"Long before it's in the papers"
August 21, 2012

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Scientists see evidence of planet destroyed by parent star

Aug. 21, 2012
Courtesy of Penn State University
and World Science staff

As­tro­no­mers have re­ported the first ev­i­dence of a plan­et’s de­struc­tion by its own, ag­ing host star.

Ev­i­dence in­di­cates a plan­et was de­voured as the star be­gan ex­pand­ing in­to a “red gi­ant” — the stel­lar equiv­a­lent of ad­vanced age, sci­en­tists say. 

Artist's impression of a red supergiant engulfing a Jupiter-like planet as it expands. (Credit: NASA)
“A si­m­i­lar fate may await the in­ner plan­ets in our so­lar sys­tem, when the Sun be­comes a red gi­ant and ex­pands all the way out to Earth’s or­bit some five bil­lion years from now,” said Al­ex Wol­szczan of Penn State Uni­vers­ity, one of the mem­bers of the re­search team, who is al­so cred­ited as the dis­cov­er­er of the first plan­et found out­side our so­lar sys­tem.

The as­tro­no­mers al­so iden­ti­fied a mas­sive plan­et in a sur­pris­ingly el­lip­ti­cal or “stretched” or­bit around the same red-gi­ant star, named BD+48 740, which is old­er than the Sun and about 11 times wid­er. 

Wol­szczan and col­leagues iden­ti­fied ev­i­dence of the plan­etary de­mise while us­ing the Hobby-Eberly Tel­e­scope on Mt. Locke, Texas to study the ag­ing star and to search for plan­ets around it. The ev­i­dence in­cludes the star’s pe­cu­liar chem­i­cal com­po­si­tion, plus the highly un­usu­al el­lip­ti­cal or­bit of its sur­viv­ing plan­et, they said.

“Our de­tailed spec­tro­scop­ic [col­or] anal­y­sis re­veals that this red-gi­ant star, BD+48 740, con­tains an ab­nor­mally high amount of lith­i­um, a rare el­e­ment cre­at­ed pri­marily dur­ing the Big Bang 14 bil­lion years ago,” said re­search team mem­ber Monika Adamow of Nic­o­la­us Co­per­ni­cus Uni­vers­ity in To­run, Po­land.

Lith­i­um is easily de­stroyed in stars, which is why its ab­nor­mally high abun­dance in this old­er star is so un­usu­al, they said. “The­o­rists have iden­ti­fied only a few, very spe­cif­ic cir­cum­stances, oth­er than the Big Bang, un­der which lith­i­um can be cre­at­ed in stars,” Wol­szczan added. “In the case of BD+48 740, it is prob­a­ble that the lith­i­um pro­duc­tion was trig­gered by a mass the size of a plan­et that spi­raled in­to the star and heat­ed it up while the star was di­gest­ing it.”

The sec­ond piece of ev­i­dence, the as­tro­no­mers said, is the elon­gat­ed-looking or­bit of the star’s newly dis­cov­ered mas­sive plan­et, which is at least 1.6 times as heavy as Ju­pi­ter. “We dis­cov­ered that this plan­et re­volves around the star in an or­bit that is only slightly wid­er than that of Mars at its nar­row­est point, but is much more ex­tend­ed at its far­thest point,” said co-re­searcher An­drzej Nied­ziel­ski of Co­per­ni­cus Uni­vers­ity. “Such or­bits are un­com­mon in plan­etary sys­tems around evolved stars and, in fact, the BD+48 740 plan­et’s or­bit is the most el­lip­ti­cal one de­tected so far.” 

Be­cause gravita­t­ional in­ter­ac­tions be­tween plan­ets are re­spon­si­ble for such pe­cu­liar or­bits, the as­tro­no­mers sus­pect that the dive of the mis­sing plan­et to­ward the star be­fore it be­came a gi­ant could have giv­en the sur­viv­ing mas­sive plan­et a burst of en­er­gy, throw­ing it in­to an ec­cen­tric or­bit.

“Catch­ing a plan­et in the act of be­ing de­voured by a star is an al­most improb­a­ble feat to ac­com­plish be­cause of the com­par­a­tive swift­ness of the pro­cess, but the oc­cur­rence of such a col­li­sion can be de­duced from the way it af­fects the stel­lar chem­istry,” ex­plained re­search team mem­ber Eva Vil­laver of the Uni­ver­si­dad Au­tonoma de Ma­drid in Spain. “The highly elon­gat­ed or­bit of the mas­sive plan­et we dis­cov­ered around this lith­i­um-polluted red-gi­ant star is ex­actly the kind of ev­i­dence that would point to the star’s re­cent de­struc­tion of its now-mis­sing plan­et.”

A pa­per on the find­ings ap­pears in an early on­line edi­tion of the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters. The Hobby-Eberly Tel­e­scope is a joint proj­ect of the Uni­vers­ity of Tex­as at Aus­tin, Penn State Uni­vers­ity, Ludwig-Maxim­i­lians-Uni­vers­i­tat Mun­chen, and Georg-Au­gust-Uni­vers­i­tat Got­tin­gen.

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Astronomers have reported the first evidence of a planet’s destruction by its aging host star. Evidence indicates a planet was devoured as the star began expanding into a “red giant” — the stellar equivalent of advanced age, scientists say. “A similar fate may await the inner planets in our solar system, when the Sun becomes a red giant and expands all the way out to Earth’s orbit some five billion years from now,” said Alex Wolszczan of Penn State University, one of the members of the research team, who is also credited as the discoverer of the first planet found outside our solar system. The astronomers also identified a massive planet in a surprisingly elliptical or “stretched” orbit around the same red-giant star, named BD+48 740, which is older than the Sun and about 11 times wider. Wolszczan and colleagues identified evidence of the planetary demise while using the Hobby-Eberly Telescope to study the aging star and to search for planets around it. The evidence includes the star’s peculiar chemical composition, plus the highly unusual elliptical orbit of its surviving planet, they said. “Our detailed spectroscopic [color] analysis reveals that this red-giant star, BD+48 740, contains an abnormally high amount of lithium, a rare element created primarily during the Big Bang 14 billion years ago,” said research team member Monika Adamow of Nicolaus Copernicus University in Torun, Poland. Lithium is easily destroyed in stars, which is why its abnormally high abundance in this older star is so unusual. “Theorists have identified only a few, very specific circumstances, other than the Big Bang, under which lithium can be created in stars,” Wolszczan added. “In the case of BD+48 740, it is probable that the lithium production was triggered by a mass the size of a planet that spiraled into the star and heated it up while the star was digesting it.” The second piece of evidence, the astronomers said, is the elongated-looking orbit of the star’s newly discovered massive planet, which is at least 1.6 times as heavy as Jupiter. “We discovered that this planet revolves around the star in an orbit that is only slightly wider than that of Mars at its narrowest point, but is much more extended at its farthest point,” said co-researcher Andrzej Niedzielski of Copernicus University. “Such orbits are uncommon in planetary systems around evolved stars and, in fact, the BD+48 740 planet’s orbit is the most elliptical one detected so far.” Because gravitational interactions between planets are responsible for such peculiar orbits, the astronomers suspect that the dive of the missing planet toward the star before it became a giant could have given the surviving massive planet a burst of energy, throwing it into an eccentric orbit. “Catching a planet in the act of being devoured by a star is an almost improbable feat to accomplish because of the comparative swiftness of the process, but the occurrence of such a collision can be deduced from the way it affects the stellar chemistry,” explained research team member Eva Villaver of the Universidad Autonoma de Madrid in Spain. “The highly elongated orbit of the massive planet we discovered around this lithium-polluted red-giant star is exactly the kind of evidence that would point to the star’s recent destruction of its now-missing planet.” A paper on the findings appears in an early online edition of the Astrophysical Journal Letters The Hobby-Eberly Telescope is a joint project of the University of Texas at Austin, Penn State University, Ludwig-Maximilians-Universitat Munchen, and Georg-August-Universitat Gottingen.