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.

This is a partial chart; click for complete chart. The orbits of some extrasolar planets and brown dwarfs are shown in relation to their stars' habitable zones. A light horizontal line shows how close and far a body ranges from its star; eccentric orbits cause large ranges. The large dot marks the mean distance from the star. The orbits of Mercury, Venus, Earth, and Mars are shown for comparison. Bodies that do not stray too far into the red zone (Runaway Greenhouse) or blue zone (Icebox) are prime candidates for harboring habitable moons.

Sky & Telescope diagram.

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.

The Future

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.

Moons of extrasolar planets won't be spotted any time soon. Even NASA's proposed Terrestrial Planet Finder, which will look for Earth-like planets orbiting nearby stars, won't have the resolution needed to see satellite bodies.

Courtesy NASA.

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.

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