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January 25, 2013

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For signs of life, some strange planetary systems may be most promising

Jan. 25, 2013
Special to World Science  

To detect signs of life on dis­tant plan­ets, some of the strang­est plan­e­tary sys­tems may be the best bet, two sci­en­tists pro­pose.

The plan­ets in ques­tion are ones whose “sun” is not much fur­ther from them than our moon is to us.

These host stars would be “white dwarfs,” or “ex­pired” stars that have be­come dras­tic­ally shrunk­en ver­sions of their form­er selves. But they are still hot and lin­ger for bil­lions of years that way, cool­ing very slow­ly. Such con­sid­er­ations led the as­tron­o­mer Er­ic Agol Uni­vers­ity of Wash­ing­ton, Se­at­tle, to pro­pose two years ago that white dwarfs are good places to look for hab­it­a­ble plan­ets.

To get warmth suit­a­ble for liq­uid wa­ter and per­haps life, it’s es­ti­mat­ed that a plan­et near one of these hum­bled stars would have to cir­cle it so closely that it lies with­in the form­er bound­aries of the star it­self. Thus it would be a plan­et that mi­grat­ed to­ward the star af­ter its de­mise as an or­di­nary star. Plan­et migra­t­ion is be­lieved to be fairly com­mon.

Agol cal­cu­lat­ed that in such a sce­nar­i­o, the dis­tance be­tween a white dwarf and a hab­it­a­ble com­pan­ion world would be be­tween twice, and eight times the Earth-moon dis­tance. This hab­it­a­ble state, he added, could per­sist at least three bil­lion years, two-thirds the cur­rent age of Earth. 

The weird ar­range­ment could al­so cre­ate an ad­van­tage for Earth­lings hop­ing to de­tect chem­i­cal res­i­due that points to life, ac­cord­ing to as­t­ro­phys­i­cists Abra­ham Loeb of Har­vard Uni­vers­ity and Dan Maoz of Tel Aviv Uni­vers­ity, who have con­ducted a fol­low­up stu­dy.

To de­tect such chem­i­cals, as­tron­o­mers want to look at star­light that passes through a plan­et’s at­mos­phere. The way the star­light changes after going through re­veals what is in that at­mos­phere. So as­tron­o­mers look for plan­e­tary “tran­sits,” events in which the plan­et passes in front of the star from our point of view.

Be­cause a white dwarf star is very small, an Earth-sized plan­et pass­ing in front of it would af­fect the vis­i­ble star­light in a way that would­n’t oc­cur for an or­di­nar­y-sized star, ac­cord­ing to Loeb and Maoz. As a result, chem­i­cals in the at­mos­phere that be­tray life forms be­low, such as ox­y­gen, would come in­to great­er re­lief for our tele­scopes. 

Earth-sized plan­ets or­bit­ing white dwarfs at dis­tances suit­a­ble for life, may there­fore “of­fer the best prospects for de­tecting bio-signatures with­in the com­ing decade,” the pair wrote in a re­search pa­per. Plan­ets pass­ing in front of white dwarfs will “en­joy a much high­er con­trast of their at­mos­pher­ic trans­mis­sion sig­nal above the back­ground light of their host stars.”

The re­port is posted on­line and has been sub­mit­ted for con­sid­era­t­ion to the jour­nal Monthly No­tices of the Roy­al As­tronomical So­ci­e­ty.

For most types of plan­e­tary sys­tems, it will be a problem to de­tect these chem­i­cal “biomark­ers,” or in­di­ca­tors of pos­si­ble life, the re­search­ers wrote. But they pro­pose that con­duct­ing such a search with white dwarfs would be fea­si­ble us­ing the James Webb Space Tel­e­scope, planned as the suc­ces­sor to the Hub­ble Space Tel­e­scope. NASA cur­rently aims to launch the new sat­el­lite in 2018.

Loeb and Maoz es­ti­mate that the Webb tel­e­scope would have to be trained on a white dwarf through­out 160, two-minute pas­sages of a plan­et in front of it in or­der to pick up enough da­ta to dis­cern the biomarker con­sid­ered the most prom­is­ing, at­mos­pher­ic ox­y­gen. This gas mostly comes from land plants and would dis­ap­pear with­in a mil­lion years or so “if all life on Earth ceased,” they not­ed. 

Some oth­er at­mos­pher­ic in­gre­di­ents, such as wa­ter, will be eas­i­er to de­tect and are al­so po­ten­tial sig­nals of life be­low, they added. Dur­ing a typ­i­cal “tran­sit,” or pas­sage of the plan­et in front of the star, Loeb and Maoz es­ti­mate that the plan­et would cov­er up about half the white dwar­f’s sur­face as seen from far away.

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To look for signs of life around distant planets, some of the strangest planetary systems may be the best, two scientists propose. Some of the most promising environments for the search, they say, could be planets whose “sun” is not much further from them than our Moon is to us. These suns would be “white dwarfs,” or “expired” stars that have become drastically shrunken versions of their former selves. But they are still hot and linger for billions of years that way, cooling very slowly. This has led the astronomer Eric Agol University of Washington, Seattle, to propose two years ago that white dwarf stars are suitable places to look for habitable planets. His study on the subject was published in the April 20, 2011 issue of the journal Astrophysical Journal Letters. But there was more to come. To get warmth suitable for liquid water and perhaps life, it’s estimated that a planet near one of these humbled stars would have to circle so close by it that it lies within the former boundaries of the star itself. Thus it would be a planet that migrated toward the star after its demise as an ordinary star. Planet migration is believed to be fairly common. Agol calculated that in such a scenario, the distance between a white dwarf and a habitable companion world would be between twice and eight times the Earth-moon distance. This habitable state, he added, could persist at least three billion years, two-thirds the current age of Earth. The weird arrangement could also create an advantage for Earth-bound sky-gazers hoping to detect chemical residue that points to life, according to astrophysicists Abraham Loeb of Harvard University and Dan Maoz of Tel Aviv University, who have conducted a followup study. To detect such chemicals, astronomers want to look at starlight that passes through a planet’s atmosphere. The way the starlight changes as a result reveals what’s in that atmosphere. So astronomers look for planetary “transits,” events in which the planet passes in front of the star from our point of view. Because a white dwarf star is very small, an Earth-sized planet passing in front of it would affect the visible starlight in a way that wouldn’t occur for an ordinary-sized star, according to Loeb and Maoz. As a rsult, chemicals in the atmosphere that betray life forms below, such as oxygen, would come into greater relief for our telescopes. Earth-sized planets orbiting white dwarfs at distances suitable for life, may therefore “offer the best prospects for detecting bio-signatures within the coming decade,” the pair wrote in a research paper. Planets passing in front of white dwarfs will “enjoy a much higher contrast of their atmospheric transmission signal above the background light of their host stars.” The report is posted online and has been submitted for consideration to the journal Monthly Notices of the Royal Astronomical Society. For most types of planetary systems, it will be difficult to impossible to detect these chemical “biomarkers,” or indicators of possible life, the researchers wrote. But they propose that conducting such a search with white dwarfs would be feasible using the James Webb Space Telescope, planned as the successor to the Hubble Space Telescope. NASA currently aims to launch the new satellite in 2018. Loeb and Maoz estimate that the Webb telescope would have to be trained on a white dwarf throughout 160, two-minute passages of a planet in front of it in order to pick up enough data to discern the likely most promising biomarker, atmospheric oxygen. This gas mostly comes from land plants and would disappear within a million years or so “if all life on Earth ceased,” they noted. Some other atmospheric ingredients, such as water, will be easier to detect and are also potential signals of life below, they added. During a typical “transit,” or passage of the planet in front of the star, Loeb and Maoz estimate that the planet would cover up about half the white dwarf’s surface as seen from our distance. most promising