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Moon systems, not planets, may be place to find life

Alien life might be both and ea­sier and more in­ter­est­ing to find through a new strategy, a study sug­gests

Feb. 19, 2008
Special to World Science  

Many sci­en­tists in search of alien life pur­sue the ob­vi­ous strat­e­gy: look for po­ten­tially hab­it­a­ble plan­ets. But a new study suggests an­oth­er plan might work as well or bet­ter, and perhaps yield more in­tri­guing re­sults. 

As­tro­no­mers should study gi­ant plan­ets that, while un­inhab­it­a­ble, lie in ar­eas suitably warm for life, the authors say. These may turn out to have moons—some of which might in­deed sup­port life.

Artist's concept of a moon near a Jupiter-like planet. (Courtesy NASA/JPL)


One ad­van­tage of this ap­proach, the re­search­ers ar­gue, is that big worlds are far eas­i­er to find than the small, Earth-like ones on which most hab­it­a­ble-pla­net searches fo­cus. Un­like the Earth-like breed, sev­er­al suitably warm gi­ants, dubbed “tem­per­ate Ju­pi­ters,” have been re­ported found al­ready.

Such bod­ies can “act as ‘sign-posts’ for fu­ture stud­ies look­ing for po­ten­tially hab­it­a­ble worlds,” said the Uni­ver­s­ity of Flor­i­da’s Scott Flem­ing, lead auth­or of the stu­dy.

If their moons do harbor life forms, these might even be more in­ter­est­ing in some ways than their plan­et-dwel­ling coun­ter­parts.

For in­stance, if one moon of a dis­tant “Ju­pi­ter” had life, so might oth­ers, as they would all lie in the same gen­er­al tem­per­a­ture zone. If ad­vanced enough, the in­hab­itants might even reg­u­larly ex­ploit the proxim­ity among moons to trav­el or com­mu­ni­cate among them. Or, bi­zarre ev­o­lu­tion­ary ex­pe­ri­ments might arise when me­te­ors ran­domly plop small or­gan­isms from one moon on­to an­oth­er.

On the oth­er hand, a large percentage of moons may be hos­tile to life, for in­stance be­cause they’re too small to hold at­mo­spheres. But giv­en ap­pro­pri­ate con­di­tions, life, trav­el and com­mu­nica­t­ion among a “tem­per­ate Ju­pi­ter’s” moons “would in­deed be very pos­si­ble,” said Flem­ing, a doctoral stu­dent. His re­search on de­tec­tion of “tem­per­ate Ju­pi­ters,” with col­leagues at the Space Te­le­s­cope Science In­sti­tute in Baltimore and at other ins­ti­tu­tions, is to ap­pear in a fu­ture is­sue of the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty. The paper is also on­line here.

One “tem­per­ate Ju­pi­ter” re­ported found last year or­bits the star HD 75898, which is vis­i­ble with bin­oc­u­lars in the faint north­ern con­stella­t­ion Lynx. Tech­no­logy used to date has gen­erally been in­ca­pable of find­ing moons of such pla­nets. But Flem­ing said this could change next year with the launch of a NA­SA sa­tel­lite, Kep­ler: its instru­ments can de­tect both Earth-sized planets and moons, which would re­veal themselves through their gra­vi­ta­tional pulls on the planets. But there are con­cerns that Kep­ler’s mis­sion will be too short to find many moons, Flem­ing added.

Ju­pi­ter-like plan­ets, re­gard­less of tem­per­a­ture zone, are con­sid­ered un­inhab­it­a­ble them­selves, in part be­cause they’re made com­pletely of gas.

Yet if our So­lar Sys­tem is any guide, they’re rich in moons. Ju­pi­ter has 61 known moons. These range from the big­gest known—a­bout Mer­cury’s size, some 5,300 km (3,300 miles) wide—to chunks 2 km (1.2 miles) across, too small to even pull them­selves in­to round shapes with their gra­vity. Sat­urn, which is a bit like a some­what smaller Ju­pi­ter with rings, has 31 known moons, in a si­m­i­lar size range. In fact, most plan­ets in our sys­tem have moons, so as­tro­no­mers sup­pose the same would be true else­where.

Any residents of a Ju­pi­ter-like plan­et’s moon might find that trav­el to oth­er moons is rath­er easy com­pared to interplan­e­tary trav­el. The smaller size of moons greatly re­duces the en­er­gy re­quire­ments. For in­stance, it takes less than one-twentieth the en­er­gy to leave our moon as it does to leave Earth. As for com­mu­nica­t­ions among moons—it would take about three min­utes for ra­dio or light waves to trav­el be­tween the two furthest-apart of Ju­pi­ter’s moons. Res­i­dents of a multi-lu­nar sys­tem, Flem­ing said, could li­terally all tune in to the same ra­dio sta­tion.
 

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In search of places where alien life may lurk, many scientists pursue the obvious strategy: looking for potentially habitable planets. But some astronomers say another plan might work as well or better: study giant planets that, while uninhabitable, lie in areas suitably warm for life. These may well turn out to have dozens of moons—some of which might indeed support life. Giant planets are also far easier to find than the small, Earth-like ones on which many searches for signs of habitability have focused. Unlike these, several of the suitably warm giants, dubbed “temperate Jupiters,” have been reported found already. Such bodies can “act as ‘sign-posts’ for future studies looking for potentially habitable worlds,” said University of Florida astronomer Scott Fleming. If life forms inhabit their moons, these might even be more intriguing than planetary creatures. For instance, if one moon of a distant “Jupiter” had life, so might others, as they would all lie in the same general temperature zone. If advanced enough, the inhabitants might even regularly exploit the proximity among moons to travel or communicate among them. Or, bizarre evolutionary experiments might arise when meteors randomly drop small organisms from one moon on another. On the other hand, many moons may be hostile to life, for instance because they’re too small to hold atmospheres. But given appropriate conditions, life, travel and communication among a temperate Jupiter’s moons “would indeed be very possible,” said Fleming. His research on detection of “temperate Jupiters,” with colleagues, is to appear in a future issue of the research journal Monthly Notices of the Royal Astronomical Society. One “temperate Jupiter” already reported found orbits the star HD 75898, which is visible with binoculars in the faint northern constellation Lynx. Jupiter-like planets, regardless of temperature zone, are considered uninhabitable themselves, in part because they’re made completely of gas. Yet if our Solar System is any guide, they’re rich in moons. Jupiter has 61 known moons. These range from the biggest known—about Mercury’s size, some 5,300 km (3,300 miles) wide—to mere chunks 2 km (1.2 miles) across, too small to even pull themselves into round shapes with their gravity. Saturn, which is like a somewhat smaller Jupiter, has 31 known moons, in a similar size range. In fact, every planet in our system has moons, so astronomers assume most others would also. Any inhabitants of a Jupiter-like planet’s moon might find that travel to other moons is rather easy compared to interplanetary travel. The smaller size of moons greatly reduces the energy requirements. For instance, it takes less than one-twentieth the energy to leave our moon than it does to leave Earth. As for communications among moons—it would take about three minutes for radio or light waves to travel between the two furthest-apart of Jupiter’s moons. An open question is whether current technology is adequate to find moons in alien solar systems. “Not easily,” Fleming said. The European Space Agency recently planned a project to do just that, though; but it was canceled in 2003 for lack of funds. Dubbed Eddington, the mission involved first finding planets around other suns, then detecting details of the planets’ motions that would betray the gravitational tug of moons.