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Off-kilter planetary system surprises astronomers

May 24, 2010
Courtesy of the McDonald Observatory
and World Science staff

As­tro­no­mers have found an “out of whack” plan­e­tary sys­tem in which two plan­ets’ or­bits are at a steep an­gle to each oth­er—com­p­licat­ing the study of how plan­e­tary sys­tems evolve, they say.

Sci­en­tists had gen­er­ally as­sumed that when more than one plan­et or­bit a star, the or­bits share one plane. This can be vis­u­al­ized as an im­ag­i­nary disk sur­round­ing the par­ent star: the or­bits are all in the same, flat re­gion of space, much the way Sat­urn’s rings are. The new­found plan­e­tary sys­tem, by con­trast, can be vis­u­al­ized as akin to the rings of an im­ag­i­nary Sat­urn whose rings are strongly tilted with re­spect to each oth­er.

Artist's il­lus­tra­tion of the Up­si­lon An­dromedae A plan­e­tary sys­tem, where three Jupiter-type plan­ets or­bit the yellow-white star Up­si­lon An­dro­me­dae A. The or­bits of two of the plan­ets are in­clined by 30 de­grees with re­spect to each oth­er. (Cred­it: NA­SA, ESA, and A. Feild (STScI))


The find­ing was re­ported May 24 by a team of as­tro­no­mers led by Bar­ba­ra McArthur of The Uni­vers­ity of Tex­as at Aus­tin’s Mc­Don­ald Ob­serv­a­to­ry. The dis­cov­ery shows that some vi­o­lent events can dis­rupt plan­ets’ or­bits af­ter a plan­e­tary sys­tem forms, say re­search­ers. “As­tro­no­mers can no long­er as­sume all plan­ets or­bit their par­ent star in a sin­gle plane,” McArthur ex­plained.

McArthur and her team used da­ta from the Hub­ble Space Tel­e­scope, the gi­ant Hobby-Eberly Tel­e­scope, and oth­er ground-based tele­scopes com­bined with ex­ten­sive mod­el­ing to un­earth in­forma­t­ion on the plan­e­tary sys­tem sur­round­ing the near­by star Up­si­lon An­dro­me­dae.

McArthur re­ported the find­ings in a press con­fer­ence at the an­nu­al meet­ing of the Amer­i­can As­tro­nom­i­cal So­ci­e­ty in Mi­ami. The work is al­so to be pub­lished in the June 1 edi­tion of the As­t­ro­phys­i­cal Jour­nal.

For just over a dec­ade, as­tro­no­mers have known that three Jupiter-type plan­ets or­bit the yellow-white dwarf star Up­si­lon An­dro­me­dae. Si­m­i­lar to our Sun, Up­si­lon An­dro­me­dae lies about 44 light-years away; a light-year is the dis­tance light trav­els in a year. The star is a bit young­er, heav­i­er and brighter than the Sun.

Com­bin­ing dif­fer­ent types of da­ta from Hub­ble and ground-based tele­scopes, McArthur’s team de­ter­mined the weights of two of the three known plan­ets, dubbed Ups c and d. They al­so found that the or­bits of plan­ets c and d are in­clined by 30 de­grees with re­spect to each oth­er. “Most probably Up­si­lon An­dro­me­dae had the same forma­t­ion pro­cess as our own so­lar sys­tem, al­though there could have been dif­fer­ences in the late forma­t­ion,” McArthur said.

Con­ven­tion­al wis­dom has been that a big cloud of gas col­lapses down to form a star, and plan­ets are a nat­u­ral by­prod­uct. Left over ma­te­ri­al from the star form­a­tion settles in a disk that gives rise to plan­ets. In our so­lar sys­tem, there’s a fos­sil of that crea­t­ion event be­cause all of the eight ma­jor plan­ets or­bit in nearly the same plane.

Sev­er­al dif­fer­ent types of gravita­t­ional in­ter­ac­tions could be re­spon­si­ble for the sur­pris­ingly in­clined or­bits in Up­si­lon An­dro­me­dae, as­tro­no­mers said. “Pos­si­bil­i­ties in­clude in­ter­ac­tions oc­cur­ring from the in­ward migra­t­ion of plan­ets, the ejec­tion of oth­er plan­ets from the sys­tem through plan­et-plan­et scat­ter­ing, or dis­ruption from the par­ent star’s bi­na­ry com­pan­ion star, Up­si­lon An­dro­me­dae B,” McArthur said.


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Astronomers have found an “out of whack” planetary system in which two planets’ orbits are at a steep angle to each other—complicating the study of how planetary systems evolve, they say. Scientists had generally assumed that when more than one planet orbit a star, the orbits share one plane. This can be visualized as an imaginary disk surrounding the parent star: the orbits are all in the same, flat region of space, the way Saturn’s rings are. The newfound planetary system, by contrast, can be visualized as akin to an imaginary Saturn whose rings are strongly tilted with respect to each other. The finding was reported May 24 a team of astronomers led by Barbara McArthur of The University of Texas at Austin McDonald Observatory. The discovery shows that some violent events can disrupt planets’ orbits after a planetary system forms, say researchers. “Astronomers can no longer assume all planets orbit their parent star in a single plane,” McArthur said. McArthur and her team used data from Hubble Space Telescope, the giant Hobby-Eberly Telescope, and other ground-based telescopes combined with extensive modeling to unearth a landslide of information about the planetary system surrounding the nearby star Upsilon Andromedae. McArthur reported the findings in a press conference at the annual meeting of the American Astronomical Society in Miami. The work is also to be published in the June 1 edition of the Astrophysical Journal. For just over a decade, astronomers have known that three Jupiter-type planets orbit the yellow-white dwarf star Upsilon Andromedae. Similar to our Sun, Upsilon Andromedae lies about 44 light-years away; a light-year is the distance light travels in a year. The star is a bit younger, heavier and brighter than the Sun. Combining different types of data from Hubble and ground-based telescopes, McArthur’s team determined the weights of two of the three known planets, Ups And c and d. They also found that the orbits of planets c and d are inclined by 30 degrees with respect to each other. “Most probably Upsilon Andromedae had the same formation process as our own solar system, although there could have been differences in the late formation,” McArthur said. Until now the conventional wisdom has been that a big cloud of gas collapses down to form a star, and planets are a natural byproduct. Left over material forms a disk. In our solar system, there’s a fossil of that creation event because all of the eight major planets orbit in nearly the same plane. Several different types of gravitational interactions could be responsible for the surprisingly inclined orbits in Upsilon Andromedae, astronomers said. “Possibilities include interactions occurring from the inward migration of planets, the ejection of other planets from the system through planet-planet scattering, or disruption from the parent star’s binary companion star, Upsilon Andromedae B,” McArthur said.