As astronomical instrumentation becomes more sophisticated, we are rapidly approaching a crossroads in the search for extraterrestrial life, according to a leading planetary scientist. It's also "inevitable" that alien life exists in the universe given the preponderance of extrasolar planets that are being discovered - it's up to us to seek out the extraterrestrial biosignatures.
These conclusions are outlined by Sara Seager, Professor of Planetary Science and Physics at the Massachusetts Institute of Technology (MIT), in a paper published in the journal Proceedings of the National Academy of Sciences on Aug. 4.
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"In the coming decade or two, we will have a lucky handful of potentially habitable exoplanets with atmospheres that can be observed in detail with the next generation of sophisticated space telescopes," writes Seager, pointing out that NASA's James Webb Space Telescope (JWST) and a planned direct-imaging space telescope will be able to seek out biosignatures (i.e. chemicals created by extraterrestrial biology) in the atmospheres of nearby exoplanets. The JWST is set for launch in 2018.
"Life can be inferred by the presence of atmospheric biosignature gases - gases produced by life that can accumulate to detectable levels in an exoplanet atmosphere," she writes.
To date, a handful of exoplanetary atmospheres have been studied through the analysis of their host star's light passing through their atmospheres. As an alien world orbits its star, from our perspective, it may block some of the starlight from view and be registered as a "transit." The transit method is used by NASA's Kepler space telescope and has so far confirmed the detection of hundreds of exoplanets. But this method can also help us analyze the chemicals contained in exoplanetary atmospheres.
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During a transit, if that exoplanet has an atmosphere, some of the starlight is filtered through its atmosphere. Some wavelengths of that light are absorbed by specific chemicals, leaving a spectroscopic ‘fingerprint' in the starlight we detect. Although only the largest class of exoplanets have so far had their atmospheres analyzed in this way (gas giants with tight orbits around their stars known as "hot-Jupiters"), Seager argues that with the advent of advanced space telescopes, the composition of smaller worlds' atmospheres could also studied. Habitable "super-Earths" fall into this category.
Once this happens, we can begin to observe small rocky worlds, potentially detecting spectroscopic signatures of chemicals associated with life.
Although the next generation of space telescopes may be able to detect biosignatures in nearby exoplanets, Seager urges caution.
"(M)any different gases are produced by life, but the anticipated diversity of exoplanet atmosphere composition and host star environments may yield different detectable biosignature gases than the terrestrial examples. Even with excellent data, false positives will drive a permanent ambiguity in many cases," she adds.
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Molecules such as methane can be generated through biological (methanogenic) and geological (volcanic) processes, so the detection of methane in an exoplanetary atmosphere may not indicate life. To find out what is generating that gas, astronomers will need to study the atmosphere in its entirety to avoid jumping to conclusions about that world's biological potential. The identification of these "false positives," using advanced instrumentation, will be critical when seeking out genuine biosignatures.
The advances in space-based observatories are tantalizing and, with the launch of JWST and other advanced direct imaging telescopes (such as the "star shade" concept), we could start studying small habitable worlds with atmospheres and teasing hints as to any biosignatures within the next couple of decades.
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But to fully investigate this exciting class of exoplanet, "we require the ability to directly image exoplanets orbiting 1,000 or more of the nearest sun-like stars." Such an endeavor would require a huge space-borne observatory - an optical telescope with a diameter exceeding 10 meters. Considering the Hubble Space Telescope is only 2.4 meters in diameter, the exoplanetary atmosphere telescopes of the future will require some huge innovative leaps before they become a reality.
One thing seems certain, however. The longer we gaze into the stars, the more certain we become about the possibility for life beyond Earth.
"Our own Galaxy has 100 billion stars, and our Universe has upwards of 100 billion galaxies - making the chance for life elsewhere seem inevitable based on sheer probability," writes Seager. "We can say with certainty that, for the first time in human history, we are finally on the verge of being able to search for signs of life beyond our solar system around the nearest hundreds of stars."
Publication: The future of spectroscopic life detection on exoplanets, Sara Seager, 2014. doi:10.1073/pnas.1304213111
via The Guardian