Detecting biomarkers on faraway planets

Sep 10, 2013
After more than four years of observations using the most successful low-mass exoplanet hunter in the world, the HARPS spectrograph attached to the 3.6-metre ESO telescope at La Silla, Chile, astronomers have discovered in this system the lightest exoplanet found so far: Gliese 581e (foreground) is only about twice the mass of our Earth. The Gliese 581 planetary system now has four known planets, with masses of about 1.9 (planet e, left in the foreground), 16 (planet b, nearest to the star), 5 (planet c, centre), and 7 Earth-masses (planet d, with the bluish colour). The planet furthest out, Gliese 581d, orbits its host star in 66.8 days, while Gliese 581 e completes its orbit in 3.15 days. Credit: ESO/L. Calçada

( —On Earth, life leaves telltale signals in the atmosphere. Photosynthesis is ultimately responsible for the high oxygen levels and the thick ozone layer. Microbes emit methane and nitrous oxide into the atmosphere, and seaweeds emit chloromethane gas. These chemicals, when present in sufficient quantities, are indicators of life and are known as atmospheric biomarkers. Detecting them in the atmosphere of an exoplanet should, in theory, be a means of discovering whether life exists on any alien worlds.

While have never been spotted in observations of an , because their signal is so faint, the new generation of telescopes being planned today, such as the European Extremely Large Telescope, may be sensitive enough to detect them. New research presented to the European Planetary Science Congress at UCL by Lee Grenfell (DLR) 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 explains. "To do this, 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 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 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 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 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 an extremely promising one for detecting potential signs of life on .

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?"

This research has been submitted to the journal Planetary & Space Science (2013) "Planetary Evolution and Life" Special Issue.

Explore further: Novel technique boosts hunt for water on planets around other stars

Provided by European Planetary Science Congress

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5 / 5 (7) Sep 10, 2013
Can't wait for ESO's, ESPRESSO instrument to follow up on HARPS world leading success by 2016. Able to accurately detect earth sized planets in earth sized orbits with ESPRESSO is a real revolution. Together with the current installation of ESO's revolutionary SPHERE planet finding instrument this will be big for getting closer to finding alien life.

But also can`t wait for the European Sace Agency GAIA to launch just 2 months from now. What a revolution that is as well. Calculations show it could at the very least detect all Jupiter sized planets within 600 light years. We are talking about at least tens of thousands of planets it could find. Building the first large and accurate space maps of planets within our galaxy region.

Then we have future telescopes like the European Extremely Large Telescope that will even succeed on that. Amazing time to be alive. To explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before.
5 / 5 (3) Sep 10, 2013
What I would like to see is an Earth-orbit sized system of interferometers that would be able to directly image other planets. Seeing something akin to city lights would not only answer the question of alien life, but intelligence as well.
5 / 5 (3) Sep 11, 2013
What I would like to see is an Earth-orbit sized system of interferometers that would be able to directly image other planets. Seeing something akin to city lights would not only answer the question of alien life, but intelligence as well.

The problem with that is synchronising the timing to a fraction of a wavelength between the components, something that is well beyond our current technology.
5 / 5 (1) Sep 11, 2013
something that is well beyond our current technology.

Yep, but I'd still like to see it. =)

Hm. If we ever get the quantum teleportation thing from 100 km to a couple of AU's, that could get the timing in sync...
4.5 / 5 (2) Sep 14, 2013
something that is well beyond our current technology.

Yep, but I'd still like to see it. =)

Hm. If we ever get the quantum teleportation thing from 100 km to a couple of AU's, that could get the timing in sync...

Yes, that would indeed be excellent. But as excited as I am about the search for extraterrestrial life, it may be good if such earth-shattering revelations were not made with the current tenuous state of global affairs. At that point, new millennial sects would be formed and rapidly find a following, the existing religious sects that postulate that the end of civilization is soon to arrive (Jehovah's Witnesses, LDS, et cetera) and there would be general chaos in the streets, the souring of diplomatic relations, and whatnot.

I do sincerely hope that our first interaction with extraterrestrial life is with something that can easily be studied, such as bacteria and tube worms from Europa, rather than with the discovery of city lights on an alien planet.

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