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White dwarfs could be key to the discovery of extraterrestrial life

A new theoretical study of Earth-like planets orbiting white dwarf stars suggests that even dying stars could host exoplanets with life. If such life exists, scientists contend that we might be able to detect it within the next decade. Researchers discovered that they could detect oxygen in the atmosphere of a white dwarf’s planet much more easily than for an Earth-like planet circling a Sun-like star.

According to NASA’s Goddard Space Flight Center, a white dwarf is what stars like the Sun became after they have exhausted their nuclear fuel. A typical white dwarf is half as massive as the Sun, yet only slightly larger than Earth.

“In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs,” said Avi Loeb, theorist at the Harvard-Smithsonian Center for Astrophysics and director of the Institute for Theory and Computation, in a statement.

Due to the fact that a white dwarf is tinier and fainter than the Sun, a planet would have to be a lot closer in to be habitable with liquid water on its surface. The scientists note that a habitable planet would orbit the white dwarf once every 10 hours at a distance of about a million miles.

Scientists recently redefined the boundaries of the “habitable zone” for alien planets. In fact, the new definition places the Earth extremely close to the inner edge of the habitable zone.

The scientists also contend that a planet would have to arrive in the habitable zone after the star turned into a white dwarf because before a star becomes a white dwarf it swells into a red giant, destroying any nearby planets.

Loeb and his colleague Dan Maoz from Tel Aviv University think that a survey of the 500 closest white dwarfs could spot one or more habitable planets. According to scientists, the best way to accomplish such a survey would be to conduct a transit search. This would be relatively simple, as a white dwarf is roughly the same size as Earth and a Earth-like planet would obscure a significant amount of its light as it transits the star.

The scientists will also be able to determine more about the atmospheres of the transiting planets by looking at the chemical fingerprints left by the white dwarf’s light as it shines through the ring of air that encircles the planet’s silhouetted disk and is absorbed by the atmosphere. These chemical fingerprints reveal whether the air holds water vapor or oxygen. The presence of significant quantities of oxygen in the atmosphere of an exoplanet would signal the likely presence of life there.

According to Loeb and Maoz, NASA’s James Webb Space Telescope would be able to detect both oxygen and water vapor in the atmosphere of an alien world with only a few hours of total observation time. They came to this conclusion after using a synthetic spectrum to replicate what JWST would spot if it looked at a habitable planet circling a white dwarf.

“JWST offers the best hope of finding an inhabited planet in the near future,” said Maoz.

Their findings have significant implications for astronomers searching for habitable planets.

“Although the closest habitable planet might orbit a red dwarf star, the closest one we can easily prove to be life-bearing might orbit a white dwarf,” said Loeb.