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New evidence cited that rocky, watery planets are common

April 13, 2010
Courtesy of the Royal Astronomical Society
and World Science staff

The va­por­ized rem­nants of rocky, and pos­sibly wa­tery, bod­ies hang around many dead stars—sug­gest­ing plan­ets like Earth are com­mon in our gal­axy, re­search­ers say.

Al­though sci­en­tists have de­tected up­wards of 400 plan­ets out­side our so­lar sys­tem, it’s un­clear how many of these may be made up si­m­i­larly to ours, an im­por­tant fac­tor to weigh in con­sid­er­ing which ones might sup­port life.

An artist’s im­pres­sion of a mas­sive as­ter­oid belt in or­bit around a star. (Cred­it: NA­SA-JPL / Cal­tech / T. Pyle (SSC))


Jay Far­ihi of Leices­ter Uni­vers­ity in the U.K. sur­veyed white dwarfs, the com­pact rem­nants of stars once like our Sun, and found that many show signs of con­tamina­t­ion by heav­i­er el­e­ments and pos­sibly even wa­ter. 

That im­proves the prospects for ex­tra­ter­res­tri­al life, said Far­ihi, who pre­s­ents his find­ings April 13 at the U.K. Roy­al As­tro­nom­i­cal So­ci­ety’s Na­tional As­tron­o­my Meet­ing in Glas­gow, Scot­land.

More than nine in 10 stars in our Milky Way gal­axy, in­clud­ing our own, are ex­pected to end up as white dwarfs, or have done so al­ready. White dwarfs should the­o­ret­ic­ally have es­sen­tially pure hy­dro­gen or pure he­li­um at­mo­spheres. If heav­i­er el­e­ments are pre­s­ent, com­monly in­clud­ing such sub­stances as cal­ci­um, mag­ne­si­um and iron, then these must be ex­ter­nal pol­lu­tants, Far­ihi ex­plained.

For dec­ades, it was be­lieved that the thin gas be­tween the stars was the source of these pol­lu­tants.

Far­ihi and his team used da­ta from the Sloan Dig­it­al Sky Sur­vey, a proj­ect that aims to sur­vey the sky in in­fra­red light, im­ag­ing more than 100 mil­lion ob­jects. The team fol­lowed up on a mil­lion of these by ob­tain­ing their spec­tra, the ex­act break­down of their col­ors.

By ex­am­in­ing the po­si­tions, mo­tions and spec­tra of the white dwarfs, Far­ihi and his team con­clud­ed that the pol­lu­tants can’t come from the in­ter­stel­lar gas. In­stead, rocky plan­e­tary de­bris is al­most cer­tainly the main cul­prit, they said. Their re­search in­di­cates that be­tween 3 and 20 per­cent of white dwarfs are con­tam­i­nated in this way.

This im­plies that a si­m­i­lar pro­por­tion of stars like our Sun, as well as some­what big­ger stars, build ter­res­tri­al plan­e­tary sys­tems, Far­ihi main­tains.

Most of the con­taminants in ques­tion probably came from as­ter­oids rath­er than full-fledged plan­ets, Far­ihi not­ed. How­ev­er, he added, the pres­ence of such bod­ies means a so­lar sys­tem has the ma­te­ri­al to form Earth-like plan­ets—particularly if wa­ter is pre­s­ent.

The sci­en­tists gauged the com­po­si­tion of the con­taminants through their chem­i­cal sig­na­ture which stands out in the oth­er­wise pure white dwarf at­mo­spheres. It seems a sig­nif­i­cant frac­tion of these stars are pol­lut­ed with ma­te­ri­al that con­tained wa­ter, with im­por­tant im­plica­t­ions for the fre­quen­cy of hab­it­a­ble plan­ets around oth­er stars, ac­cord­ing to Far­ihi’s group. 

“In our own So­lar Sys­tem with at least one wa­tery, hab­it­a­ble plan­et, the as­ter­oid belt—the left­o­ver build­ing blocks of the ter­res­tri­al plan­ets—is sev­er­al per­cent wa­ter by mass,” Far­ihi ex­plained. “From our study of white dwarfs, it ap­pears there are bas­ic si­m­i­lar­i­ties found among as­ter­oid-like ob­jects around oth­er stars; hence it is likely a frac­tion of these white dwarfs once har­bored wa­tery plan­ets, and pos­sibly life.”


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The vaporized remnants of rocky, and possibly watery, bodies hang around many dead stars—suggesting planets like Earth are common in our galaxy, researchers say. Although scientists have detected upwards of 400 planets outside our solar system, it’s unclear how many of these may be made up similarly to ours, an important factor to weigh in considering which ones might support life. Jay Farihi of Leicester University in the U.K. surveyed white dwarfs, the compact remnants of stars once like our Sun, and found that many show signs of contamination by heavier elements and possibly even water. That improves the prospects for extraterrestrial life, said Farihi, who presents his findings April 13 at the Royal Astronomical Society’s National Astronomy Meeting in Glasgow, Scotland. More than nine in 10 stars in our Milky Way galaxy, including our own, are expected to end up as white dwarfs, or have done so already. White dwarfs should theoretically have essentially pure hydrogen or pure helium atmospheres. If heavier elements are present, commonly including such substances as calcium, magnesium and iron, then these must be external pollutants, Farihi explained. For decades, it was believed that the thin gas between the stars was the source of these pollutants. Farihi and his team used data from the Sloan Digital Sky Survey, a project that aims to survey the sky in infrared light, imaging more than 100 million objects. The team followed up on a million of these by obtaining their spectra, the exact breakdown of their colors. By examining the positions, motions and spectra of the white dwarfs, Farihi and his team concluded that the pollutants can’t come from the interstellar gas. Instead, rocky planetary debris is almost certainly the main culprit, they said. Their research indicates that between 3 and 20 percent of white dwarfs are contaminated in this way. This implies that a similar proportion of stars like our Sun, as well as somewhat bigger stars, build terrestrial planetary systems, Farihi maintains. Most of the contaminants in question probably came from asteroids rather than full-fledged planets, Farihi noted. However, he added, the presence of such bodies means a solar system has the material to form Earth-like planets—particularly if water is present. The scientists gauged the composition of the contaminants through their chemical signature which stands out in the otherwise pure white dwarf atmospheres. It seems a significant fraction of these stars are polluted with material that contained water, with important implications for the frequency of habitable planets around other stars, according to Farihi’s group. “In our own Solar System with at least one watery, habitable planet, the asteroid belt—the leftover building blocks of the terrestrial planets—is several percent water by mass,” Farihi explained. “From our study of white dwarfs, it appears there are basic similarities found among asteroid-like objects around other stars; hence it is likely a fraction of these white dwarfs once harbored watery planets, and possibly life.”