Making a habitable moon
The critical minimum location, Barnes said, is about half the distance that Earth is from the Sun. So, in theory, a big planet orbiting a star at roughly one Earth-distance (a nice comfortable region of space, radiation-wise) could have a moon zipping around it. In fact, Barnes said, a moon the size of Earth is plausible, one big enough to retain a nifty atmosphere. But, he notes, there is no proof that moons grow that large.
Barnes' model is based on Sun-like stars. Stars with less mass are more common, however.
In other research, scientists have figured that a planet (either an Earth or a Jupiter) could maintain a stable orbit very close to a cooler, low-mass star, close enough to be in a warm, habitable zone. But physics dictates that such a planet would always show one face to the star -- just as our Moon shows one face to Earth. One side of the satellite would be overheated, the other frozen.
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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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There's a possible loophole, however, to this problem of extremes.
In the same sort of location very near a low-mass star, an Earth-sized moon orbiting a giant planet would benefit from day-night cycles, Barnes said. The moon would remain in a stable orbit for about 4.5 billion years. That's just about the age of Earth, and it's just about how long it took life here to evolve into something smart enough to begin looking for distant moons.
Moon hunting
Before any large-scale searches for other moons begin, the quest for terrestrial planets will be well underway.
While all known extrasolar planets are huge, astronomers fully expect to find Earth-sized planets over the next decade as telescopes become more advanced. Already, planet hunters are finding worlds whose orbits look like those of Jupiter and Saturn. Theorist Gregory Laughlin of the University of California, Santa Cruz, says these extrasolar systems, from an orbital mechanics point of view, could contain Earth-sized planets in stable, Earth-like orbits.
NASA's Kepler mission, set for launch in 2007, will seek to create a census of somewhat distant terrestrial places in one region of the sky. In subsequent missions, finer instruments will be used to photograph similar rocky, Earth-sized planets closer to home and probe their atmospheres for signs of biological activity.
Meanwhile, a handful of other astronomers will be searching for otherworldly moons. The task will not be simple, but it is possible with present technology.
"It will be difficult but cannot be ruled out," said Boss, the Carnegie Institution theorist.
One way to spot an extrasolar moon is to examine the light coming from a star that harbors a planet known to cross in front of the star, as seen from our point of view. The planet will cause a tiny dip in starlight, and the moon will generate a separate dip, even smaller.
Another technique is to watch a transiting planet's motion across the star. If there is a moon, the planet's orbital timing will change as it is tugged to-and-fro by its satellite.
The techniques have been tried already on HD 209458, the only star presently known to have a planet that transits in front of it. Researchers saw no evidence for moons, but they did make the
of an extrasolar planet's atmosphere, using the Hubble Space Telescope and illustrating that astronomy has a habit of moving forward with surprising leaps into the unknown.
Caltech's David Charbonneau, who led the investigation into HD 209458, said Hubble could have found an Earth-sized moon around the planet, had it been there. Smaller moons would have eluded detection, however.
If it were to occur, the discovery of large, rocky satellite orbiting in the shadow of a gas giant planet around a Sun-like star would be of obvious great interest to astrobiologists, especially if it were at an Earth-distance from the star.
"That would be a very interesting place to look for life," Charbonneau said.
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