Dying stars could serve as havens for life

In "Too Hard For Science?" I interview scientists about ideas they would love to explore that they don't think could be investigated. For instance, they might involve machines beyond the realm of possibility, such as devices as big as galaxies, or they might be completely unethical, such as experimenting on children like lab rats. This feature aims to look at the impossible dreams, the seemingly intractable problems in science. However, the question mark at the end of "Too Hard For Science?" suggests that nothing might be impossible.

The scientist: Eric Agol, associate professor of astronomy at the University of Washington.

The idea: Although scientists are looking for Earth-like worlds around stars like our sun, Agol suggests that an easier place to find planets that could host life might be dying stars known as white dwarfs.

White dwarfs make up about 10 percent of all stars in our galaxy, making them roughly as common as sunlike stars. Our sun, and more than 90 percent of all stars in the Milky Way, will one day end up as white dwarfs, which are made of their dim, cooling hearts. These fading stars are usually about 40 to 90 percent of the mass of our sun but only about the same volume as Earth.

This Hubble Space Telescope image shows the "last hurrah" of a star like our sun, the outer layers of gas being cast off and leaving behind the burned out white dwarf, the white dot in the center. Credit: NASA/European Space Agency

Although white dwarfs are cooling, Agol calculates they would still be warm enough to have so-called habitable zones — orbits where liquid water can exist on the surfaces of worlds in those areas. These zones are considered prime habitats for life, as there is life virtually everywhere liquid water exists on Earth. Since white dwarfs are relatively cold, planets would have to be very close to one to lie within its habitable zone, perhaps just 800,000 to 3.2 million kilometers away — just far enough way for the dying star's gravitational field to not rip these worlds apart. In comparison, even Mercury is at least 46 million kilometers away from our sun.

White dwarfs should be habitable for at least 3 billion years, Agol says. Earth-like planets around these stars might not only be potential sites for alien life, but they could also be potential havens for humanity if Earth for some reason became uninhabitable, he adds.

The problem: It remains uncertain whether planets could exist in the habitable zones of white dwarfs because of their violent pasts. Before a star dwindles to become a white dwarf, it usually swells to become a red giant, destroying any worlds as close as Earth before shedding its outer layers of gas to leave behind a pale ember of star. In principle, more distant planets could survive this metamorphosis and eventually migrate into the white dwarf's habitable zone, or new worlds could originate in the ring of debris left over by the star's transformation, but how likely either prospect is remains to be calculated, Agol says.

The solution? If these planets exist, detecting them should actually be easy, Agol suggests. Since white dwarfs are so tiny, if even an Earth-sized planet eclipsed one from the point of view of Earth, the resulting dimming would be significant. A habitable-zone Earth-sized planet passing in front of a white dwarf would lead to a dip in light of 50 percent — in comparison, Earth would only cause a 0.01 percent dip if it passed in front of the sun. The eclipses caused by an Earth-sized planet in a white dwarf's habitable zone be enough to detect using ground-based telescopes, instead of the more expensive space-based observatories employed to spot the dimming resulting from Earth-sized planets passing in front of sun-like stars.

Agol proposes that researchers survey the 20,000 white dwarfs within a distance of about 325 light years from here. (The nearest white dwarf to Earth is Sirius B, which is 8.5 light years distant.) A network of 20 telescopes each just 1-meter wide could survey all these in about 14 years, he calculates. A bigger telescope, such as the Large Synoptic Survey Telescope planned for operations later this decade in Chile, could examine about 10 million white dwarfs over the course of 10 years.

"The biggest hurdle is likely to be funding," Agol says. Still, "there has been a very positive response, and at least three people have expressed significant interest, including Wayne Rosing, founder of the Las Cumbres Observatory Global Telescope Network."


If you have a scientist you would like to recommend I question, or you are a scientist with an idea you think might be too hard for science, email me at toohardforscience@gmail.com.