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Sunless but livable planets may be detectable

Sept. 10, 2007
Special to World Science  

A strange breed of plan­et, de­void of any sun yet able to sus­tain life, may soon be de­tecta­ble, a study has con­clud­ed.

A sci­ent­ist first suggested the ex­ist­ence of these gloomy though habita­ble worlds in 1999, but held out mea­ger chances that they could be de­tected with in­stru­ments then availa­ble.

In 1977, sci­en­tists dis­cov­ered com­mu­ni­ties of or­gan­isms miles be­low the ocean sur­face. They draw nour­ish­ment from hot, mineral-rich wa­ter welling up from the ocean floor. Re­search­ers have spec­u­lat­ed that si­m­i­lar en­er­gy sources from deep with­in a plan­et could sus­tain life on a world with no sun. (Cour­te­sy NA­SA)


Tech­no­log­i­cal ad­vanc­es are im­prov­ing the out­look, how­ev­er, a new study has found. It al­so pro­poses that such plan­ets could gain ad­di­tion­al heat for life through the pres­ence of a moon, whose gravita­t­ional force would partly be con­vert­ed in­to heat early in the plan­et’s ev­o­lu­tion. 

This, com­bined with a plan­et’s own in­ter­nal heat, “can pro­vide the con­di­tions ne­ces­sary for liq­uid wa­ter to per­sist” on a roughly Earth-sized plan­et, “mak­ing it a po­ten­tial site for life,” sci­ent­ists wrote in a pa­per ac­cept­ed for pub­lica­t­ion in the re­search jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters. A draft of the pa­per is al­so posted on­line.

The study ex­am­ined the pos­si­bil­ity that a roughly Earth-sized plan­et would be tossed out of a young so­lar sys­tem, along with its moon. This could hap­pen in a close en­coun­ter with a gas gi­ant plan­et like Sat­urn or Ju­pi­ter. 

The gravita­t­ional force of these ob­jects can un­der cer­tain cir­cum­stances kick smaller bod­ies out in­to open space, send­ing them off to wan­der the cos­mos alone and sun­less.

But not nec­es­sarily un­de­tecta­ble, wrote the re­search­ers, John Deb­es of the Car­ne­gie In­sti­tute of Wash­ing­ton, D.C. and Steinn Sig­urds­son of Penn State Un­ivers­ity in Un­ivers­ity Park, Penn. The plan­ets, they pro­posed, could be found us­ing a space tel­e­scope that de­tects ce­les­tial ob­jects through their abil­ity to slightly bend pass­ing light with their gra­vity. The phe­nom­e­non is called mi­cro­lens­ing.

An in­stru­ment with the req­ui­site ca­pa­bil­i­ties was pro­posed as “fea­si­ble” in a pa­per in the Au­gust 2002 is­sue of The As­t­ro­phys­i­cal Jour­nal, but has­n’t been built yet.

Based on some reasona­ble as­sump­tions, Deb­es and Sig­urds­son wrote, such a tel­e­scope could de­tect up to two drift­ing plan­et-moon pairs, around the size of Earth and its moon, with a good chance of be­ing habita­ble. At a min­i­mum, they added, this type of mis­sion has a one-in-ten chance of de­tecting at least one such sys­tem.

The real like­li­hood of de­tecting or­phaned, habita­ble worlds is bet­ter than these es­ti­mates sug­gest, they con­tin­ued, partly be­cause the es­ti­mates omit two types of sys­tems that could in­clude such ob­jects. One type is a plan­et-moon pair whose plan­et is a gas gi­ant. The oth­er is a plan­et with no part­ner; the orig­i­nal study sev­en years ago sug­gested such an abode could be habita­ble even with­out a moon.

In that stu­dy, pub­lished the July 1, 1999 is­sue of the jour­nal Na­ture, Dave Ste­venson of the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy in Pas­a­de­na, Calif., laid out the bas­ic rea­sons why a sun­less plan­et might har­bor life. Ste­venson rea­soned that such bod­ies are most likely to be tossed out of their so­lar sys­tem while it’s still form­ing. Dur­ing this pe­ri­od, a so­lar sys­tem is be­lieved to be per­me­at­ed with hy­dro­gen gas. The plan­et would thus re­tain a dense hy­dro­gen at­mos­phere which would act as a heat-trapping blan­ket, like the green­house gas­es be­lieved to be re­spon­si­ble for glob­al warm­ing on Earth.

Com­bined with heat from the nat­u­ral ra­dioac­ti­vity in­side an Earth-like plan­et, this should al­low enough heat build­up to sus­tain liq­uid wa­ter, Ste­venson wrote. His work fol­lowed pre­vi­ous re­search hint­ing that such ra­dioac­ti­vity might keep water liq­uid with­in Ju­pi­ter’s moon Eu­ro­pa.

To these ideas, Deb­es and Sig­urds­son add the no­tion that “ti­dal heat­ing” from a moon could pro­vide even more warmth for an or­phaned plan­et. Tid­al heat­ing oc­curs when the gra­vity of one ob­ject ex­erts stresses on anoth­er, cre­at­ing mo­tions that heat it up.

Tid­al heat­ing grad­u­ally di­min­ishes as the moon’s or­bit slows down, the re­search­ers not­ed, but it could still more than dou­ble the ef­fects of a plan­et’s in­ter­nal heat­ing for a few hun­dred mil­lion years—pro­vid­ing cru­cial warmth to jump-start an ori­gin of life. Or­gan­isms could lat­er adapt to grad­u­ally fall­ing tem­per­a­tures, they wrote.


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A strange breed of planet devoid of any sun, yet able to sustain life, may soon be detectable, a study has concluded. A scientist first proposed the existence of these gloomy though habitable bodies in 1999, but held out meager chances that they could be detected with instruments then available. Technological advances are improving the outlook, however, a new study has found. It also proposes that such planets could gain additional heat for life through the presence of a moon, whose gravitational force would partly be converted into heat early in the planet’s evolution. This, combined with a planet’s own internal heat, “can provide the conditions necesary for liquid water to persist” on a roughly Earth-sized planet, “making it a potential site for life,” researchers wrote in a paper accepted for publication in the research journal Astrophysical Journal Letters. A draft of the paper is also published online. The study examined the possibility that a roughly Earth-sized planet would be tossed out of a young solar system, along with its moon. This could happen in a close encounter with a gas giant planet like Saturn or Jupiter. The gravitational force of these objects can under certain circumstances “slingshot” smaller bodies out into open space, sending them off to wander the cosmos alone and sunless. But not necessarily undetectable, wrote the researchers, John Debes of the Carnegie Institute of Washington, D.C. and Steinn Sigurdsson of Penn State University in University Park, Penn. The planets, they proposed, could be found using a space telescope that would detect celestial objects through their ability to slightly bend passing light, a phenomenon called microlensing, An instrument with the requisite capabilities was proposed as “feasible” in a paper in the August 2002 issue of the Astrophysical Journal, but hasn’t been built yet. Based on some reasonable assumptions, Debes and Sigurdsson wrote, such a telescope could detect up to two drifting planet-moon pairs, around the size of Earth and its moon, with a good chance of being habitable. At a minimum, they added, this type of mission has a one-in-ten chance of detecting at least one such system. The real likelihood of detecting orphaned, habitable worlds are better than these estimates suggest, they continued, partly because the estimates omit two types of systems that could include such objects. One type is a planet-moon pair whose planet is a gas giant. The other is a planet with no partner; the original study seven years ago suggested such an abode could be habitable even without a moon. In that study, published the July 1, 1999 issue of the journal Nature, Dave Stevenson of the California Institute of Technology in Pasadena, Calif., laid out the basic reasons why a sunless planet might harbor life. Stevenson reasoned that such bodies are most likely to be tossed out of their solar system while it’s still forming. During this period, a solar system is believed to be permeated with hydrogen gas. The planet would thus retain a dense hydrogen atmosphere which would act as a heat-trapping blanket, like the greenhouse gases believed to be responsible for global warming on Earth. Combined with heat from the natural radioactivity inside an Earth-like planet, this should allow enough heat buildup to sustain liquid water, Stevenson wrote. His work followed previous research hinting that natural radioactivity might maintain liquid water on Jupiter’s moon Europa. To these ideas, Debes and Sigurdsson add the notion that “tidal heating” from a moon could provide even more warmth for an orphaned planet. Tidal heating occurs when the gravity of one object exerts stresses on another, creating motions that heat it up. Tidal heating gradually diminishes as the moon’s orbit slows down, the researchers noted, but it could still more than double the effects of a planet’s internal heating for a few hundred million years, providing crucial warmth to jump-start the formation of life. Organisms could later adapt to gradually falling temperatures, they wrote.