"Long before it's in the papers"
January 27, 2015


Technology for detecting those distant signs of life advances

Sept. 10, 2013
Courtesy of the Eu­ro­pe­an 
Plan­e­tary Sci­ence Con­gress
and World Science staff

On Earth, life leaves tell­tale sig­nals in the at­mos­phe­re. Pho­to­syn­the­sis, the pro­cess by which plants get en­er­gy from light, ul­ti­mately leads to the high ox­y­gen lev­els and the thick ozone lay­er. Mi­crobes emit meth­ane and ni­trous ox­ide, and sea­weeds emit chlorometh­ane gas. 

These chem­i­cals, when abun­dant enough, are in­di­ca­tors of life and are known as at­mos­pher­ic “biomark­ers.” De­tect­ing them in the at­mos­phere of a dis­tant plan­et should, in the­o­ry, help re­veal wheth­er life ex­ists there.

An art­ist's im­pres­sion of the plan­e­tary sys­tem Gliese 581 (Cour­tesy ESO)

Biomark­ers have nev­er been spot­ted in ob­serva­t­ions of an exo­plan­et, of plan­et from an­oth­er so­lar sys­tem, be­cause their sig­nal is so faint. But the new genera­t­ion of tele­scopes be­ing planned, such as the Eu­ro­pe­an Ex­tremely Large Tel­e­scope, may be sen­si­tive enough to de­tect them, sci­en­tists say. 

New re­search pre­sented to the Eu­ro­pe­an Plan­e­tary Sci­ence Con­gress at Uni­vers­ity Col­lege Lon­don this week aims to ex­plore how such biomark­ers might be de­tected.

“The main aim of our work is to as­sess the pos­si­ble range of biomarker sig­nals that might be de­tected by fu­ture tele­scopes,” Lee Gren­fell of the DLR In­sti­tute of Plan­e­tary Re­search in Ber­lin, who is pre­sent­ing the find­ings. “We de­vel­oped com­put­er mod­els of exoplan­ets which sim­u­late the abun­dances of dif­fer­ent biomark­ers and the way they af­fect the light shin­ing through a plan­et’s at­mos­phe­re.”

Chem­i­cals in a plan­et’s at­mos­phere af­fect light that passes through it, leav­ing char­ac­ter­is­tic chem­i­cal fin­ger­prints in the star’s spec­trum. Us­ing this tech­nique, as­tro­no­mers have al­ready de­duced a wealth of in­forma­t­ion about the con­di­tions pre­s­ent in (large, hot) exo­plan­ets. Biomark­ers would be de­tected in much the same way, but here the sig­nal is ex­pected to be so weak that sci­en­tists will need a sol­id un­der­stand­ing based on the­o­ret­i­cal mod­els be­fore they can hope to de­ci­pher the ac­tu­al da­ta.

“In our sim­ula­t­ions, we mod­eled an exo­plan­et si­m­i­lar to the Earth, which we then placed in dif­fer­ent or­bits around stars, cal­cu­lat­ing how the biomarker sig­nals re­spond to dif­fer­ing con­di­tions,” Gren­fell said. “We fo­cused on red-dwarf stars, which are smaller and faint­er than our Sun, since we ex­pect any biomarker sig­nals from plan­ets or­bit­ing such stars to be eas­i­er to de­tect.”

For de­tections of the biomarker ozone, the team said there seems to be a ‘Goldilocks’ ef­fect when it comes to the amount of ul­tra­vi­o­let, or UV, radia­t­ion from the star to which the plan­et is ex­posed. With weak UV radia­t­ion, less ozone is pro­duced and its de­tection is hard. Too much UV leads to in­creased heat­ing in the mid­dle at­mos­phere that de­stroys the sig­nal. At in­ter­me­diate UV, the con­di­tions are ‘just right’ for de­tecting ozone.

There are oth­er lim­ita­t­ions to the meth­od. For ex­am­ple, it assumes that any life-bearing plan­ets would be like Earth, which isn’t guar­an­teed. And sci­en­tists will have to be cer­tain that ap­par­ent biomarker sig­nals they find truly arose from life, and not from oth­er, non-living pro­cesses. Fi­nal­ly, dim red dwarf stars may not be the most suit­a­ble for life. Still, Gren­fell said, “for the first time we are reach­ing a point where se­ri­ous sci­en­tif­ic de­bate can be ap­plied to ad­dress the age-old ques­tion: are we alone?”

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On Earth, life leaves telltale signals in the atmosphere. Photosynthesis, the process by which plants get energy from light, ultimately leads to the high oxygen levels and the thick ozone layer. Microbes emit methane and nitrous oxide, and seaweeds emit chloromethane gas. These chemicals, when abundant enough, are indicators of life and are known as atmospheric “biomarkers.” Detecting them in the atmosphere of a distant planet, should, in theory, be a means of discovering whether life exists on any alien worlds. Biomarkers have never been spotted in observations of an exoplanet, of planet from another solar system, because their signal is so faint. But the new generation of telescopes being planned today, such as the European Extremely Large Telescope, may be sensitive enough to detect them, scientists say. New research presented to the European Planetary Science Congress at University College London by Lee Grenfell of the DLR Institute of Planetary Research in Berlin aims to explore how such biomarkers might be detected in future. “The main aim of our work is to assess the possible range of biomarker signals that might be detected by future telescopes,” Grenfell said. “We developed computer models of exoplanets which simulate the abundances of different biomarkers and the way they affect the light shining through a planet’s atmosphere.” Chemicals in a planet’s atmosphere affect light that passes through it, leaving characteristic chemical fingerprints in the star’s spectrum. Using this technique, astronomers have already deduced a wealth of information about the conditions present in (large, hot) exoplanets. Biomarkers would be detected in much the same way, but here the signal is expected to be so weak that scientists will need a solid understanding based on theoretical models before they can hope to decipher the actual data. “In our simulations, we modeled an exoplanet similar to the Earth, which we then placed in different orbits around stars, calculating how the biomarker signals respond to differing conditions,” Grenfell explains. “We focused on red-dwarf stars, which are smaller and fainter than our Sun, since we expect any biomarker signals from planets orbiting such stars to be easier to detect.” For detections of the biomarker ozone, the team confirms that there appears to be a ‘Goldilocks’ effect when it comes to the amount of ultraviolet, or UV, radiation from the star to which the planet is exposed. With weak UV radiation, less ozone is produced in the atmosphere and its detection is challenging. Too much UV leads to increased heating in the middle atmosphere that weakens the vertical gradient and destroys the signal. At intermediate UV, the conditions are ‘just right’ for detecting ozone. “We find that variations in the UV emissions of red-dwarf stars have a potentially large impact on atmospheric biosignatures in simulations of Earth-like exoplanets. Our work emphasizes the need for future missions to characterize the UV emissions of this type of star,” said Grenfell. There are other limitations on using this method to detect signs of life. For example, it is assuming that any life-bearing planets would be identical to Earth, which is not guaranteed. Moreover, scientists will have to be certain that apparent biomarker signals they find truly arose from life, and not from other, non-living processes. Finally, dim red dwarf stars may not be the most suitable for the onset and maintenance of life. Nevertheless, this technique is extremely promising for detecting potential signs of life on alien worlds, researchers claim. Grenfell concludes: “For the first time we are reaching a point where serious scientific debate can be applied to address the age-old question: are we alone?”