February 25, 2013
Dying White Dwarfs Could Host Exoplanets With Life
Lee Rannals for redOrbit.com - Your Universe Online
Scientists are on the hunt to find habitable exoplanets, and a new discovery might mean there are even more candidates out there than previously thought.A new theoretical study suggests even dying stars might be able to have orbiting exoplanets that are hospitable to extraterrestrial life. As a star dies, its outer layers puff off, leaving behind a hot core known as a white dwarf which is typically about the size of Earth. Although white dwarfs slowly cool and fade over time, they can retain heat long enough to warm a nearby planet for a billion years.
Since white dwarfs are smaller and fainter than the Sun, a life-hosting planet would need to be closer to its host star in order to maintain liquid water, an essential ingredient for life as we know it. A planet circling a white dwarf every 10 hours, at a distance of about a million miles, would be able to host life.
"In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs," said Avi Loeb, a researcher at the Harvard-Smithsonian Center for Astrophysics (CfA) and director of the Institute for Theory and Computation.
In order to become a white dwarf, a star first swells into a red giant and engulfs any nearby planets in its path. Thus, in order for a planet to be circling a white dwarf, it would have had to evolve after a red giant has subsided into a white dwarf. The researchers suggest such a planet could potentially form from leftover dust and gas, or by migrating inward from further away in the star´s solar system.
The astronomers wrote in the Monthly Notices of the Royal Astronomical Society they believe the 500 closest white dwarfs could contain one or more habitable exoplanets. To find these potential planets, astronomers say they must first perform a transit search, which is the process of hunting for planets by looking for a star that periodically dims as an orbiting planet crosses in front of it.
Because white dwarfs are about the same size as Earth, an Earth-sized planet would block a large fraction of its light and create an obvious signal. When a white dwarf's light shines through the ring of air around the planet, the atmosphere absorbs starlight, leaving a chemical fingerprint that shows whether that air contains water vapor or oxygen.
NASA is currently developing its James Webb Space Telescope which researchers expect will help in the search for these dwarf-orbiting exoplanets. Scientists from the project replicated what NASA's James Webb Space Telescope (JWST) will be able to see once it becomes operational, and they said it would only take a few hours to detect oxygen and water vapor.
"JWST offers the best hope of finding an inhabited planet in the near future," said Dan Maoz Tel Aviv University.