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Two-sunned planets don’t form easily, study finds

Feb. 3, 2014
Courtesy of the University of Bristol
and World Science staff

Plan­ets with two stars—like Luke Sky­walk­er’s home plan­et from Star Wars—don’t form that eas­i­ly, a new study con­cludes.

Sky­walk­er’s home world, Tatooine, would have had to form much far­ther away from the two stars than where it ap­pears in the film, as­tro­nom­ers say, though it could have lat­er mi­grat­ed. The study ex­am­ined real-world coun­ter­parts of Tatooine ob­served through the Kep­ler space tel­e­scope.

An artist's con­cep­tion of Kepler-34b which or­bits a double-star sys­tem
(Im­age by Da­vid A. Aguilar, Har­vard-Smith­son­ian Cen­tre for As­tro­phys­ics)


Like the fic­tion­al Star Wars plan­et, Kep­ler-34(AB)b is a cir­cum­bi­nary plan­et, so called be­cause its or­bit en­com­passes two stars. 

There are few en­vi­ron­ments more ex­treme for plan­et form­a­t­ion, ac­cord­ing to the sci­en­tists. Pow­er­ful gra­vi­ta­t­ional per­turba­t­ions from the two stars on the rocky build­ing blocks of plan­ets lead to de­struc­tive col­li­sions that grind down the ma­te­ri­al. So, how can the pres­ence of such plan­ets be ex­plained?

In re­search pub­lished this week in As­t­ro­phys­i­cal Jour­nal Let­ters, Zo­e Lein­hardt and col­leagues at the Uni­vers­ity of Bris­tol, U.K. made com­put­er sim­ula­t­ions of the early stages of plan­et forma­t­ion around bi­na­ry or double stars. They cal­cu­lat­ed the ef­fect of gra­vity and of col­li­sions among a mil­lion plan­etary build­ing blocks.

They found that the ma­jor­ity of these plan­ets must have formed much fur­ther away from the cen­tral bi­na­ry stars and then mi­grat­ed to their cur­rent loca­t­ion. 

Most plan­ets are be­lieved to form in disk-shaped clouds of dust and gas sur­rounding par­ent stars. But for two stars, this disk “is a hos­tile en­vi­ron­ment,” Lein­hardt said. “Kep­ler 34(AB)b would have strug­gled to grow where we find it now.”

It seems like­ly, the re­search­ers said, that all of the cur­rently known cir­cum­bi­nary plan­ets have al­so mi­grat­ed sig­nif­i­cantly from their forma­t­ion loca­t­ion­s—with the pos­si­ble ex­cep­tion of Kep­ler-47 (AB)c which is fur­ther away from the bi­na­ry stars than any of the oth­er cir­cum­bi­nary plan­ets.

“Circumbi­na­ry plan­ets have cap­tured the ima­gina­t­ion of many science-fiction writ­ers and film­makers. Our re­search shows just how re­mark­a­ble such plan­ets are,” said Ste­fan Lines, lead au­thor of the stu­dy. He added that un­der­stand­ing more about where they form will help “in the hunt for earth-like plan­ets in bi­na­ry star sys­tems.”


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Planets with two stars—like Luke Skywalker’s home planet from Star Wars—don’t form that easily, a new study concludes. Skywalker’s home world, Tatooine, would have had to form much farther away from the two stars than where it appears in the film, researchers say, though it could have later migrated. The study examined real-world counterparts of Tatooine observed through the Kepler space telescope. Like the fictional Star Wars planet, Kepler-34(AB)b is a circumbinary planet, so called because its orbit encompasses two stars. There are few environments more extreme for planet formation, according to the scientists. Powerful gravitational perturbations from the two stars on the rocky building blocks of planets lead to destructive collisions that grind down the material. So, how can the presence of such planets be explained? In research published this week in Astrophysical Journal Letters, Zoe Leinhardt and colleagues at the University of Bristol, U.K. made computer simulations of the early stages of planet formation around the binary stars, calculating the effect of gravity and collisions among a million planetary building blocks. They found that the majority of these planets must have formed much further away from the central binary stars and then migrated to their current location. Most planets are believed to form in disk-shaped clouds of dust and gas surround parent stars. But for two stars, this disk “is a hostile environment,” Leinhardt said. “Kepler 34(AB)b would have struggled to grow where we find it now.” It seems likely, the researchers said, that all of the currently known circumbinary planets have also migrated significantly from their formation locations—with the possible exception of Kepler-47 (AB)c which is further away from the binary stars than any of the other circumbinary planets. “Circumbinary planets have captured the imagination of many science-fiction writers and film-makers—our research shows just how remarkable such planets are,” said said Stefan Lines, lead author of the study. He added that understanding more about where they form will help “in the hunt for earth-like planets in binary star systems.”