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Clouds may make many more planets livable than thought

July 1, 2013
Courtesy of 
Steve Koppes/University of Chicago,
Megan Fellman/ Northwestern University
and World Science staff

A new study dou­bles the es­ti­mat­ed num­ber of po­ten­tially hab­it­a­ble plan­ets or­bit­ing red dwarfs—the most com­mon type of stars—by con­sid­er­ing how clouds af­fect those plan­ets.

The find­ing means that in our Milky Way gal­axy alone, 60 bil­lion plan­ets may be cir­cling these faint lit­tle stars at dis­tances suit­a­ble for life, as­tro­no­mers say. 

The re­search­ers at the Uni­vers­ity of Chi­ca­go and North­west­ern Uni­vers­ity in Il­li­nois car­ried out com­put­er sim­ula­t­ions of cloud be­hav­ior for their stu­dy, pub­lished in the July 10 is­sue of the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters

Da­ta from NASA’s Kep­ler space ob­serv­a­to­ry sug­gest each red dwarf has about one Earth-size plan­et in its “hab­it­a­ble zone”—the ar­ea around the star where plan­et tem­per­a­tures would be suit­a­ble for liq­uid sur­face wa­ter. 

The new study dou­bles that num­ber be­cause it in­di­cates clouds act as a temperature-stabilizing “ther­mostat,” whose ef­fects are most pro­nounced where it would be oth­er­wise too hot.

The sim­ula­t­ions found that if a plan­et has sur­face wa­ter, clouds re­sult. These will cool down plan­ets in the in­ner por­tion of the hab­it­a­ble zone, where it’s rel­a­tively hot, en­a­bling plan­ets to sus­tain sur­face wa­ter much clos­er to their sun than oth­er­wise.

The for­mu­la for cal­cu­lat­ing the hab­it­a­ble zone of al­ien plan­ets has re­mained much the same for dec­ades. But the for­mu­la largely ne­glects clouds.

“Most of the plan­ets in the Milky Way or­bit red dwarfs,” said co-author Ni­co­las Cowan of North­west­ern. “A ther­mo­stat that makes such plan­ets more clem­ent means we don’t have to look as far [from the star] to find a hab­it­a­ble plan­et.”

Because red dwarfs are faint, plan­ets need to orbit them much clos­er than the Earth does our sun to get the requi­site warmth. Thus, esti­mates show the year on a red-dwarf orbit­ing, habit­able plan­et would last a mere month or two in Earth time.


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A new study doubles the estimated number of potentially habitable planets orbiting red dwarfs—the most common type of stars—by considering how clouds affect those planets. The finding means that in our Milky Way galaxy alone, 60 billion planets may be circling these faint little stars at distances suitable for life, astronomers say. The researchers at the University of Chicago and Northwestern University in Illinois carried out computer simulations of cloud behavior for their study, published in the July 10 issue of the journal Astrophysical Journal Letters. Data from NASA’s Kepler space observatory suggest each red dwarf has about one Earth-size planet in its “habitable zone”—the area around the star where planet temperatures would be suitable for liquid surface water. The new study doubles that number because it indicates clouds act as a temperature-stabilizing “thermostat,” whose effects are most pronounced where it would be otherwise too hot. The simulations found that if there the planet has surface water, clouds result. These will cool down planets in the inner portion of the habitable zone, where it’s relatively hot, enabling planets to sustain surface water much closer to their sun than otherwise. The formula for calculating the habitable zone of alien planets has remained much the same for decades. But the formula largely neglects clouds. “Most of the planets in the Milky Way orbit red dwarfs,” said co-author Nicolas Cowan of Northwestern. “A thermostat that makes such planets more clement means we don’t have to look as far [from the star] to find a habitable planet.”