"Long before it's in the papers"
June 22, 2012

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In planetary odd couple, two worlds within plain sight distance of each other

June 22, 2012
Courtesy of the Har­vard-Smith­son­ian 
Cen­ter for As­t­ro­phys­ics
and World Science staff


Few night­time sights of­fer more dra­ma than the full Moon ris­ing over the ho­ri­zon. 

Now im­ag­ine that in­stead of the Moon, a gas gi­ant world span­ning three times more sky loomed over a mol­ten land­scape of a la­va plan­et. And im­ag­ine that this ce­les­tial vis­i­tor is­n’t a moon, but an­oth­er plan­et en­tire­ly.

This al­ien vis­ta ex­ists, as­tro­no­mers say, in a newly disco­vered so­lar sys­tem called Kep­ler-36.

In this artist's con­cep­tion, a "hot Nep­tune" known as Kepler-36c looms in the sky of its neigh­bor, the rocky world Kepler-36b. The two plan­ets have re­peat­ed close en­coun­ters, eve­ry 97 days on av­er­age, sci­ent­ists say. At that time, they are sep­a­rat­ed by less than five Earth-Moon dis­tances. Such close ap­proaches stir up tre­men­dous grav­i­ta­tion­al tides that squeeze and stretch both plan­ets, which may pro­mote ac­tive vol­can­ism on Kepler-36b. (Cred­it: Da­vid A. Agui­lar (CfA))
“These two worlds are hav­ing close en­coun­ters,” said Josh Cart­er, a Hub­ble Fel­low at the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass. “They are the clos­est to each oth­er of any plan­etary sys­tem we’ve found,” added co-author Er­ic Agol of the Uni­vers­ity of Wash­ing­ton.

Cart­er, Agol and col­leagues re­ported their find­ings June 21 in the ad­vance on­line edi­tion of the re­search jour­nal Sci­ence. They spot­ted the plan­ets in da­ta from NASA’s Kep­ler sat­el­lite, which can de­tect a plan­et when it passes in front of, and briefly re­duces the light com­ing from, its par­ent star.

The new­found sys­tem is be­lieved to con­tain two plan­ets cir­cling a “sub-gi­ant” star much like the Sun ex­cept sev­er­al bil­lion years old­er. The in­ner world, Kep­ler-36b, is a rocky plan­et es­ti­mat­ed to be 1.5 times the size, and 4.5 times the weight, of Earth. It or­bits the star about eve­ry 14 days at an av­er­age dis­tance of less than 11 mil­lion miles.

The out­er world, Kep­ler-36c, is a gas­e­ous plan­et 3.7 times the size of Earth and weigh­ing eight times as much. This “hot Nep­tune” or­bits once each 16 days at a dis­tance of 12 mil­lion miles.

The two plan­ets ex­pe­ri­ence a “con­junc­tion” eve­ry 97 days on av­er­age, as­tro­no­mers say. At that time, they’re sep­a­rat­ed by less than five Earth-Moon dis­tances. Since Kep­ler-36c is much larg­er than the Moon, it pre­s­ents a spec­tac­u­lar view in its neigh­bor’s sky. Co­in­ci­den­tally, the smaller Kep­ler-36b would ap­pear about the size of the Moon when viewed from Kep­ler-36c. 

Such close ap­proaches would stir up tre­men­dous gravita­t­ional tides that squeeze and stretch both plan­ets.

Re­search­ers are strug­gling to un­der­stand how these two very dif­fer­ent worlds ended up in such close or­bits. In our so­lar sys­tem, rocky plan­ets re­side close to the Sun while the gas gi­ants re­main dis­tant.

“We’re won­der­ing how many more [sys­tems] like this are out there,” said Agol. “We found this one on a first quick look,” added Cart­er. “We’re now comb­ing through the Kep­ler da­ta to try to lo­cate more.”

The find­ing was made pos­si­ble with as­ter­o­seis­mol­ogy, the study of stars through their nat­u­ral os­cilla­t­ions. Sun­like stars res­o­nate like mu­si­cal in­stru­ments, due to sound waves trapped in­side them. And just like a mu­si­cal in­stru­ment, the larg­er the star, the “deep­er” are its res­o­nances. This trapped sound makes the stars gently vi­brate.

“Kep­ler-36 shows beau­ti­ful os­cilla­t­ions. By meas­ur­ing the os­cilla­t­ions we were able to meas­ure the size, mass and age of the star to ex­quis­ite pre­ci­sion,” said study co-author Bill Chap­lin of the Uni­vers­ity of Bir­ming­ham in the U.K. “With­out as­ter­o­seis­mol­ogy, it would not have been pos­si­ble to place such tight con­straints on the prop­er­ties of the plan­ets.”

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Few nighttime sights offer more drama than the full Moon rising over the horizon. Now imagine that instead of the Moon, a gas giant world spanning three times more sky loomed over a molten landscape of a lava planet. And imagine that this celestial visitor isn’t a moon, but another planet entirely. This alien vista exists, astronomers say, in a newly discovered solar system called Kepler-36. “These two worlds are having close encounters,” said Josh Carter, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “They are the closest to each other of any planetary system we’ve found,” added co-author Eric Agol of the University of Washington. Carter, Agol and colleagues reported their findings June 21 in the advance online edition of the research journal Science. They spotted the planets in data from NASA’s Kepler satellite, which can detect a planet when it passes in front of, and briefly reduces the light coming from, its parent star. The newfound system is believed to contain two planets circling a “sub-giant” star much like the Sun except several billion years older. The inner world, Kepler-36b, is a rocky planet estimated to be 1.5 times the size, and 4.5 times the weight, of Earth. It orbits the star about every 14 days at an average distance of less than 11 million miles. The outer world, Kepler-36c, is a gaseous planet 3.7 times the size of Earth and weighing eight times as much. This “hot Neptune” orbits once each 16 days at a distance of 12 million miles. The two planets experience a “conjunction” every 97 days on average, astronomers say. At that time, they’re separated by less than five Earth-Moon distances. Since Kepler-36c is much larger than the Moon, it presents a spectacular view in its neighbor’s sky. (Coincidentally, the smaller Kepler-36b would appear about the size of the Moon when viewed from Kepler-36c.) Such close approaches stir up tremendous gravitational tides that squeeze and stretch both planets. Researchers are struggling to understand how these two very different worlds ended up in such close orbits. Within our solar system, rocky planets reside close to the Sun while the gas giants remain distant. “We’re wondering how many more [systems] like this are out there,” said Agol. “We found this one on a first quick look,” added Carter. “We’re now combing through the Kepler data to try to locate more.” The finding was made possible with asteroseismology, the study of stars through their natural oscillations. Sunlike stars resonate like musical instruments, due to sound waves trapped inside them. And just like a musical instrument, the larger the star, the “deeper” are its resonances. This trapped sound makes the stars gently vibrate. “Kepler-36 shows beautiful oscillations. By measuring the oscillations we were able to measure the size, mass and age of the star to exquisite precision,” said study co-author Bill Chaplin of the University of Birmingham in the U.K. “Without asteroseismology, it would not have been possible to place such tight constraints on the properties of the planets.” sight distance of each other