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September 23, 2009

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Exotic life forms: looking for life as we don’t know it

Sept. 23, 2009
Courtesy Europlanet Media Centre
and World Science staff

Sci­en­tists at a new re­search in­sti­tute are work­ing to find out how life might evolve us­ing chem­i­cals not found in Earth-based life forms. 

They’re stu­dy­ing how organ­isms might emp­loy al­ter­na­tive sol­vents—that is, oth­er liq­uids that could play the role that wa­ter does in fa­mil­iar life forms.

Bodies of liquid on Ti­tan, a moon of Sa­turn. (Cour­tesy NA­SA)
The Un­ivers­ity of Vi­en­na es­tab­lished the re­search group Al­ter­na­tive Sol­vents as a Ba­sis for Life Sup­port­ing Zones in (Exo-)Plan­e­tary Sys­tems last May un­der the lead­er­ship of as­tron­o­mer Ma­ria Firneis. Re­search by the group was pre­sented at the Eu­ro­pe­an Plan­e­tary Sci­ence Con­gress in Pots­dam, Ger­ma­ny on Sept. 18.

Tra­di­tion­ally, plan­ets that might sus­tain life are sought in “hab­it­able zone,” the re­gions around stars in which Earth-like plan­ets with car­bon di­ox­ide, wa­ter va­pour and ni­tro­gen at­mo­spheres could main­tain liq­uid wa­ter on their sur­faces.

Sci­en­tists have been seek­ing chem­i­cal sig­na­tures pro­duced by ex­tra­ter­res­tri­al life with metabolisms re­sem­bling the ter­res­tri­al ones, where the build­ing blocks of life, ami­no ac­ids, are based on car­bon and ox­y­gen dis­solved in wa­ter.

But “it can­not be ruled out that life forms have evolved some­where that nei­ther rely on wa­ter nor on a car­bon- and ox­y­gen-based metabolis­m,” said re­search group mem­ber Jo­han­nes Leit­ner. “It is time to make a rad­i­cal change in our pre­s­ent ‘geo­cen­tric’ mind­set.”

A life-sup­porting sol­vent must re­main liq­uid over a large tem­per­a­ture range. Wa­ter is liq­uid be­tween 0 and 100 de­grees Cel­si­us, but some oth­er sol­vents are liq­uid over more than 200 de­grees. Such a sol­vent would al­low an ocean on a plan­et clos­er to the cen­tral star, re­search­ers say. 

The re­verse sce­nar­i­o is al­so pos­si­ble – a liq­uid ocean of am­mo­nia could ex­ist much fur­ther from a star. Fur­ther­more, sul­phu­ric ac­id can be found with­in the cloud lay­ers of Ve­nus and lakes of meth­ane or eth­ane cov­er parts of the sur­face of the Sa­tur­ni­an moon Ti­tan.

The re­search group, with in­terna­t­ional col­la­bo­ra­tors, plans to study the prop­er­ties of a range of sol­vents oth­er than wa­ter, in­clud­ing their abun­dance in space, ther­mal and bio­chem­i­cal char­ac­ter­is­tics as well as their abil­ity to sup­port the or­i­gin and ev­o­lu­tion of life-sup­porting metabolisms. Al­though known most exoplan­ets, or plan­ets out­side our so­lar sys­tem, are com­posed of gas, “it is a mat­ter of time un­til smaller, Earth-size exoplan­ets are discov­ered,” said Leit­ner.

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Scientists at a new research institute are working to uncover how life might evolve using chemicals not used by to Earth-based life forms. They’re studying how life forms might make use of alternative solvents—that is, other liquids that could play the role that water does in familiar life forms. The University of Vienna established the research group Alternative Solvents as a Basis for Life Supporting Zones in (Exo-)Planetary Systems last May under the leadership of astronomer Maria Firneis. Research by the group was presented at the European Planetary Science Congress in Potsdam, Germany on Sept. 18. Traditionally, planets that might sustain life are sought in “habitable zone,” the regions around stars in which Earth-like planets with carbon dioxide, water vapour and nitrogen atmospheres could maintain liquid water on their surfaces. Scientists have been seeking chemical signatures produced by extraterrestrial life with metabolisms resembling the terrestrial ones, where the building blocks of life, amino acids, are based on carbon and oxygen dissolved in water. But “it cannot be ruled out that life forms have evolved somewhere that neither rely on water nor on a carbon- and oxygen-based metabolism,” said research group member Johannes Leitner. “It is time to make a radical change in our present ‘geocentric’ mindset for life as we know it.” A life-supporting solvent must remain liquid over a large temperature range. Water is liquid between 0 and 100 degrees Celsius, but some other solvents are liquid over more than 200 degrees. Such a solvent would allow an ocean on a planet closer to the central star, researchers say. The reverse scenario is also possible – a liquid ocean of ammonia could exist much further from a star. Furthermore, sulphuric acid can be found within the cloud layers of Venus and lakes of methane or ethane cover parts of the surface of the Saturnian satellite Titan. The research group, with international collaborators, plans to study the properties of a range of solvents other than water, including their abundance in space, thermal and biochemical characteristics as well as their ability to support the origin and evolution of life supporting metabolisms. Although known most exoplanets, or planets outside our solar system, are composed of gas, “it is a matter of time until smaller, Earth-size exoplanets are discovered,” said Leitner.