atmospheric temperature on geologic timescales.
This important cycle maintains a roughly constant level of carbon dioxide in an atmosphere across many millions of years, due to a feedback effect between temperature, the weathering of rocks on continents, the deposition of weathered carbonates in ocean sediments, and the release of CO2 from buried sediments by volcanic activity. Without this self-regulating cycle, an otherwise habitable world will fall into a perpetual ice age, much as Mars is in today.
In solar system's terrestrial planets, the most important source of internal heat today is the decay of radioactive isotopes. This heat source decreases with time, however, and small bodies cool faster than large ones. As a result, large planets like Earth can support the carbonate-silicate cycle longer than small planets like Mars.
While the amount of internal heat required is still a subject of debate, it is estimated that a world's mass must be at least 25 percent that of Earth to maintain this cycle for 4.6 billion years, if radioactivity is the only heat source.
But large moons orbiting an extrasolar giant have an additional source of internal energy that our terrestrial planets lack: tidal heating. A spectacular example is Jupiter's moon Io. The constant flexing of Io's body as it is tugged between Jupiter and the planet's other moons generates enough heat to make it the most volcanically active body in the solar system.
Europa seems to experience a lesser degree of tidal heating, enough to maintain the ocean of liquid water that apparently exists beneath its icy shell.
Ganymede has a magnetic field and evidence of past geologic activity that both suggest this largest moon of Jupiter also underwent tidal heating as its orbit evolved through resonances with its siblings over the past few billion years.
The amount of tidal heating depends on a number of factors — such as the masses of the moon and its planet, the moon's internal structure, the size and eccentricity of its orbit, and the orbits of its near neighbors. Large moons of extrasolar giants probably experience some tidal heating that could help maintain good living conditions far longer than much larger terrestrial planets maintain in isolation.
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Length of Day
Another potential problem is the length of a moon's day. Computer models show that any large moon orbiting a giant planet or brown dwarf becomes locked into synchronous rotation — one side of the moon always facing the planet — within a few hundred million years. This, of course, has happened with Earth's own Moon and many other large moons in the solar system.
Assuming that large moons typically have orbital periods of 2 to 16 days, any potentially habitable moon would have a "day" several times longer than Earth's. Simple calculations by Stephen Dole of the Rand Corporation in the 1960s showed that a body with an Earth-like atmosphere would become uninhabitable when the period of rotation exceeds 4 days, due to large swings in surface temperature.
In reality the situation for slowly rotating moons is probably not so bleak. Monoj Joshi and Robert Haberle (NASA/Ames Research Center) and their colleagues have investigated the effects of synchronous rotation on the livability of planets closely orbiting red-dwarf stars. Such a planet would become tidally locked so that one hemisphere always faces its sun while the other experiences perpetual night. Joshi and Haberle's computer models have shown that an atmosphere with a carbon-dioxide pressure of only 1 to 1.5 bars (a bar is the atmospheric pressure on Earth) not only maintains habitable conditions on a synchronously rotating planet but even allows liquid water on the planet's perpetually dark side. A thick carbon-dioxide atmosphere (an infrared-trapping "greenhouse") retains heat better than a thin Earth-like (nitrogen-oxygen) atmosphere and also transfers this heat to the night side via global circulation.
The situation with a slowly rotating moon should be less extreme than for a synchronously rotating planet. While simulations with such a moon have yet to be performed, even modest additions of carbon dioxide to a moon's atmosphere (probably a natural consequence of the carbonate-silicate cycle) could keep it clement despite having a day as long as several weeks. Clouds and large bodies of water, which were not taken into account in the models, should further moderate temperature extremes.
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Problems of Eccentricity
A number of recently discovered extrasolar planets have mean orbital distances that lie within the habitable zones of their stars. For instance, companions of 47 Ursae Majoris and HD 29587 in Perseus, while near the outer limits of their systems' habitable zones, could possibly support water-bearing moons. Unfortunately, most exoplanets have eccentric orbits that would complicate the situation due to large swings in the amount of sunlight reaching them. The mean insolation of the planet orbiting 16 Cygni B, for example, is about half that of Earth — but ranges from 20 percent to 260 percent of the sunlight on Earth because of the planet's eccentric orbit. Living things would have a hard time being repeatedly deep-frozen and oven-roasted.
As with moons having long days, however, a dense carbon-dioxide atmosphere could lessen the extremes. Since the companion to 16 Cygni B spends most of its time in the outer portion of its system's habitable zone, any large moon it possesses could have the required dense atmosphere as a result of the carbonate-silicate weathering cycle. Other candidates in this category include the brown dwarfs orbiting HD 110833, BD +04°782, HD 18445, and HD 217580.
Detecting moons suitable for life will probably be more difficult than detecting terrestrial planets with similar sizes. None of the space-telescope systems that have been proposed to hunt for Earth-size planets around nearby stars will be able to separate the image of a moon from its primary. Photometric searches for planetary transits across the faces of stars, such as the planned Kepler mission, might have better luck. But the ever-changing positions of moons in relation to their primaries will require the observation of many transits to isolate a moon's signature. Given the difficulties, the unambiguous detection of any extrasolar moon is probably decades away.
Much theoretical work remains to be done to get a better grasp on how commonly giant planets ought to have giant moons. The distribution of volatiles such as water in a moon system, and how this is affected by the thermal history of the primary, will also have to be better understood.
Still, there may be hundreds of millions of more-or-less Earthlike moons in our galaxy. Given that large moons generally occur in groups among the gas giants in our solar system, habitable moons could also occur in sets of two or more per planet. It's anyone's guess what the implications may be for the abundance of life and the possible development of extraterrestrial intelligence.