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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

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.

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?’“