Living alongside a gas giant planet is likely rough on moons and could inhibit the emergence of life, say the researchers. If the moon is too close to the planet, tidal forces can make it so hot that the surface is always erupting volcanically. Jupiter's innermost major satellite Io is an example of this.
A water-laden moon with a thick atmosphere might suffer from a runaway greenhouse effect. The water vapor, combined with other greenhouse gasses such as methane and carbon dioxide might make the moon too warm, as is the case of Venus. The closer the moon is to its gas giant planet, the more radiation it will absorb, simply from starlight reflected off the planet.
But if the moons are farther out they are not protected from galactic cosmic rays by the planet's magnetic field, and are too small to generate a substantial field of their own. This radiation might sterilize the surface of life.
By analogy with the circumstellar habitable zone, these constraints define a circum-planetary "habitable edge," say the researchers. They conclude that if either exoplanet hosted a moon at a distance greater than 10 planetary radii, then it could be habitable.