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August 03, 2010

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Livable worlds abound, simulations find

Sept. 7, 2006
Courtesy University of Colorado at Boulder
and World Science staff

New com­pu­ter sim­u­la­tions by as­tronomers have found that Earth-like pla­n­ets, warm and wet enough for life, should be common.

The si­m­u­la­tions fo­cused on a com­mon type of so­lar sys­tem iden­t­i­fied be­fore, fea­tur­ing pla­n­ets called Hot Ju­pi­ters. A third of these sys­tems al­so pro­duce Earth-like worlds in the com­put­er re­en­act­ments, the re­searchers said.

Artist's im­pres­sion of the sur­face of an ocean-co­vered plan­et in the hab­it­a­ble zone of a sys­tem with a "hot Ju­pi­ter." A "hot Earth" and the hot Ju­pi­ter ap­pear close to the set­ting sun. Great waves and storms wash over the plan­et, a pos­si­ble re­sult of a lack of con­ti­nents. (Cred­it: Nahks Tr'Enhl, 2006)

The Hot Jupiter­s—gas gi­ants that or­bit scorch­ing­ly close to their sun—are be­lieved to ori­g­i­n­ate fur­ther out from the star, then mi­grate in­ward. This churns up ma­te­ri­al along the way in the dust­y, plan­et-forming disk of ma­te­r­i­al en­cir­cl­ing the star. 

Tra­di­tion­ally, as­tro­no­mers thought this jour­ney would dis­rupt the for­m­a­tion of oth­er pla­n­ets in the path. 

The sim­u­la­tions found the op­po­site: as they tra­vel in­ward, Hot Jupiters fling rock­y de­bris out­ward, where it eas­i­ly clumps in­to Earth-like plan­ets. Mean­while, tur­bu­lence in clouds slows down small, icy bod­ies or­biting far off, so they spir­al in­ward and dump wa­ter on the young worlds. These can even­tu­al­ly ac­cu­mu­late oceans miles deep and set­tle in­to a life-friend­ly “ha­b­it­able zone.”

The ha­bi­t­able zone is a theo­rized re­gion nei­ther too close, nor too far, from the sun for temp­er­a­tures to sup­port life. 

“These gas gi­ants cause quite a ruckus,” said Sean Ray­mond of the Uni­ver­si­ty of Col­o­rad­o at Boul­der, a mem­ber of the re­search team. But “we now think there is a new class of ocean-covered, and pos­si­bly hab­it­a­ble, plan­ets in so­lar sys­tems un­like our own.”

In the Sept. 8 is­sue of the re­search jour­nal Sci­ence, the sci­en­tists de­s­cribed si­m­u­la­tions last­ing more than eight months each on more than a doz­en desk­top com­put­ers. The crea­tors plugged in data re­flec­ting cur­rent the­o­ries on pla­net form­a­tion, start­ing with “proto-plan­etary disks” con­tain­ing more than 1,000 moon-sized, rocky and icy chunks. They played out some 200 mil­lion years of plan­et ev­o­lu­tion.

 Hot Jupiters make up some 40 per­cent of pla­n­ets found to date be­yond our So­lar Sys­tem, Ray­mond said, though this num­ber is so high pro­ba­bly be­cause their size and lo­ca­tions make them easy to find.

“I think there are def­i­nite­ly hab­it­a­ble plan­ets out there,” he ad­ded. “But any life on these plan­ets could be very dif­fer­ent from ours.”

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Simulations by astronomers have found that many solar systems should harbor Earth-like planets with temperatures suitable for life. And many of these would have deep oceans to enhance the possibility. The computer simulations focused on a common type of solar system that researchers have found before, containing planets called Hot Jupiters. Fully a third of these systems also generate Earth-like worlds in the models, the researchers said. The Hot Jupiters—gas giants that orbit scorchingly close to their parent stars—account for some 40% of planets so far identified outside our Solar System, the scientists added, though this may not be representative of the planet population as a whole. Hot Jupiters are believed to originate further out from the star, then migrate inward, churning up planet-forming material along the way in the dusty, planet-forming disc that surrounds the star. Traditionally, astronomers thought this journey would disrupt the formation of other planets in the path. The simulations found the opposite: as they migrate in toward the star, hot Jupiters fling rocky debris outward where it easily clumps into Earth-like planets. Meanwhile, turbulence in clouds slows down small, icy bodies orbiting further off, making them spiral inward and dump water on the fledgling planets. These can eventually accumulate oceans miles deep and settle into a “habitable zone” conducive to life. “These gas giants cause quite a ruckus,” said Sean Raymond of the University of Colorado at Boulder, a member of the research team. But “we now think there is a new class of ocean-covered, and possibly habitable, planets in solar systems unlike our own.” Writing in the Sept. 8 issue of the research journal Science, the scientists described detailed simulations lasting more than eight months each on more than a dozen desktop computers. They started with “proto-planetary disks” containing more than 1,000 moon-sized, rocky and icy chunks. The models were based on current theories of how planets form and simulated about 200 million years of planet evolution. “I think there are definitely habitable planets out there,” said Raymond. “But any life on these planets could be very different from ours. There are a lot of evolutionary steps in between the formation of such planets in other systems and the presence of life forms looking back at us.”