Sept. 10, 2007
Special to World Science
A strange breed of plan?et, de?void of any sun yet able to sus?tain life, may soon be de?tecta?ble, a study has con?clud?ed.
A sci?ent?ist first suggested the ex?ist?ence of these gloomy though habita?ble bod?ies in 1999, but held out mea?ger chances that they could be de?tected with in?stru?ments then availa?ble.
In 1977, sci?en?tists dis?cov?ered com?mu?ni?ties of or?gan?isms miles be?low the ocean sur?face. They draw nour?ish?ment from hot, mineral-rich wa?ter welling up from the ocean floor. Re?search?ers have spec?u?lat?ed that si?m?i?lar en?er?gy sources from deep with?in a plan?et could sus?tain life on a world with no sun. (Cour?te?sy NA?SA)
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Tech?no?log?i?cal ad?vanc?es are im?prov?ing the out?look, how?ev?er, a new study has found. It al?so pro?poses that such plan?ets could gain ad?di?tion?al heat for life through the pres?ence of a moon, whose gravita?t?ional force would partly be con?vert?ed in?to heat early in the plan?et?s ev?o?lu?tion.
This, com?bined with a plan?et?s own in?ter?nal heat, ?can pro?vide the con?di?tions ne?ces?sary for liq?uid wa?ter to per?sist? on a roughly Earth-sized plan?et, ?mak?ing it a po?ten?tial site for life,? sci?ent?ists wrote in a pa?per ac?cept?ed for pub?lica?t?ion in the re?search jour?nal As?t?ro?phys?i?cal Jour?nal Let?ters. A draft of the pa?per is al?so posted on?line.
The study ex?am?ined the pos?si?bil?ity that a roughly Earth-sized plan?et would be tossed out of a young so?lar sys?tem, along with its moon. This could hap?pen in a close en?coun?ter with a gas gi?ant plan?et like Sat?urn or Ju?pi?ter.
The gravita?t?ional force of these ob?jects can un?der cer?tain cir?cum?stances kick smaller bod?ies out in?to open space, send?ing them off to wan?der the cos?mos alone and sun?less.
But not nec?es?sarily un?de?tecta?ble, wrote the re?search?ers, John Deb?es of the Car?ne?gie In?sti?tute of Wash?ing?ton, D.C. and Steinn Sig?urds?son of Penn State Un?ivers?ity in Un?ivers?ity Park, Penn. The plan?ets, they pro?posed, could be found us?ing a space tel?e?scope that de?tects ce?les?tial ob?jects through their abil?ity to slightly bend pass?ing light with their gra?vity. The phe?nom?e?non is called mi?cro?lens?ing.
An in?stru?ment with the req?ui?site ca?pa?bil?i?ties was pro?posed as ?fea?si?ble? in a pa?per in the Au?gust 2002 is?sue of The As?t?ro?phys?i?cal Jour?nal, but has?n?t been built yet.
Based on some reasona?ble as?sump?tions, Deb?es and Sig?urds?son wrote, such a tel?e?scope could de?tect up to two drift?ing plan?et-moon pairs, around the size of Earth and its moon, with a good chance of be?ing habita?ble. At a min?i?mum, they added, this type of mis?sion has a one-in-ten chance of de?tecting at least one such sys?tem.
The real like?li?hood of de?tecting or?phaned, habita?ble worlds is bet?ter than these es?ti?mates sug?gest, they con?tin?ued, partly be?cause the es?ti?mates omit two types of sys?tems that could in?clude such ob?jects. One type is a plan?et-moon pair whose plan?et is a gas gi?ant. The oth?er is a plan?et with no part?ner; the orig?i?nal study sev?en years ago sug?gested such an abode could be habita?ble even with?out a moon.
In that stu?dy, pub?lished the July 1, 1999 is?sue of the jour?nal Na?ture, Dave Ste?venson of the Cal?i?for?nia In?sti?tute of Tech?nol?o?gy in Pas?a?de?na, Calif., laid out the bas?ic rea?sons why a sun?less plan?et might har?bor life. Ste?venson rea?soned that such bod?ies are most likely to be tossed out of their so?lar sys?tem while it?s still form?ing. Dur?ing this pe?ri?od, a so?lar sys?tem is be?lieved to be per?me?at?ed with hy?dro?gen gas. The plan?et would thus re?tain a dense hy?dro?gen at?mos?phere which would act as a heat-trapping blan?ket, like the green?house gas?es be?lieved to be re?spon?si?ble for glob?al warm?ing on Earth.
Com?bined with heat from the nat?u?ral ra?dioac?ti?vity in?side an Earth-like plan?et, this should al?low enough heat build?up to sus?tain liq?uid wa?ter, Ste?venson wrote. His work fol?lowed pre?vi?ous re?search hint?ing that such ra?dioac?ti?vity might keep water liq?uid with?in Ju?pi?ter?s moon Eu?ro?pa.
To these ideas, Deb?es and Sig?urds?son add the no?tion that ?ti?dal heat?ing? from a moon could pro?vide even more warmth for an or?phaned plan?et. Tid?al heat?ing oc?curs when the gra?vity of one ob?ject ex?erts stresses on anoth?er, cre?at?ing mo?tions that heat it up.
Tid?al heat?ing grad?u?ally di?min?ishes as the moon?s or?bit slows down, the re?search?ers not?ed, but it could still more than dou?ble the ef?fects of a plan?et?s in?ter?nal heat?ing for a few hun?dred mil?lion years?pro?vid?ing cru?cial warmth to jump-start an ori?gin of life. Or?gan?isms could lat?er adapt to grad?u?ally fall?ing tem?per?a?tures, they wrote.
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