White dwarfs might be fertile ground for other Earths

"Long before it's in the papers"
April 11, 2011

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White dwarfs might be fertile ground for other Earths

March 30, 2011
Courtesy of the University of Washington
and World Science staff

An artist's conception of the evolution of our Sun (left) through the red giant stage (center) and onto a white dwarf (right). (Credit: NASA). On the homepage, an image of the white dwarf Sirius B, located 8.6 light years from Earth. (Courtesy NASA, ESA, H. Bond (STScI) & M. Barstow (U. Leicester))

In a new paper published online Tuesday in the research journal Astrophysical Journal Letters, University of Washington astronomer Eric Agol suggests that potentially habitable planets orbiting white dwarfs could be much easier to find – if they exist – than other planets located outside our solar system so far.

White dwarfs, cooling stars believed to be in the final stage of life, typically weigh as much as about 60 percent the sun, but are only about the size of Earth. Though born hot, they eventually become cooler than the sun and emit just a fraction of its energy. So the zone around a white dwarf with warmth suitable for life would be much closer than Earth is to the sun.

“If a planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface – if it has water at all – and that’s a big factor for habitability,” Agol said. A planet so close to its star could be observed using an Earth-based telescope as small as one meter (yard) across, as the planet passes in front of, and dims the light from, the white dwarf, he said.

White dwarfs evolve from stars like the sun. When such a star’s core can no longer produce nuclear reactions that convert hydrogen to helium, it starts burning hydrogen outside the core. That begins the transformation to a red giant, with a greatly expanded outer atmosphere that typically envelops – and destroys – any planets as close as Earth.

Finally the star sheds its outer atmosphere, leaving the glowing, gradually cooling, core as a white dwarf, with a surface temperature around 5,000 degrees Celsius (about 9,000 degrees Fahrenheit). At that point, the star produces heat and light in the same way as a dying fireplace ember, though the star’s ember could last for three billion years.

Once the red giant sheds its outer atmosphere, more distant planets that were beyond the reach of that atmosphere could begin to migrate closer to the white dwarf, Agol said. New planets also might form from a ring of debris left behind by the star’s transformation. Either way, a planet would have to move very close to the white dwarf to be habitable, perhaps 500,000 to 2 million miles from the star. That’s less than one percent of the distance from Earth to the sun (93 million miles) and substantially closer than Mercury is to the sun.

“From the planet, the star would appear slightly larger than our sun, because it is so close, and slightly more orange, but it would look very, very similar to our sun,” Agol said. The planet also would be tidally locked, so the same side would always face the star and the opposite side would always be in darkness. The likely areas for habitation, he said, might be toward the edges of the light zone, nearer the dark side of the planet.

The nearest white dwarf to Earth is Sirius B at a distance of about 8.5 light years (a light year is about 6 trillion miles). It is believed to once have been five times more massive than the sun, but now it has about the same mass as the sun packed into the same volume as Earth.

Agol is proposing a survey of the 20,000 white dwarfs closest to Earth. Using a one-meter ground telescope, he said, one star could be surveyed in 32 hours of observation. If there is no telltale dimming of light from the star in that time, it means no planet orbiting closely enough to be habitable is passing in front of the star so that it is easily observable from Earth. Ideally, the work could be carried out by a network of telescopes that would make successive observations of a white dwarf as it progresses through the sky.

“This could take a huge amount of time, even with such a network,” he said.

The same job could be done, he added, by larger specialty telescopes such as the Large Synoptic Survey Telescope that is planned for operations later this decade in Chile, of which the University of Washington is a founding partner. If the number of white dwarfs with potential Earthlike planets turns out to be very small – say one in 1,000 – that telescope still would be able to track them down efficiently, Agol argued.

Finding an Earthlike planet around a white dwarf could provide a meaningful place to look for life, Agol said. But it also would be a potential lifeboat for humanity if Earth, for some reason, becomes uninhabitable. “Those are the reasons I find this project interesting,” he said. “And there’s also the question of, ‘Just how special is Earth?’“

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Planet hunters have found hundreds of planets outside the solar system in the last decade, though it’s unclear whether even one might be habitable. But it may be that the best place to look for habitable planets is around dim, dying stars called white dwarfs, a scientist argues. In a new paper published online Tuesday in The Astrophysical Journal Letters, Eric Agol, a University of Washington astronomy, suggests that potentially habitable planets orbiting white dwarfs could be much easier to find – if they exist – than other planets located outside our solar system so far. White dwarfs, cooling stars believed to be in the final stage of life, typically weigh as much as about 60 percent the sun, but are only about the size of Earth. Though born hot, they eventually become cooler than the sun and emit just a fraction of its energy. So the zone around a white dwarf with warmth suitable for life would be much closer than Earth is to the sun. “If a planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface – if it has water at all – and that’s a big factor for habitability,” Agol said. A planet so close to its star could be observed using an Earth-based telescope as small as one meter (yard) across, as the planet passes in front of, and dims the light from, the white dwarf, he said. White dwarfs evolve from stars like the sun. When such a star’s core can no longer produce nuclear reactions that convert hydrogen to helium, it starts burning hydrogen outside the core. That begins the transformation to a red giant, with a greatly expanded outer atmosphere that typically envelops – and destroys – any planets as close as Earth. Finally the star sheds its outer atmosphere, leaving the glowing, gradually cooling, core as a white dwarf, with a surface temperature around 5,000 degrees Celsius (about 9,000 degrees Fahrenheit). At that point, the star produces heat and light in the same way as a dying fireplace ember, though the star’s ember could last for three billion years. Once the red giant sheds its outer atmosphere, more distant planets that were beyond the reach of that atmosphere could begin to migrate closer to the white dwarf, Agol said. New planets also might form from a ring of debris left behind by the star’s transformation. Either way, a planet would have to move very close to the white dwarf to be habitable, perhaps 500,000 to 2 million miles from the star. That’s less than one percent of the distance from Earth to the sun (93 million miles) and substantially closer than Mercury is to the sun. “From the planet, the star would appear slightly larger than our sun, because it is so close, and slightly more orange, but it would look very, very similar to our sun,” Agol said. The planet also would be tidally locked, so the same side would always face the star and the opposite side would always be in darkness. The likely areas for habitation, he said, might be toward the edges of the light zone, nearer the dark side of the planet. The nearest white dwarf to Earth is Sirius B at a distance of about 8.5 light years (a light year is about 6 trillion miles). It is believed to once have been five times more massive than the sun, but now it has about the same mass as the sun packed into the same volume as Earth. Agol is proposing a survey of the 20,000 white dwarfs closest to Earth. Using a one-meter ground telescope, he said, one star could be surveyed in 32 hours of observation. If there is no telltale dimming of light from the star in that time, it means no planet orbiting closely enough to be habitable is passing in front of the star so that it is easily observable from Earth. Ideally, the work could be carried out by a network of telescopes that would make successive observations of a white dwarf as it progresses through the sky. “This could take a huge amount of time, even with such a network,” he said. The same job could be done, he added, by larger specialty telescopes such as the Large Synoptic Survey Telescope that is planned for operations later this decade in Chile, of which the University of Washington is a founding partner. If the number of white dwarfs with potential Earthlike planets turns out to be very small – say one in 1,000 – that telescope still would be able to track them down efficiently, Agol argued. Finding an Earthlike planet around a white dwarf could provide a meaningful place to look for life, Agol said. But it also would be a potential lifeboat for humanity if Earth, for some reason, becomes uninhabitable. “Those are the reasons I find this project interesting,” he said. “And there’s also the question of, ‘Just how special is Earth?’“