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First planet with two suns reported found

Sept. 15, 2011
Courtesy of NASA
and World Science staff


The ex­ist­ence of a world with a dou­ble sun­set, as por­trayed in the film Star Wars more than 30 years ago, is now a known fact, as­tro­no­mers say. 

Sci­en­tists an­nounced Sept. 15 that NASA’s Kep­ler mis­sion has made the first clear de­tec­tion of a “cir­cumbi­na­ry” plan­et, a plan­et or­bit­ing two stars. The body is 200 light-years from Earth; a light-year is the dis­tance light trav­els in a year.

Artist's im­age of a plan­et or moon in a sys­tem with two suns. (Courtesy NASA JPL)


Un­like Star Wars’ Tatooine, the plan­et is cold, gas­e­ous and not thought to har­bor life, but its dis­cov­ery demon­strates the di­vers­ity of plan­ets in our gal­axy, ac­cord­ing to re­search­ers. Pre­vi­ous re­search had hinted at the ex­ist­ence of cir­cum­bi­nary plan­ets, but clear con­firma­t­ion proved elu­sive. A 2005 study re­port­ed a planet in a three-star sys­tem, but it was ap­par­ently only or­bit­ing one of those stars.

Kep­ler de­tected the plan­et, known as Kep­ler-16b, by ob­serv­ing tran­sits, events in which the bright­ness of a par­ent star dims from the plan­et cross­ing in front of it. “This dis­cov­ery con­firms a new class of plan­etary sys­tems that could har­bor life,” Kep­ler prin­ci­pal in­ves­ti­ga­tor Wil­liam Borucki said. “Given that most stars in our gal­axy are part of a bi­na­ry sys­tem, this means the op­por­tun­i­ties for life are much broader than if plan­ets form only around sin­gle stars. This mile­stone dis­cov­ery con­firms a the­o­ry that sci­en­tists have had for dec­ades but could not prove un­til now.” 

A re­search team led by Lau­rance Doyle of the SETI In­sti­tute in Moun­tain View, Calif., used da­ta from the Kep­ler space tel­e­scope, which meas­ures dips in the bright­ness of more than 150,000 stars, to search for tran­sit­ing plan­ets. Kep­ler is the first NASA mis­sion ca­pa­ble of find­ing Earth-size plan­ets in or near the “hab­it­able zone,” the re­gion in a plan­etary sys­tem where liq­uid wa­ter can ex­ist on the sur­face of the or­bit­ing plan­et.

Sci­en­tists de­tected the new plan­et in the Kep­ler-16 sys­tem, a pair of or­bit­ing stars that eclipse each oth­er from our van­tage point on Earth. When the smaller star par­tially blocks the larg­er star, a pri­ma­ry eclipse oc­curs, and a sec­ond­ary eclipse oc­curs when the smaller star is oc­culted, or com­pletely blocked, by the larg­er star.

As­tro­no­mers fur­ther ob­served that the bright­ness of the sys­tem dipped even when the stars were not eclips­ing one anoth­er, hint­ing at a third body. The ad­di­tion­al dim­ming in bright­ness events, called the ter­tiary and qua­ter­nary eclipses, reap­peared at ir­reg­u­lar in­ter­vals of time, in­di­cat­ing the stars were in dif­fer­ent po­si­tions in their or­bit each time the third body passed. This showed the third body was cir­cling, not just one, but both stars, in a wide cir­cumbi­nary or­bit.

The gravita­t­ional tug on the stars, meas­ured by changes in their eclipse times, was a good in­di­ca­tor of the mass of the third body. Only a very slight gravita­t­ional pull was de­tected, one that only could be caused by a small mass. The find­ings are de­scribed in a new study pub­lished Sept. 16 in the jour­nal Sci­ence.

“Most of what we know about the sizes of stars comes from such eclips­ing bi­na­ry sys­tems, and most of what we know about the size of plan­ets comes from tran­sits,” said Doyle, who al­so is the lead au­thor of the study and a Kep­ler par­ti­ci­pat­ing sci­ent­ist. “Kep­ler-16 com­bines the best of both worlds, with stel­lar eclipses and plan­etary tran­sits in one sys­tem.” 

Kep­ler-16b is an in­hos­pi­ta­b, cold world about the size of Sat­urn and thought to be made up of about half rock and half gas, as­tro­no­mers said. The par­ent stars are smaller than our Sun. One is 69 per­cent the mass of the Sun and the oth­er only 20 per­cent. Kep­ler-16b or­bits around both stars every 229 days, si­m­i­lar to Venus’ 225-day or­bit, but lies out­side the sys­tem’s hab­it­a­ble zone, where liq­uid wa­ter could ex­ist on the sur­face, be­cause the stars are cool­er than our Sun.

“Work­ing in film, we of­ten are tasked with cre­at­ing some­thing nev­er be­fore seen,” said vis­u­al ef­fects su­per­vi­sor John Knoll of In­dus­t­ri­al Light & Mag­ic, a di­vi­sion of Lu­cas­film Ltd., in San Fran­cis­co. “How­ever, more of­ten than not, sci­en­tif­ic dis­cov­er­ies prove to be more spec­tac­u­lar than an­y­thing we dare im­ag­ine. There is no doubt these dis­cov­er­ies in­flu­ence and in­spire sto­ry­tellers. Their very ex­ist­ence serves as cause to dream big­ger and open our minds to new pos­si­bil­i­ties be­yond what we think we ‘know.’”


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The existence of a world with a double sunset, as portrayed in the film Star Wars more than 30 years ago, is now a known fact, astronomers say. Scientists announced Sept. 15 that NASA’s Kepler mission has made the first clear detection of a “circumbinary” planet, a planet orbiting two stars. The body is 200 light-years from Earth; a light-year is the distance light travels in a year. Unlike Star Wars’ Tatooine, the planet is cold, gaseous and not thought to harbor life, but its discovery demonstrates the diversity of planets in our galaxy, according to researchers. Previous research had hinted at the existence of circumbinary planets, but clear confirmation proved elusive. Kepler detected the planet, known as Kepler-16b, by observing transits, events in which the brightness of a parent star dims from the planet crossing in front of it. “This discovery confirms a new class of planetary systems that could harbor life,” Kepler principal investigator William Borucki said. “Given that most stars in our galaxy are part of a binary system, this means the opportunities for life are much broader than if planets form only around single stars. This milestone discovery confirms a theory that scientists have had for decades but could not prove until now.” A research team led by Laurance Doyle of the SETI Institute in Mountain View, Calif., used data from the Kepler space telescope, which measures dips in the brightness of more than 150,000 stars, to search for transiting planets. Kepler is the first NASA mission capable of finding Earth-size planets in or near the “habitable zone,” the region in a planetary system where liquid water can exist on the surface of the orbiting planet. Scientists detected the new planet in the Kepler-16 system, a pair of orbiting stars that eclipse each other from our vantage point on Earth. When the smaller star partially blocks the larger star, a primary eclipse occurs, and a secondary eclipse occurs when the smaller star is occulted, or completely blocked, by the larger star. Astronomers further observed that the brightness of the system dipped even when the stars were not eclipsing one another, hinting at a third body. The additional dimming in brightness events, called the tertiary and quaternary eclipses, reappeared at irregular intervals of time, indicating the stars were in different positions in their orbit each time the third body passed. This showed the third body was circling, not just one, but both stars, in a wide circumbinary orbit. The gravitational tug on the stars, measured by changes in their eclipse times, was a good indicator of the mass of the third body. Only a very slight gravitational pull was detected, one that only could be caused by a small mass. The findings are described in a new study published Friday, Sept. 16, in the journal Science. “Most of what we know about the sizes of stars comes from such eclipsing binary systems, and most of what we know about the size of planets comes from transits,” said Doyle, who also is the lead author of the study and a Kepler participating scientist. “Kepler-16 combines the best of both worlds, with stellar eclipses and planetary transits in one system.” Kepler-16b is an inhospitable, cold world about the size of Saturn and thought to be made up of about half rock and half gas, astronomers said. The parent stars are smaller than our Sun. One is 69 percent the mass of the Sun and the other only 20 percent. Kepler-16b orbits around both stars every 229 days, similar to Venus’ 225-day orbit, but lies outside the system’s habitable zone, where liquid water could exist on the surface, because the stars are cooler than our Sun. “Working in film, we often are tasked with creating something never before seen,” said visual effects supervisor John Knoll of Industrial Light & Magic, a division of Lucasfilm Ltd., in San Francisco. “However, more often than not, scientific discoveries prove to be more spectacular than anything we dare imagine. There is no doubt these discoveries influence and inspire storytellers. Their very existence serves as cause to dream bigger and open our minds to new possibilities beyond what we think we ‘know.’” reported found