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Planets outside our solar system more hospitable to life than thought

January 16, 2015

TORONTO, ON — A study by astro­physi­cists at the Uni­ver­si­ty of Toron­to sug­gests that exo­plan­ets — plan­ets out­side our solar sys­tem — are more like­ly to have liq­uid water and be more hab­it­able than we thought.

“Plan­ets with poten­tial oceans could have a cli­mate that is much more sim­i­lar to Earth’s than pre­vi­ous­ly expect­ed,” said Jérémy Lecon­te, a post­doc­tor­al fel­low at the Cana­di­an Insti­tute for The­o­ret­i­cal Astro­physics (CITA) at the Uni­ver­si­ty of Toron­to, and lead author of a study pub­lished today in Sci­ence Express.

Sci­en­tists have thought that exo­plan­ets behave in a man­ner con­trary to that of Earth — that is they always show their same side to their star. If so, exo­plan­ets would rotate in sync with their star so that there is always one hemi­sphere fac­ing it while the oth­er hemi­sphere is in per­pet­u­al cold dark­ness.

Leconte’s study sug­gests, how­ev­er, that as exo­plan­ets rotate around their stars, they spin at such a speed as to exhib­it a day-night cycle sim­i­lar to Earth.

“If we are cor­rect, there is no per­ma­nent, cold night side on exo­plan­ets caus­ing water to remain trapped in a gigan­tic ice sheet. Whether this new under­stand­ing of exo­plan­ets’ cli­mate increas­es the abil­i­ty of these plan­ets to devel­op life remains an open ques­tion.”

Lecon­te and his team reached their con­clu­sions via a three-dimen­sion­al cli­mate mod­el they devel­oped to pre­dict the effect of a giv­en planet’s atmos­phere on the speed of its rota­tion, which results in changes to its cli­mate,” said Lecon­te. “Atmos­phere is a key fac­tor affect­ing a planet’s spin, the impact of which can be of enough sig­nif­i­cance to over­come syn­chro­nous rota­tion and put a plan­et in a day-night cycle.”

Though astronomers are still await­ing obser­va­tion­al evi­dence, the­o­ret­i­cal argu­ments sug­gest that many exo­plan­ets should be able to main­tain an atmos­phere as mas­sive that of Earth. In Earth’s case — with its rel­a­tive­ly thin atmos­phere — most of the light from the Sun reach­es the sur­face of the plan­et, max­i­miz­ing the effect of heat­ing through­out the atmos­phere and pro­duc­ing a more mod­er­ate cli­mate across the plan­et. By cre­at­ing tem­per­a­ture dif­fer­ences at the sur­face, between day and night and between equa­tor and poles, the solar heat­ing dri­ves winds that redis­trib­ute the mass of the atmos­phere.

The impact is so sig­nif­i­cant that it over­comes the effect of tidal fric­tion exert­ed by a star on what­ev­er satel­lite is orbit­ing it, much like Earth does on the Moon.

“The Moon always shows us the same side, because the tides raised by Earth cre­ate a fric­tion that alters its spin,” said Lecon­te. “The Moon is in syn­chro­nous rota­tion with Earth because the time it takes to spin once on its axis equals the time it takes for it to orbit around Earth. That is why there is a dark side of the moon. The tidal the­o­ry, how­ev­er, neglects the effects of an atmos­phere.”

The researchers say that a large num­ber of known ter­res­tri­al exo­plan­ets should not be in a state of syn­chro­nous rota­tion, as ini­tial­ly believed. While their mod­els show that they would have a day-night cycle mak­ing them much more sim­i­lar to Earth, the dura­tion of their days could last between a few weeks and few months.

The find­ings are report­ed in the paper “Asyn­chro­nous rota­tion of Earth-mass plan­ets in the hab­it­able zone of low­er-mass stars” pub­lished today in Sci­ence Express. The work was sup­port­ed by grants from the Nat­ur­al Sci­ences and Engi­neer­ing Research Coun­cil of Cana­da.

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For more infor­ma­tion, con­tact:

Sean Bet­tam
Com­mu­ni­ca­tions Offi­cer
Fac­ul­ty of Arts and Sci­ence
416–946-7950
s.bettam@utoronto.ca