Planetary systems dominated by huge ‘hot Jupiters’ — the kind of systems we’ve found so far — are unlikely to contain Earth-like worlds. Massive gas giants close to their star would probably disrupt stable planetary orbits further out, in the habitable zone. But systems with large planets in the 5 AU range, where our Solar System’s Jupiter resides, may well have small, rocky inner worlds. And Greg Laughlin, an assistant professor of astronomy and astrophysics at UC Santa Cruz, thinks we’ll soon be discovering plenty of systems like these.

Laughlin is interested in what he calls ‘metallicity,’ which he believes is the determining factor in whether or not a system will have gas giants. In fact, the vast majority of planets detected around other stars, at least so far, have circled stars that are metal-rich. The planetary cores have accreted from hydrogen and helium, to be sure, but also from heavier elements that allow the cores to form fairly quickly, within a period of a few million years, attracting more hydrogen and helium from the gas cloud out of which the planet has formed. Clouds with low metallicity grow such cores more slowly, and don’t attract the dense atmospheres of gas giants.

According to an article in Astrobiology Magazine called “Extrasolar Planets: A Matter of Metallicity,” the majority of stars extrasolar planet hunters are now studying have roughly half the metallicity of the Sun, enough to form large, rocky planets like Neptune, but probably not enough to produce many Jupiters. These are systems that may well pay off in our search for planets that could sustain life.

From the article:

So what are the prospects of finding solar systems that contain Earth-like planets? Pretty good, according to Laughlin… [A] solar system with a large planet in a circular orbit at 5 AU – even a Neptune-sized planet – is a solar system in which a habitable Earth-like planet could exist quite comfortably.

Indeed, Laughlin believes that, when all the data are in, we’ll have discovered hundreds of nearby stars with solar systems much like our own, although the majority of them will have a Neptune or a Saturn at 5 AU rather than a Jupiter. True, planet hunters haven’t found any such planets yet. But that doesn’t mean they’re not there. Astronomers just haven’t been looking long enough to confirm their presence. With current planet-hunting techniques, Laughlin says, “it’s not like you discover a planet – boom!” – in a single observation. “The planets emerge gradually,” as a result of many, many observations over time.

In 2002, Laughlin worked with Geoff Marcy and Paul Butler on the planetary system around 55 Cancri, determining on the basis of their data that an Earth-sized planet could exist between the two gas giants found there. Here’s a link to a NASA piece on that subject.