Dying stars may have planets with life, U.S. astronomers say, and it might even be easier to detect such planets than around a sun-like star. Scientists at the Harvard-Smithsonian Center for Astrophysics say a new theoretical study of Earth-like planets orbiting white dwarf stars suggests we could detect oxygen in the atmosphere of a white dwarf's planet much more easily than for an Earth-like planet orbiting a sun-like star. "In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs," theorist Avi Loeb said. White dwarfs are stars that start out like the sun but puff off their outer layers as they die, leaving a hot white core about the size of the Earth. Although it slowly cools and fades over time, a white dwarf can retain heat long enough to warm a nearby world for billions of years, the researchers said. The best method for finding planets orbiting distant stars is a transit search -- looking for periodic dimming of a star's light as an orbiting planet crosses in front of it, and since a white dwarf is about the same size as Earth, an Earth-sized planet would block a large fraction of its light and create an obvious signal, they said. Also, when the white dwarf's light shines through the ring of atmosphere that surrounds the transiting planet's silhouetted disk, the atmosphere absorbs some starlight, leaving chemical fingerprints showing whether that air contains water vapor, or even signatures of life such as oxygen.