E.T. Signatures

The Mysteries of a Dying Star

Researchers found that 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.

Even dying stars could host planets with life, and if such life exists, we might be able to detect it within the next decade. This encouraging result comes from a new theoretical study of Earth-like planets orbiting white dwarf stars. Researchers found that 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," said Avi Loeb from the Harvard-Smithsonian Center for Astrophysics in Cambridge. When a star like the Sun dies, it puffs off its outer layers, leaving behind a hot core called a white dwarf. A typical white dwarf is about the size of Earth. It slowly cools and fades over time, but it can retain heat long enough to warm a nearby world for billions of years.

Since a white dwarf is much smaller and fainter than the Sun, a planet would have to be much closer in to be habitable with liquid water on its surface. A habitable planet would circle the white dwarf once every 10 hours at a distance of about a million miles. If planets exist in the habitable zones of white dwarfs, astronomers would need to find them before they could study them. The abundance of heavy elements on the surface of white dwarfs suggests that a significant fraction of them have rocky planets. Loeb and his colleague Dan Maoz from Tel Aviv University in Israel estimate that a survey of the 500 closest white dwarfs could spot one or more habitable Earths. The best method for finding such planets is a transit search, looking for a star that dims as an orbiting planet crosses in front of it.

 

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.

 

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