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    Science in Mind

    In search for habitable planets, scientists rethink signs

    NASA/Tiago Campante/Peter Devine
    Artist’s conception of another solar system called Kepler-444 , home to five small planets in very compact orbits.

    In the 1960s, scientists began writing papers about what must have seemed like pure science fiction to many: how to detect life on other planets. What gases in the atmosphere would be the tell-tale signs of alien life? Was it blindly Earth-centric of us to limit our search to life as we know it when probing extraterrestrial terra incognita?

    Rather suddenly, what seemed like a fun, though extremely speculative, thought experiment is becoming a nitty-gritty practical challenge for astronomers searching the galaxy for Earth-like worlds.

    The hunt for habitable planets has gone into turbo since the first exoplanet — a planet circling other stars — was detected two decades ago. The list of confirmed exoplanets discovered by NASA’s Kepler space telescope numbers more than 1,000 at last count; more than 4,000 others are still being evaluated.


    “What excites me the most is for the first time in human history, we have the capability to answer this question of, ‘Are we alone in the universe?’ ” said Sarah Rugheimer, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “We’ve been thinking about it as humans for millennia, but it’s just now that we can do it and that’s why it’s such an exciting time to be a scientist in this field and to be alive on this planet.”

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    In a review article published Friday in the journal Science Advances, Massachusetts Institute of Technology astrophysicist Sara Seager lays out the opportunities and the formidable challenges that lie ahead. As scientists have found more worlds that sit in their stars’ “Goldilocks zone,” where it is neither too hot nor too cold for life, the issue of what biosignature to look for is becoming a matter of practical importance.

    “Wouldn’t that be a shame, if we were able to find a whole bunch of planets but we didn’t really know what we’re looking for?” Seager said. “We want to go through all this work now so we’re ready.”

    Exoplanets can be detected through various means but are often found because they cross between their star and Earth, temporarily blocking part of the light of the star. Taking detailed measurements of such transits can tell scientists a lot about the exoplanet — the size, the density, and even clues about the atmosphere.

    It might seem like a straightforward task, then. Scientists simply need to train a powerful telescope on a faraway world long enough to discern whether it has an atmosphere indicative of life.


    Scientists have long hoped that oxygen would be a good litmus test, but Seager said it has become increasingly clear that a combination of gases will need to be detected. That’s because oxygen can be produced by other processes that don’t involve living things.

    She points to the complexity of an exoplanet called GJ 1214b. Initial measurements suggested that planet is either a water world, a rocky world with a hydrogen atmosphere, or a gaseous “sub-Neptune.”

    More precise measurements of the planet didn’t exactly clear things up. After significant extra study and telescope time, scientists didn’t winnow down the possibilities — in fact, they came up with more possible explanations.

    “We may find that we have too little data on a planet of interest or the data may not tell us what we know,” Seager wrote.

    Rugheimer said it has only become more clear as claims about exoplanets have been made, overturned, and refined, that measuring just one gas won’t be the tell-tale sign of life. Understanding which ones we need to look for aids in the design of future telescopes.


    Rugheimer said that her work is guided by excitement over the search paired with the hypercritical self-examination necessary to make sure data that seem like a sign of life can’t be explained some other way. She says she often thinks of a quote from the Nobel prize-winning physicist, Richard Feynman: “The first principle is that you must not fool yourself, and you are the easiest person to fool.”

    Carolyn Y. Johnson can be reached at Follow her on Twitter @carolynyjohnson.