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Signatures of Life

In 1961, NASA was preparing to send a robotic probe to Mars. One of its missions was to look for signs of life. The agency asked independent scientist and inventor James Lovelock to develop a set of sensitive instruments that might support this effort. In preparation for the task, Lovelock found himself intrigued by the Martian atmosphere, rather than its surface. He reasoned that the likelihood of life on the red planet could be deduced without sending instruments there if the atmosphere were properly analyzed.

Lovelock suggested that a planetary atmosphere would have certain elements present if life existed on that planet, and that the atmosphere would tend toward a stable state, or equilibrium, if life were absent.

NASA sent the probe to Mars anyway, and continues to do so in its search for evidence of past or present life on that planet. However, more than 40 years later, Lovelock’s basic idea is being used to look for signs of life in the atmospheres of exoplanets.

Planets outside our solar system are too far away for us to visit, or even send a robot probe that could return within a typical human life time. For scientists interested in exoplanets, the challenge is to detect signs of life without leaving the Earth.

How might we do that? The answer is to look for biosignatures. A biosignature is anything that indicates the presence of life -- in the past or present -- on a planet. The most promising candidates for biosignatures on distant planets are gases in their atmospheres.

Which gases, then, would we like to detect in our hunt for life? Let’s consider the one planet where we know life exists, our own Earth. The remarkable aspect of Earth’s atmosphere, as compared with the atmospheres of all other bodies in the solar system, is the presence of large amounts of oxygen.

As Sara Seager, an MIT professor and leader in the search for life in the universe, told the House Committee on Science, Space, and Technology in December of 2013:

"In exoplanet research, we define biosignature gases as gases produced by life that can accumulate in a planetary atmosphere to levels detectable remotely by large telescopes. We make the assumption that life uses chemistry to capture and store energy, and that life’s chemistry generates gaseous products. Oxygen is Earth’s most robust and abundant biosignature gas, produced by plants and photosynthetic bacteria. Oxygen fills Earth’s atmosphere to 20 percent by volume, but without photosynthetic life, our planet would be virtually anoxic, with only the faintest trace of oxygen in its air."

Living things do produce other gases in addition to oxygen, including methane, nitrous oxide, methyl chloride and dimethylsulfide. However, as Seager also points out in her testimony, these gases are only produced in trace amounts, not enough to be detected by remote observation. Oxygen, or its by-product ozone, remains the best candidate for detecting life.

Thus, scientists know what they are trying to find in the atmospheres of distant planets. The question then becomes how to detect these biosignatures with our telescopes.

None of our current detection methods are sensitive enough to look at the atmospheres of Earth-like planets to determine if they contain biosignatures. Two new telescopes, the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST), will soon be able to help. TESS launches in 2017, and JWST in 2018.

TESS will use the same transiting method of detection that Kepler has employed, but will survey the entire sky, focusing on nearby stars. It is not expected to find many Earth-sized planets, but will be able to see Super-Earths. With JWST, astronomers will be able study a planet’s atmosphere using transit spectroscopy, in which measurements of the exoplanet atmosphere can be made as it is backlit by starlight. JWST will be able to probe the atmospheres of Super-Earths orbiting bright stars, and some of the planets found by TESS will be excellent targets for study with JWST.

Like all other aspects of searching for life in the universe, detecting biosignatures is a challenge. However, the payoff, if the search is successful, will be enormous.

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