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Debris of Earth-like planets found to float around dead stars

March 26, 2014
Courtesy of the University of Leicester
and World Science staff

Sci­en­tists say they have “solved” a dec­ades-old space mys­tery in a way that om­i­nously points to the ul­ti­mate fate of Earth, but also sug­gests rocky plan­ets like ours are common.

Sci­en­tists from the Uni­vers­ity of Leices­ter in the U.K. and Uni­vers­ity of Ar­i­zo­na stud­ied hot, young, white dwarfs—super-compact re­mains of Sun-like stars that ran out of fu­el and shrank to about the size of the Earth. The re­search ap­pears in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

Artist's impression of de­bris around a white dwarf. (Cred­it: NA­SA, ESA, STScI, G. Ba­con (STScI))


As­tro­no­mers have long known that many hot white dwarfs’ at­mo­spheres, es­sen­tially of pure hy­dro­gen or pure he­li­um, are “con­tam­i­nat­ed” by oth­er el­e­ments like car­bon, sil­i­con and iron. What was­n’t known was the ori­gins of these el­e­ments.

Their pre­cise or­i­gin “has re­mained a mys­tery and ex­treme dif­fer­ences in their abun­dance be­tween stars could not be ex­plained,” said Mar­tin Bar­stow, a pro-vice-chancellor at the Uni­vers­ity of Leices­ter, who led the re­search.

“It was be­lieved that this ma­te­ri­al was 'le­vi­tat­ed' by the in­tense radia­t­ion from deeper lay­ers in the star,” he added, but it now turns out that many such stars show signs of con­tamina­t­ion by rocky ma­te­ri­al—left­overs from a plan­etary sys­tem.

The re­search­ers sur­veyed 89 white dwarfs, us­ing a tel­e­scope known as the Far Ul­tra­vi­o­let Spec­tro­scop­ic Ex­plor­er to ob­tain their spec­tra, or dis­tri­bu­tion of col­ors. In these spec­tra the “fin­ger­prints” of car­bon, sil­i­con, phos­pho­rous and sul­fur are vis­i­ble.

“We found that in stars with pol­lut­ed at­mo­spheres the ra­tio of sil­i­con to car­bon matched that seen in rocky ma­te­ri­al, much high­er than found in stars or in­ter­stel­lar gas,” Bar­stow said.

“The new work in­di­cates that at around a one-third of all hot white dwarfs are con­tam­i­nated in this way, with the de­bris most likely in the form of rocky mi­nor plan­et ana­logues,” he added. This im­plies, he said, that a about a third of stars like our Sun, as well as stars that are a lit­tle heav­i­er like Ve­ga and For­mal­haut, build sys­tems con­tain­ing plan­ets that are rocky, like Earth. 

“This work is a form of ce­les­tial ar­chae­o­lo­gy where we are stu­dy­ing the ‘ru­ins’ of rocky plan­ets and/or their build­ing blocks, fol­low­ing the de­mise of the main star,” he said. The stars “are swal­low­ing up the left­overs from plan­etary sys­tems, per­haps like our own,” he went on, and per­haps “follow-up work will be able to tell us about the com­po­si­tion of rocky plan­ets or­bit­ing oth­er stars.”

The study al­so points to the ul­ti­mate fate of the Earth bil­lions of years from now, he said—ending up as a con­tamina­t­ion with­in the white dwarf Sun.


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Scientists say they have “solved” a decades-old space mystery in a way that ominously points to the ultimate fate of our own planet. Scientists from the University of Leicester in the U.K. and University of Arizona studied hot, young, white dwarfs—super-compact remains of Sun-like stars that ran out of fuel and shrank to about the size of the Earth. The research appears in the journal Monthly Notices of the Royal Astronomical Society. Astronomers have long known that many hot white dwarfs atmospheres, essentially of pure hydrogen or pure helium, are “contaminated” by other elements like carbon, silicon and iron. What wasn’t known was the origins of these elements. Their precise origin “has remained a mystery and extreme differences in their abundance between stars could not be explained,” said Martin Barstow, a pro-vice-chancellor at the University of Leicester, who led the research. “It was believed that this material was “levitated” by the intense radiation from deeper layers in the star,” he added, but it now turns out that many such stars show signs of contamination by rocky material—leftovers from a planetary system. The researchers surveyed 89 white dwarfs, using a telescope known as the Far Ultraviolet Spectroscopic Explorer to obtain their spectra, or distribution of colors. In these spectra the “fingerprints” of carbon, silicon, phosphorous and sulphur are visible. “We found that in stars with polluted atmospheres the ratio of silicon to carbon matched that seen in rocky material, much higher than found in stars or interstellar gas,” Barstow said. “The new work indicates that at around a one-third of all hot white dwarfs are contaminated in this way, with the debris most likely in the form of rocky minor planet analogues,” he added. This implies, he said, that a about a third of stars like our Sun, as well as stars that are a little heavier like Vega and Fomalhaut, build systems containing planets that are rocky, like Earth. “This work is a form of celestial archaeology where we are studying the ‘ruins’ of rocky planets and/or their building blocks, following the demise of the main star,” he said. The stars “are swallowing up the leftovers from planetary systems, perhaps like our own,” he went on, and perhaps “follow-up work will be able to tell us about the composition of rocky planets orbiting other stars.” The study also points to the ultimate fate of the Earth billions of years from now, he said—ending up as a contamination within the white dwarf Sun.