Some Earth-like exoplanets that are seemingly habitable may be missing magnetic shielding, exposing them to damaging radiation, researchers in Colombia say. To support life as we know it, planets need thick, water-rich atmospheres and liquid surface water, Jorge Zuluaga at the University of Antioquia and colleagues said, but water can get blasted away by stellar winds unless the planet has a strong magnetic field. In our own solar system Mars and Venus do not have magnetic fields, and it is thought stellar winds stripped away the bulk of Mars's atmosphere while Venus was left with one of mostly carbon dioxide, making it toxic, they said. In most planets it is a churning molten core that generates a magnetic field, so the researchers calculated how long it would take a rocky, Earth-like planet to cool to the point that it's internal magnetic generator stopped working, NewScientist.com reported Wednesday. They applied their calculations three well-known exoplanets considered to be potentially habitable: Gliese 581d, HD 40307g and GJ 667Cc. The first two might have magnetic fields just barely strong enough to protect the planets from dangerous stellar radiation, they said, but the third is doomed. The presence or absence of magnetic fields needs to be taken into account when considering whether a planet might be capable of supporting life, Zuluaga said.